Factors affecting the within-river spawning migration of Atlantic salmon, with emphasis on human impacts

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FactorsAffectingthewithin-RiverSpawningMigrationofAtlanticSalmon,withEmphasisonHumanImpacts

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DOI:10.1007/s11160-007-9076-4

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RESEARCH PAPER

Factors affecting the within-river spawning migrationof Atlantic salmon, with emphasis on human impacts

Eva B. Thorstad Æ Finn Økland Æ Kim Aarestrup ÆTor G. Heggberget

Received: 8 May 2007 / Accepted: 19 October 2007 / Published online: 20 November 2007

� Springer Science+Business Media B.V. 2007

Abstract We review factors affecting the within-

river spawning migration of Atlantic salmon. With

populations declining across the entire distribution

range, it is important that spawners survive in the last

phase of the spawning migration. Knowledge on the

factors affecting migration is essential for the

protection of populations, and to increase the success

of reintroduction programmes. A number of studies

have documented that the upstream migration may be

delayed for many weeks at man-made obstacles such

as power station outlets, residual flow stretches,

dams, weirs and fishways. The fish may also be

delayed at natural migration barriers. Often, the

magnitude of delay is not predictable; fish may be

considerably delayed at barriers that appear to

humans to be easily passable, or they may quickly

pass barriers that appear difficult. Stressful events

like catch-and-release angling may affect upstream

migration. Impacts of human activities may also

cause altered migration patterns, affect the within-

river distribution of the spawning population, and

severe barriers may result in displacement of the

spawning population to other rivers. Factors docu-

mented to affect within-river migration include

previous experience, water discharge, water temper-

ature, water velocity, required jump heights, fish size,

fish acclimatisation, light, water quality/pollution,

time of the season, and catch and handling stress.

How each of these factors affects the upstream

migration is to a varying extent understood; however,

the effects may differ among different river sections

and sites. There are likely a number of additional

important factors, and the relationship between

different factors is complex. The understanding of

general mechanisms stimulating fish within-river

migration are still lacking, and it cannot be reliably

predicted under which conditions a fish will pass a

given migration barrier or which conditions are

needed to stimulate migration at different sites. The

strong focus on the effects of water discharge in past

work may have hampered consideration of other

factors. Exploration of the influence of these other

factors in future studies could improve our under-

standing of what controls the upstream migration.

Keywords Atlantic salmon � Salmo salar �Upstream migration � Spawning � Water discharge �Human impacts

Introduction

Humans have exploited fishes during their migrations

for several thousand years (Lucas and Baras 2001).

E. B. Thorstad (&) � F. Økland � T. G. Heggberget

Norwegian Institute for Nature Research (NINA),

Tungasletta 2, 7485 Trondheim, Norway

e-mail: [email protected]

K. Aarestrup

Department of Inland Fisheries, Danish Institute for

Fisheries Research, Vejlsøvej 39, 8600 Silkeborg,

Denmark

123

Rev Fish Biol Fisheries (2008) 18:345–371

DOI 10.1007/s11160-007-9076-4

Today, many migrating species have a high economic

value. In many areas draining to the North Atlantic

Ocean, the Atlantic salmon (Salmo salar) is one of

the most valuable species, both economically and

culturally.

Atlantic salmon life history

Most Atlantic salmon populations are anadromous

(Klemetsen et al. 2003). The migration between

freshwater and sea is seen as a strategy of adaptive

value, with individuals utilising the best suited

habitat during different stages of the life cycle to

increase individual fitness (Lucas and Baras 2001).

Atlantic salmon spawn in rivers in autumn and

winter, and juveniles remain in freshwater for

1–8 years before they migrate to sea for feeding

(Klemetsen et al. 2003). At sea, they are distributed

over large areas in the North Atlantic Ocean

(Hansen and Quinn 1998). After 1–5 years, they

return to freshwater for spawning (Klemetsen et al.

2003). Generally, Atlantic salmon return with a

high precision to their home river (Hasler 1966;

Harden Jones 1968), although a small percentage

stray to other rivers (Stabell 1984; Jonsson et al.

1991a). Precise homing may form and maintain

local adaptations through natural selection, and

salmon populations in different rivers differ both

ecologically and genetically (Taylor 1991; Klemet-

sen et al. 2003; Verspoor et al. 2005; Garcia de

Leaniz et al. 2007). Moreover, Atlantic salmon

apparently return to the same area of the river

where they spent their pre-smolt period, and

ecological and genetic differences among subpop-

ulations within rivers are also documented

(Heggberget et al. 1986, 1988; Summers 1996;

Verspoor et al. 2005; Primmer et al. 2006). Atlantic

salmon may spawn repeatedly; however, the mor-

tality is high and most individuals spawn once or

twice (Jonsson et al. 1991b; Klemetsen et al. 2003).

During the upstream migration, Atlantic salmon do

not feed, and energy reserves are used to fuel body

maintenance, gonad growth and migration (Jonsson

et al. 1997). Total energy loss due to migration and

spawning may be more than 60% of the body

reserves prior to upstream migration (Jonsson et al.

1997).

Atlantic salmon spawning migration

Atlantic salmon typically start entering coastal home

waters and rivers from the sea several months prior to

spawning, and timing of the run is highly variable both

within and among populations (Fleming et al. 1996;

Klemetsen et al. 2003). Most Atlantic salmon in

Norway and Canada enter the rivers from May to

October (Klemetsen et al. 2003), with a general

tendency for large multi-sea-winter salmon to enter

the rivers earlier in the season than smaller one-sea-

winter fish (Power 1981; Jonsson et al. 1990). In

Scotland and other parts of the UK, salmon can enter

the rivers in all months of the year, with some

individuals entering more than a year prior to spawn-

ing (Klemetsen et al. 2003). In rivers on the Kola

Peninsula in Russia, such as the River Varzuga, there

is a summer run of salmon spawning the same year,

and an autumn run of salmon remaining in the river

until the spawning period the year after (Lysenko

1997). Run timing has been associated with several

river characteristics including hydrological condi-

tions, temperature regime, length and difficulty of

ascent and sea age at maturity (Fleming 1996;

Klemetsen et al. 2003). There is, however, no satis-

factory adaptive explanation for the early entry time of

Atlantic salmon, which results in lost feeding oppor-

tunities at sea, and thus reduced growth and ultimately

reduced reproductive success (Fleming et al. 1996).

Control of the timing of migration depends on an

interaction between the internal physiological state of

the fish and external triggering factors in the environ-

ment (Northcote 1984). Information from the

environment can affect migratory behaviour in two

ways; the environmental stimulus may alter fish

orientation, and also change the intensity of move-

ment (Northcote 1984). Water discharge appears to be

an important proximate factor stimulating adult

Atlantic salmon to enter rivers from the sea, but acts

in combination with other environmental factors

(reviewed by Banks 1969; Jonsson 1991).

Rationale

Information on the within-river migration patterns of

Atlantic salmon has previously been limited, and

mainly based on records of fish passage in fishways,

mark-recapture studies and catch statistics (the

346 Rev Fish Biol Fisheries (2008) 18:345–371

123

limitations of these methods are discussed below).

The availability of telemetry methods the two last

decades has considerably expanded our knowledge

on upstream migration patterns within rivers and the

factors affecting this phase of the spawning

migration.

There are several reasons that the upstream migra-

tion phase in the river is important and has received

much focus in recent years. Factors affecting spawn-

ing migrations may lead to reduced spawning success

and survival, and hence declining populations (Lucas

and Baras 2001). Within the whole distribution range,

Atlantic salmon populations are in decline, despite

reduced exploitation by marine fisheries (Parrish et al.

1998; Klemetsen et al. 2003; ICES 2006). Human

impacts such as overexploitation, acid deposition,

transfer of parasites and diseases, pollution, aquacul-

ture, freshwater habitat degradation, hydropower

development and other river regulations are likely to

have contributed to this decline, but their exact roles

are poorly understood. With declining populations, it

is important that adult Atlantic salmon succeed in the

last phase of the return migration and reach the

spawning areas. Power stations, dams and flow

regulations can and have caused major passage

problems for Atlantic salmon (discussed below), and

in several regulated rivers, salmon populations have

been eliminated or substantially reduced due to the

loss of areas for natural production (Karlsson and

Karlstrom 1994; Larinier 1998; Northcote 1998;

Anonymous 1999; NRC 2004). For reintroduction

programmes to succeed, and for the protection of

existing populations, knowledge on factors affecting

salmon migrations and access to spawning grounds is

essential. Moreover, upstream migration in the river is

the foundation for the popular and economically

important recreational salmon angling industry. In

many rivers, a high proportion of the returning

spawning population are exploited through angling

(Mills 1991; Anonymous 1999; ICES 2006), further

emphasizing the importance of a strong knowledge

base for management decisions concerning this

migration phase.

Objective

The objective of this paper is to synthesise and

review various factors affecting the within-river

spawning migration of Atlantic salmon, with special

emphasis on human impacts. The review is based on

published literature, and should serve as a knowledge

base for scientists, and in the management and

conservation of salmon stocks. The synthesis covers

effects of intrinsic factors, previous experience,

hydropower installations, other man-made obstacles,

water discharge on different river sections, day and

night, water temperature, other natural environmental

factors, water pollution and catch-and-release

angling. Although these factors are discussed in

separate sections, the high level of complexity

associated with the factors initiating and controlling

upstream migration advise against considering each

factor in isolation. The different methods used to

study within-river migrations restrict comparisons of

the results, and the methods are therefore briefly

described and evaluated. The consequences of factors

affecting the upstream migration are discussed in the

final part of the paper, and the requirements and

directions of future research are outlined.

Methods of studying upstream migration patterns

Upstream migration of Atlantic salmon in rivers has

traditionally been studied by counting fish passing

fences and traps, either with manual counting or

using automatic fish counters, by analysing catch

statistics, or performing mark and recapture studies

(Banks 1969; Jonsson 1991). Such methods are

limited for studying the effects of various factors on

the upstream migration patterns.

The problem with correlating counts of fish

passing a fixed location in a river with different

environmental factors is the lack of information on

how many fish are present downstream of the

counting site (Trepanier et al. 1996). Even if envi-

ronmental conditions are favourable for upstream

migration, count data may show little migration

activity if there are no fish available in the area

during this period. Likewise, an increase in upstream

counts may not mean that conditions are necessarily

optimal, but could reflect increased fish abundance

in the area due to other reasons (such as recently

favourable conditions for fish entering the river

from sea, or for passage at a downstream obstacle).

The serial correlation, or temporal autocorrelation,

Rev Fish Biol Fisheries (2008) 18:345–371 347

123

that characterizes time series must therefore be

considered in the statistical analyses (Ostrom 1990;

Trepanier et al. 1996). If social factors, such that

moving upstream in groups, is more important to the

fish than the environmental factors studied, it poses a

further dilemma to scientists and managers how to

statistically weight large groups of fish passing the

counter. When observing groups of Atlantic salmon it

seems that the movement of one individual may

stimulate others to follow (personal observation from

bridges and fishways), but such social mechanisms

have never been studied and considered in analyses of

factors affecting the upstream migration. Carr et al.

(2004) designed a migration study in which they

attempted to examine how the presence of fellow

travellers influenced migration patterns of hatchery-

reared, wild-strain-origin maturing salmon released

to a river to undertake spawning migration. The

presence of fellow travellers had no effect, possibly

because of the hatchery experience of the fish and the

attraction to the outlet pipe of the hatchery. More-

over, fish counters are usually placed in fishways, and

the environmental factors important to stimulate

salmon to pass such sites may be specific for each

artificial construction and different from natural river

sections with other or no migration barriers (Banks

1969; Smith et al. 1997).

Results based on angling catches are generally not

suitable for studying factors affecting the upstream

migration, because the susceptibility of fish to capture

and changes in catches may not correspond to

changes in migratory activity (Hayes 1953; Alabaster

1970; Solomon et al. 1999).

Mark and recapture studies have generally pro-

vided important information on fish migrations

(Lucas and Baras 2001). However, the method

provides little information on the migration pattern

between the mark and recapture site and, thus,

provides limited information on factors affecting

migration patterns. The method also limits informa-

tion to the individuals being recaptured, which

usually constitute a small proportion of the tagged

individuals. If recaptured individuals have an

increased likelihood of being recaptured compared

to non-recaptured individuals because they possess a

particular migration pattern, results on migration

behaviour may not be representative of the

population.

Fish telemetry using radio and acoustic transmitters

to study movements of individual fish, is a well suited

method for studying upstream migration in adult

Atlantic salmon and other fish species. Movements of

tagged individuals can be recorded by following

signals from the transmitter using a portable, manual

receiver, or by installing fixed automatic listening

stations recording fish within the range of the station.

More detailed movements within an area can be

automatically recorded by using several antennas or

receivers, or by using CDMA (code division multiple

access) technology (Thorstad et al. 2003b; Cooke

et al. 2005). Radio transmitters are best suited in rivers,

because acoustic transmitters often have reduced range

in turbulent and flowing water (Thorstad et al. 2000c).

Information on, for example, water temperature, fish

depth, muscle activity, swimming speed, and heart rate

can also be recorded using telemetry transmitters (e.g.

Økland et al. 1997; Anderson et al. 1998; Thorstad

et al. 2000b; Cooke et al. 2004). The most common

limitation in telemetry studies is low sample sizes.

Transmitters are relatively expensive, it may be

difficult and expensive to catch a large number of fish

for tagging, and the tracking effort may be challenging

and time-consuming when following a large number of

individuals. Furthermore, it is important to minimise

the impacts of the capture, handling and tagging

procedures on the fish, to avoid biasing the study

(reviewed by Jepsen et al. 2002; 2005b; Bridger and

Booth 2003). External attachment, surgical implanta-

tion and stomach implants may all be suitable tagging

methods in upstream migration studies of Atlantic

salmon (Smith et al. 1998; Thorstad et al. 2000a, 2001;

Rivinoja et al. 2006).

Catching, handling and tagging Atlantic salmon

after they have entered the rivers may affect the

upstream migration pattern by causing stress-related

delays and/or downstream movements after release

(Gerlier and Roche 1998; Webb 1998; Makinen et al.

2000; Jokikokko 2002; Thorstad et al. 2005a). Catch-

ing salmon in the sea prior to river entry is therefore

preferred method for telemetry studies investigating

the patterns of upstream migration of salmon. This

method will not always be practical, but where

possible bag nets, hoop nets and trap nets have

proven to be gentle catch methods that can be applied

in fjord areas and river mouths (Thorstad et al. 1998,

2003b, 2005a, Jokikokko 2002).

348 Rev Fish Biol Fisheries (2008) 18:345–371

123

Factors affecting the upstream migration pattern

Migration pattern in pristine rivers without

significant migration barriers

When studying anthropogenic factors affecting the

upstream migration patterns, comparative informa-

tion is needed on timing, migration speed and arrival

to the spawning area as a reference on migratory

behaviour of fish in pristine rivers without significant

migration barriers.

In generally undisturbed systems, the riverine

migration of Atlantic salmon has been reported to take

place in two or three successive phases before spawn-

ing: (1) a migration phase with steady progress upriver

with periods of swimming alternating with stationary

resting periods, (2) a search phase with movements

both up and down river at or close to the position held at

spawning, followed by (3) a long residence period, also

termed the holding phase (Hawkins and Smith 1986;

Heggberget et al. 1988, 1996; Karppinen et al. 2004;

Laughton 1989; Webb 1989; Bagliniere et al. 1990,

1991; Laughton 1991; Økland et al. 2001; Finstad

et al. 2005). In the Norwegian River Tana, the number

of resting periods during the upstream migration phase

(0–9) increased with migratory distance, and the

resting periods lasted on average 5–8 days (Økland

et al. 2001). After the upstream migration, the Tana

fish had a holding period of more than 50 days with

little or no movement until spawning. The proportion

of time spent on the migratory phase increased, while

the proportion of time spent on the holding phase

decreased with increasing distance to the spawning

area in this large river (Økland et al. 2001). In contrast,

in the River Lærdalselva, the relationship between the

time spent migrating versus resting was not significant,

indicating that a smaller part of the time budget was

spent on the migration phase in this much smaller river

(the longest distance anadromous fish can migrate from

the sea is 300 km in the River Tana and 24 km in the

River Lærdalselva) (Finstad et al. 2005). In some UK

rivers, where salmon may enter during any month of

the year and therefore stay for longer periods in the

river, resting periods during the migration phase may

last many months, with fish leaving holding pools and

migrating further upstream towards the spawning areas

in the autumn (Hawkins 1989; Laughton 1989; Clarke

et al. 1991; Laughton 1991; Solomon et al. 1999).

A distinct ‘search phase’ after the upstream migra-

tion phase has only been reported by Økland et al.

(2001), Jokikokko (2002) and Finstad et al. (2005),

and this behaviour was observed for 60–67% of the

individuals. The search phase may not necessarily

consist of several up- and downstream movements, but

may also consist of only one downriver movement,

termed an ‘overshoot’ (Økland et al. 2001; Jokikokko

2002). The length of the river section where searching

took place was on average 8 and 15 km (two study

years) in the large River Tana (Økland et al. 2001) and

only 1.7 and 1.4 km in the smaller rivers Simojoki

(Finland) and Lærdalselva, respectively (Jokikokko

2002; Finstad et al. 2005). This movement pattern may

be important in order to select a spawning area, find

potential mates or locate a position to spend time until

spawning (Økland et al. 2001). The search phase may

also be part of the orientation mechanism that facili-

tates Atlantic salmon to return to the same area of the

river where they grew up. The generally observed

migration pattern, with a relatively fast and steady

upstream migration, seemingly to a certain site,

followed by a long residence period, is consistent with

the hypothesis that Atlantic salmon home to a site

‘‘they know’’ in the river.

Maximum net ground speeds recorded during

undisturbed migration was 37 km day-1 in the Ab-

erdeenshire Dee, 15 km day-1 in the River

Lærdalselva, and 49 km day-1 for multi-sea-winter

salmon and 47 km day-1 for grilse in the River Tana

(Hawkins and Smith 1986; Økland et al. 2001;

Karppinen et al. 2004; Finstad et al. 2005). The

highest migration rates were recorded early in the

river migration phase and generally decreased as the

fish approached the spawning ground in all four

studies. Mean net ground speeds recorded in different

studies generally varied between 1.6 and 31 km per

day (Hawkins 1989; Heggberget et al. 1996; Gerlier

and Roche 1998; Johnsen et al. 1998; Thorstad et al.

1998; 2003b, 2005b; Økland et al. 2001; Rivinoja

et al. 2001; Karppinen et al. 2004). However, migra-

tion speeds are difficult to compare among studies

because of the different methods of collecting data.

Even a daily tracking programme can underestimate

migration speeds, because the fish may not have

followed the shortest route between fixes, but

migrated up- and downstream between tracking

surveys. Furthermore, the actual swim speeds vary

Rev Fish Biol Fisheries (2008) 18:345–371 349

123

with the water velocity that the fish are swimming

against. The only information available for upstream

migrating Atlantic salmon on migration speeds are

relative to the ground, and to our knowledge, no

author has tried to account for the velocity of the

water that the fish are swimming against.

In some river systems, salmon must pass through

natural lakes before reaching the spawning grounds.

No information on the migration pattern or speeds of

Atlantic salmon through lakes is available, to our

knowledge.

Intrinsic factors

Maturation stage, energy state, hormonal control and

stress level may all be physiological factors affecting

and controlling migration patterns. Such intrinsic

factors are sometimes collectively referred to as

‘motivation’ for migration (e.g. Johnsen et al. 1998;

Thorstad et al. 2005b). Although intrinsic factors

may be a key to understanding migration patterns,

there are few studies of how these factors affect

migration alone or interact with other factors. A fish

may not for instance pass a migration barrier before a

certain internal state is reached, and at the same time

certain environmental conditions must be present.

Few studies have suggested that there are sex-

related differences in migration patterns, but Karppi-

nen et al. (2004) found that females displayed a

somewhat more variable and stepwise migration

pattern in an undisturbed river than did males.

Further, males tended to be present and active on

the spawning grounds for a longer time period than

females (Webb and Hawkins 1989).

The motivation to migrate may increase as spawn-

ing time approaches. Longer delay times below

fishway entrances for early run fish have been observed

by Gowans et al. (1999b) and Laine et al. (2002).

Further, the motivation to pass a waterfall increased as

the spawning season approached (Johnsen et al. 1998).

However, both sustained and prolonged swimming

performance of Atlantic salmon seem to be reduced

during the upstream migration towards spawning,

probably linked to a combination of decreased water

temperatures and changes in body morphology and

depletion of lipid levels (Booth 1998).

Acute stress may lead to impaired swimming

performance (Wedemeyer and McLeay 1981), which

might affect migration patterns. (See also the catch

and release section below.)

Previous experience

Learning is important for the precise homing of

Atlantic salmon, and the smolts probably learn

sequentially the way back to their spawning area as

they leave the river and migrate to their feeding areas

in the ocean (Harden Jones 1968; Hansen et al.

1987). Several studies have shown that hatchery-

reared and farmed fish deprived of natural juvenile

river life differ from wild fish in their return

migration pattern (see below).

Hatchery-reared salmon are produced and

released in many rivers to support populations, for

example as compensation for lost spawning areas or

to re-establish lost populations. When released in

the river mouth or further upstream, hatchery-reared

salmon returning to the river as adults tend to have

a more erratic movement pattern in the river than

wild salmon (Power and McCleave 1980; Jonsson

et al. 1990; 1991a; Potter and Russell 1994;

Jokikokko 2002; Croze 2005; Jepsen et al. 2005a).

As a consequence of this erratic up- and down-

stream within-river migration, hatchery-reared fish

spent a longer time than wild fish in the river

before settling down in the spawning area (Jok-

ikokko 2002). However, in one study wild and

hatchery-reared fish released in the river did not

differ in migration speeds in the lower river reaches

when returing as adults (Rivinoja et al. 2001).

Furthermore, hatchery-reared salmon did not differ

from wild salmon in their ability to clear obstacles

(Croze 2005). Hatchery-reared salmon released in

the river mouth or in the lower reaches may spread

themselves further upstream than the release site

when returning as adults (Power and McCleave

1980; Jonsson et al. 1990, 1991a; McKinnell et al.

1994; Potter and Russell 1994; Jokikokko 2002),

whereas fish released higher up in the river and

within habitats usually judged to be optimal for

salmon seem to return to the site of release with a

relatively high precision (Power and McCleave

1980; Potter and Russell 1994; Rivinoja et al.

2001). Hatchery-reared salmon have also been

shown to return to the within-river outlet of the

hatchery where they were reared (Carr et al. 2004).

350 Rev Fish Biol Fisheries (2008) 18:345–371

123

Hatchery-reared salmon seem to have a reduced

spawning success compared to wild fish; hatchery-

reared fish were to a larger extent injured during the

spawning period, stayed for a shorter time in the

river and more often returned to sea without having

spawned (Jonsson et al. 1990, 1991a). A higher

mortality for hatchery-reared fish during the spawn-

ing migration has also been recorded (Jepsen et al.

2005a).

The production of farmed Atlantic salmon has

increased over the past decades, to a total in the North

Atlantic area of 831,000 t in 2004; most of the

production taking place in Norway and Scotland

(ICES 2006). Open net-pen culture in marine systems

can result in loss of farmed salmon into the wild, and

escapes from Atlantic salmon farms occur as both

repeated ‘‘trickle’’ losses of relatively small numbers

of fish, and through large-scale episodic events

(Naylor et al. 2005). Escaped farmed salmon may

enter rivers as adults, and may comprise relatively

high proportions of the spawning populations (annual

mean of 11–35% in monitored rivers in Norway

during 1989–2000, Fiske et al. 2001). Negative

effects by escaped farmed salmon on wild Atlantic

salmon populations include both ecological interac-

tions and genetic impacts of inter-breeding

(McGinnity et al. 1997; Fleming et al. 2000; McGin-

nity et al. 2003).

Escaped farmed salmon differ from wild salmon in

several ways; they may lack river imprinting, newly

escaped fish frequently have eroded fins (Fiske et al.

2005), they have less physical training and a higher

fat content (Thorstad et al. 1997), and they are

genetically different as a result of artificial selection

(Roberge et al. 2006). Despite being seemingly

physically inferior, in the few studies where farmed

fish performance was compared with that of wild fish,

escaped farmed salmon migrated as fast upstream as

wild salmon, and they distributed themselves further

upriver. This was true both for farmed fish that had

escaped before the spawning run period and stayed

for some time in nature, and for newly escaped fish

that quickly entered rivers (Heggberget et al. 1993a,

1996; Thorstad et al. 1998; Butler et al. 2005). This

physical ability was confirmed by a laboratory study,

where endurance in forced swim trials did not differ

between adult farmed and sea-ranched Atlantic

salmon (Thorstad et al. 1997). However, there are

indications that escaped farmed salmon are less

capable of passing large and difficult waterfalls than

wild salmon (Johnsen et al. 1998).

The upstream migration of escaped farmed salmon

was less affected by variation in water discharge than

that of wild salmon; unlike farmed salmon, the

number of riverine movements by wild salmon

increased significantly when variation in water flow

increased (Thorstad et al. 1998). No erratic move-

ment pattern was found in farmed compared to wild

salmon during the upstream migration phase (Heggb-

erget et al. 1996), but the farmed fish showed more

and longer up- and downstream movements during

the spawning period (Økland et al. 1995; Thorstad

et al. 1998). Despite a tendency for farmed salmon

being distributed higher up in the river during

spawning, wild and farmed salmon were not geo-

graphically separated, and farmed fish occurred in

parts of the river with important wild salmon

spawning areas (Heggberget et al. 1996; Thorstad

et al. 1998; Butler et al. 2005). However, a smaller

proportion of farmed males were recorded on the

spawning grounds (Økland et al. 1995). The distri-

bution of escaped farmed salmon high up the rivers

can be explained by their lack of previous river

experience and imprinting: they lacked a ‘stop signal’

for a particular home area of the river (Heggberget

et al. 1996; Thorstad et al. 1998). In contrast, previ-

ous studies reported farmed salmon to be more

confined to lower reaches than wild salmon (Webb

et al. 1991, 1993a, b). However, the farmed salmon

in these studies originated from a hatchery using river

water from lower reaches before being transported to

sea pens, and they were therefore probably imprinted

to the lower reaches. Farmed males were more widely

distributed throughout the river’s length than farmed

females, which may reflect a more opportunistic,

explorative behaviour of males (Webb et al. 1991).

Non-maturing escaped farmed salmon entering a

Canadian river failed to migrate to known spawning

areas, likely due to their failure to mature sexually

(Carr et al. 1997b).

A common feature of both hatchery-reared and

escaped farmed salmon seems to be their late river

entry compared to wild salmon (Heggberget et al.

1993a; Jonsson et al. 1990, 1991a, 1994; Carr et al.

1997a; Thorstad et al. 1998), resulting in upstream

migration and entering of spawning areas later in the

season. The longer migration distance of escaped

farmed salmon (see above) further adds to a late

Rev Fish Biol Fisheries (2008) 18:345–371 351

123

arrival at the spawning grounds. Another common

feature seems to be the erratic movement patterns in

the river (references above, Aarestrup et al. 2000),

which may have several explanations. The lack of

previous river experience may lead to difficulties in

selecting and defending spawning sites, which may

be intensified by their late arrival. Farmed salmon

may have less success in spawning interactions

(Fleming et al. 1996; Weir et al. 2004), and there-

fore, migrate from spawning ground to spawning

ground, or these fish could be competitively inferior

(Fleming et al. 1996; Weir et al. 2004) and may be

chased off by wild fish.

The differences in upstream migration behaviour

between hatchery-reared, farmed and wild salmon is

probably largely explained by previous experience,

but physiological, morphological and genetic differ-

ences may also add to these differences (discussed by

Jonsson et al. 1990, 1991a; Heggberget et al. 1996;

Thorstad et al. 1998).

The ability of adults to learn migration routes is

believed to be minimal (Thorstad et al. 2003b;

Hansen and Jonsson 1994), but information on

within-river migration of for instance previous

spawners compared to first-time spawners has not

been found. Adult salmon without any previous river

experience were transported downstream from a dam

and had to repeatedly pass a difficult river section

past a power station outlet and residual flow section.

The salmon that had just experienced this river

section were as delayed as naive salmon (Thorstad

et al. 2003b). In contrast, a study of Whoriskey and

Carr (2001) indicates that transplanted escaped

salmon may learn their way in the marine environ-

ment well enough to relocate a river that they tried to

ascend the first time.

Power station outlets

In regulated rivers, the often much higher water

discharge from power station tunnels frequently

attract upstream migrating Atlantic salmon and can

delay and hinder their further migration (Webb 1990;

Gerlier and Roche 1998; Rivinoja et al. 2001;

Karppinen et al. 2002; Thorstad et al. 2003b,

2005b; Lundqvist et al. 2007; Scruton et al. 2007).

Delays at power station outlets can be long (see

below).

At some power station outlets, Atlantic salmon

passage is provided in the old river stretch, which has

a reduced water discharge (residual flow section/

bypass channel) but no initial migration barrier. The

dam and fishway may be situated at some distance

upstream of the hydropower discharge. At such

power station outlets, stops of a median of 20 days

(Thorstad et al. 2003b), a mean of 42 days (Thorstad

et al. 2005b) a mean of 12 days (Lundqvist et al.

2007) and a range 1–12 days (Scruton et al. 2007)

have been recorded, depending of the study. The

salmon seem to be strongly attracted by the high

discharge from the power station. Finding and using

the bypass stretch, which typically has a much lower

water discharge, is apparently a problem for the fish

(Rivinoja et al. 2001; Thorstad et al. 2003b, 2005b;

Lundqvist et al. 2007; Scruton et al. 2007). Rivinoja

et al. (2001) speculated that smolts could have

migrated through the power station during their

outward migration and that the adults, therefore,

were attracted to the route they were imprinted to.

This could be a contributing factor to the attraction at

some power station outlets, but no studies have so far

examined this hypothesis. Fish behaviour at the

tunnel outlets seems to be dependent on the water

discharge and design of the outlet (Thorstad et al.

2005b; Lundqvist et al. 2007; Scruton et al. 2007).

When access to the power station tunnel is not closed

by a mechanical screen, salmon may even enter the

tunnel (Rivinoja et al. 2001; Thorstad et al. 2003b)

and stay for days and weeks inside the dark tunnel

(Thorstad et al. 2003b). One consequence of staying

inside the tunnel is that the fish will not be stimulated

to find and enter the bypass stretch when freshets or

other stimuli may occur, which may further extend

the delay. The tendency to enter the tunnel may be

affected by physical and environmental factors at the

tunnel outlet and in the tunnel. The tendency to stay

inside the tunnel was considerably decreased at one

power station when residual flow in the old river

stretch was increased from 3 to 5 m3 s-1 (Thorstad

et al. 2005b). However, this increase in the residual

flow was not sufficient to increase the proportion of

fish entering the residual flow section (Thorstad et al.

2005b).

At some power stations, fish are not only delayed

by attraction to the power station outlet/tailrace, but

may be further delayed when required to pass the

associated dam and fishway. At such sites, in

352 Rev Fish Biol Fisheries (2008) 18:345–371

123

different studies, stops of a mean of 24 days (Webb

1990), a mean of 15 days (Gowans et al. 1999b), up

to 137 days (Chanseau and Larinier 1998, 1999) and

up to several weeks (Karppinen et al. 2002) have

been recorded. The fish may be more attracted by the

high water discharge from the power station or over

the dam, than by the small discharge from the

fishway, and there may be an additional delay if the

fishway does not function well, even when found by

the fish (Webb 1990; Chanseau and Larinier 1998,

1999; Gowans et al. 1999b; Karppinen et al. 2002;

Larinier et al. 2005, see also below). Installation of

screens to cancel the strong counter attraction flow

from the turbine discharge increased the success of

salmon in finding and entering a fishway at one site

(Gowans et al. 1999b).

Fish unable to pass do not necessarily remain at

the power station outlet/tail race. A proportion can

migrate downstream one or more times (Webb 1990;

Chanseau and Larinier 1998, 1999; Gowans et al.

1999b; Rivinoja et al. 2001; Thorstad et al. 2003b,

2005b; Larinier et al. 2005; Lundqvist et al. 2007;

Scruton et al. 2007). Salmon may even give up

passing the outlet to spawn elsewhere (Webb 1990;

Chanseau and Larinier 1999; Rivinoja et al. 2001;

Thorstad et al. 2003b, 2005b), and in some cases both

wild and hatchery-reared salmon returned to sea

without spawning (Chanseau and Larinier 1998;

Rivinoja et al. 2001; Jepsen et al. 2005a).

In some countries, water may be withdrawn from a

river through fish farms, before entering the river

again downstream of the fish farm. Such sites may

cause similar problems as power station outlets,

where fish are attracted to the often larger water

discharge from the fish farm and may even enter the

fish farm (Jepsen et al. 2005a). Other properties of

fish farm effluent, such as pheromones, water

temperature variation and faecal matter might also

affect behaviour and attraction by wild fish to the

discharge water from fish farms. However, we are not

aware of any Atlantic salmon studies of these issues.

Man-made obstacles: dams, weirs, fishways

and reservoirs

Atlantic salmon migrating upstream are vulnerable to

delays at man-made obstacles other than power

station outlets (Gerlier and Roche 1998; Chanseau

et al. 1999; Solomon et al. 1999; Croze 2005;

Thorstad et al. 2005b; but see Smith et al. 1997),

and even dams and weirs that seem not to be

physically difficult for salmon to pass may cause

considerable delays (Gerlier and Roche 1998; Solo-

mon et al. 1999; Ovidio and Philippart 2002;

Thorstad et al. 2005b). Successive minor obstacles,

such as weirs, may cumulatively reduce a fish’s

motivation to migrate, even though no single weir can

be identified as the main obstacle (Thorstad et al.

2003b; 2005b). Similar to the pattern at power station

outlets, other man-made obstacles may also cause

erratic movement patterns downstream of the obsta-

cle (Croze 2005; Thorstad et al. 2005b). Fish may

even abandon their migration, leaving the river and

entering neighbouring watercourses. For example,

Croze (2005) found that 35% and 22% of the fish in

two successive years left the river when confronted

with an obstacle, and most of them were later

recorded in neighbouring rivers. The cumulative

effect of the 28 weirs in this river indicated that only

4% of the spawning population was likely to get

through the channellized part and reach areas suitable

for reproduction (Croze 2005).

The reasons for the reluctance of Atlantic salmon

to pass man-made obstacles are not known. For some

obstacles, maximum swimming speed, endurance or

jumping capability may be a limiting factor. Swim-

ming speeds in fish can be classified as sustained,

prolonged and burst speeds (Beamish 1978). Sus-

tained swimming speeds can be maintained for more

than 200 min, and the mobilisation of energy is

achieved through aerobic processes so that the

quantity of oxygen consumed is proportional to the

amount of work performed. Burst swimming speeds

can only be maintained for a few seconds because the

energy is mainly achieved through anaerobic pro-

cesses, resulting in an oxygen debt because the

metabolic products such as lactate must be oxidised

during the recovery phase after exercise. Prolonged

speeds can be maintained between approximately

20 s and 200 min, and the energy is achieved through

both aerobic and anaerobic processes (Beamish

1978). Hence, the higher the water velocity the fish

must swim against to pass an obstacle, the shorter

time period the fish is able to maintain this speed.

Maximum swimming speeds depend on fish body

length (Beamish 1978; Videler and Wardle 1991).

Maximum swimming speeds for Atlantic salmon

Rev Fish Biol Fisheries (2008) 18:345–371 353

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grilse (50–60 cm body length) over 18–20 m dis-

tances are at least 4 m s-1, or 8 body lengths s-1,

according to laboratory studies by Booth et al. (1996)

and Colavecchia (1998). Beamish (1978) reported

maximum swimming speeds of adult Atlantic salmon

of 4.3–6.0 m s-1, or 5.8–8.4 body lengths s-1.

Maximum swimming speeds in nature are likely

higher than those recorded under laboratory condi-

tions. Bainbridge (1958, 1960) suggested that a

100 cm rainbow trout (Oncorhynchus mykiss) may

be capable of swimming up to 9.5 m s-1, and it is not

unlikely that Atlantic salmon may be able to obtain

speeds up to 10 body lengths s-1.

Little is known about the heights of which Atlantic

salmon are capable of jumping. Physical conditions

of the environment, such as water depth and velocity

beneath the obstruction, are important for the max-

imum leaping capability (Stuart 1962). It is

particularly important that the pool beneath the

obstacle where fish have to jump is deep enough to

permit adequate acceleration (Stuart 1962). Beach

(1984) gave an example of Atlantic salmon jumping a

3.65 m waterfall, which required a launch velocity of

8.46 m s-1. Environmental variables and biological

factors such as body size (muscle power), body

proportions and nutritional condition affect swim-

ming and jumping capabilities of fish (Videler and

Wardle 1991; Beamish 1978).

However, it is clear that Atlantic salmon may also

be reluctant to pass obstacles that should not pose any

physical problem (Thorstad et al. 2003b, 2005a).

Man-made obstacles have a different design than

natural obstacles and may not stimulate passage of

fish in the same way. The obstacle may for instance

be installed in a river section with generally low

water discharge. Further, the obstacle may have a

design that provides little current masking where the

fish is supposed to enter. There may also be

competing water flows from other sources where

the fish cannot pass (e.g. spill over dams), or the

water flow over the obstacle may be laminar, which is

not often the case at natural migration barriers. It

could also be that excessive turbulence may disori-

entate the fish or prevent passage of obstacles at high

water flows. According to Banks (1969), it seems that

in salmonids there is a conflict between the need for

light in order to ascend obstacles, and a preference for

darkness or turbid water in unobstructed passages as

an antipredator device. It may be that Atlantic salmon

are reluctant to pass obstacles where they become

exposed, although generally during upstream migra-

tion Atlantic salmon are not very vulnerable to visual

predators. However, such antipredator behaviour may

be highly important in other life stages in Atlantic

salmon, and this may be a basic instinct that remains

active through all life stages.

Fishways of various designs have been provided in

many river systems to facilitate passage of Atlantic

salmon past power stations and other migration

barriers (Beach 1984). Fishways may not always be

successful in facilitating upstream migration, and the

fishway itself may act as an obstacle and delay

upstream migration (Chanseau and Larinier 1999;

Chanseau et al. 1999; Lundqvist et al. 2007). Prob-

lems are related to both finding the entrance of the

fishway (Webb 1990; Karppinen et al. 2002; Laine

et al. 2002), and with passing the entire construction

(Webb 1990; Karppinen et al. 2002). Fish may

abandon a fishway after entering it (Webb 1990;

Karppinen et al. 2002), indicating unsuitable condi-

tions in the fishway. Chanseau et al. (1999) found

pool passes and natural bypass channels to be the

most effective fishways. The location, design and

water discharge of the fishway entrance, the water

discharge and velocity in the fishway, and the fishway

design are important factors that determine how well

the fishway functions (discussed by Beach 1984;

Laine et al. 2002; Lundqvist et al. 2007).

In some regulated river systems, salmon have to

pass large reservoirs above dams, but there are few

studies recording the migration pattern and speeds

through such reservoirs. Atlantic salmon had no

difficulty in passing a 4 km long hydroelectric

reservoir, but still remained in the reservoir for up

to 67 days (mean 11 days), possibly indicating that

the reservoir was a suitable habitat for holding until

the spawning period approached (Gowans et al.

1999a). Mean residence depths of the fish dives were

3.7–4.0 m, with the deepest dive down to 20.7 m

(Gowans et al. 1999a). In contrast to this, fast

navigation and migration through a reservoir (31 km

day-1) was recorded by Karppinen et al. (2002).

Natural migration barriers

There is a striking lack of studies documenting the

effect of natural migration barriers on the upstream

354 Rev Fish Biol Fisheries (2008) 18:345–371

123

migration of Atlantic salmon compared to the large

number of studies on man-made barriers. Although it

is well known that salmon can be delayed and

congregate under large waterfalls and other natural

migration barriers (for instance documented by high

angling catch per unit effort), there are almost no

studies documenting the magnitude of such delays.

Multi-sea-winter salmon stopped on average

5–9 days below two riffle areas that were not

considered to be migration barriers for upstream

Atlantic salmon (Økland et al. 2001), whereas grilse

stopped for only 1–3 days below the same riffles in

the same study years (Karppinen et al. 2004). Sim-

ilarly, Rivinoja et al. (2001) recorded a mean stop of

five days below a rapid area. Salmon ascended a 4 m

high waterfall after staying on average 21–31 days

below the waterfall (Johnsen et al. 1998).

Upstream migrating salmon stopped on average

24 days below a 2–3 m high waterfall (not a free fall

and seemingly not too difficult for fish to pass), and

62% of the salmon below the waterfall had an erratic

movement pattern before passing, being recorded as

far as 14 km downstream after the first arrival in the

pool below the waterfall (own unpublished results).

Surprisingly, a seemingly difficult stretch in the same

river, with several waterfalls and a total fall of

approximately 15 m over a 2–3 km river section, did

not delay the upstream migration (mean stop of

2.4 days, own unpublished data). Hence, some

waterfalls may be more difficult to pass and some

may be easier than they appear.

Both Eurasian beavers (Castor fiber) and North

American beavers (Castor canadiensis) frequently

build dams on small streams to create impoundments

for their lodges and burrows (Collen and Gibson

2001). Beaver dams may interfere with fish move-

ments and distribution, and some beaver dams may

even prevent further upstream migration of Atlantic

salmon (reviewed by Collen and Gibson 2001).

Delays at river confluences are generally not

reported from studies of Atlantic salmon upstream

migration (e.g. Økland et al. 2001; Karppinen et al.

2004). However, Atlantic salmon entering smaller

tributaries may delay at the confluence and be

reluctant to enter the tributary early in the season if

there are few defined pools and poor conditions (low

water discharge) for the residence of adult fish in the

tributary (Webb 1989). Repeated excursions into the

tributary and return to the confluence have been

observed under such conditions (Webb 1989; Laugh-

ton 1991).

Water discharge

River flow is the factor most frequently reported to

control upstream migration of Atlantic salmon

(reviewed by Banks 1969; Jonsson 1991). Numerous

studies have shown that increased water discharge

stimulates Atlantic salmon to enter rivers from the

sea (e.g. Huntsman 1948; Hayes 1953; Brayshaw

1967; Clarke et al. 1991; Smith et al. 1994; Thorstad

et al. 1998), but fewer studies have examined the

effect of water discharge on the within-river spawn-

ing migration.

Water discharge may affect migration on natural

river sections without migration barriers, as well as

migration past various natural and man-made obsta-

cles. River regulations may result in river sections

with generally reduced water discharge. Furthermore,

in regulated rivers that have possibilities of control-

ling the water discharge, artificial freshets may be

used to stimulate the upstream migration. It is likely

that the effects of water discharge on the upstream

migration of Atlantis salmon will vary under each of

the above scenarios, and will therefore be reviewed as

such in the following text.

Natural river sections without large migration

barriers

The effects of water discharge on migration seem

generally limited in main stems of relatively large

rivers (Hawkins and Smith 1986; Hawkins 1989;

Webb 1989, 1990; Karppinen et al. 2004). In the

studies of Hawkins and Smith (1986) and Hawkins

(1989), Atlantic salmon moving in the river main

stem appeared to be little affected by water discharge,

both in an exceptionally wet year and during a dry

summer. The only indication of an association

between discharge and movement occurred when

fish stopped moving during periods of high flow and

turbidity (Hawkins 1989). However, increased water

discharge towards the spawning period was highly

important in allowing fish into the shallower spawn-

ing tributaries that did not provide suitable conditions

for the residence of adult salmon earlier in the season

Rev Fish Biol Fisheries (2008) 18:345–371 355

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(Hawkins 1989; Webb 1989; Webb and Hawkins

1989). There seemed not to be an annually consistent

threshold level of flow triggering entry into a

tributary, and the timing of entry during any partic-

ular autumn may depend on the flow pattern during

the preceding summer and the acclimatisation of the

fish to earlier rates of discharge (Webb and Hawkins

1989). Atlantic salmon are physically able to manage

areas with very low water discharge, and ‘instances

of what might best be described as active climbing

rather than swimming’ were described by Webb and

Hawkins (1989). This is when large fish, by major

flexing of the body and tail, moved over stones and

through shallow areas less than 10 cm deep.

Some exceptions to the conclusions above have

been documented. In Thorstad et al. 1998, the

migration speed in lower river reaches of one river

was not dependent on water discharge per se, but was

higher during increasing rather than decreasing water

discharge. Webb (1990) found that movements in

middle reaches were correlated to mean daily water

discharge. Furthermore, observations indicated that

the number of movements within the river after the

upstream migration phase, i.e., before and during the

spawning period, increased with the magnitude of

variation in water discharge (Thorstad et al. 1998). In

some UK rivers, where the fish may enter the river at

any time of the year and stay for many months in the

river before spawning, water discharges were more

important in stimulating the last stages of upstream

migration, when salmon were leaving the holding

pools heading for the spawning grounds (Laughton

1991; Solomon et al. 1999). Water discharge is

probably more important in stimulating upstream

migration in relatively small rivers, as indicated by

the results from six rivers with average daily flow

between 7 and 25 m3 s-1, where migration was

under-represented at the lowest flows (Solomon et al.

1999). However, the ‘‘threshold-flow’’ below which

migration was under-represented, varied markedly

both within and among the rivers (Solomon et al.

1999).

Webb (1989) concluded that the role of water

discharge in stimulating the resumption of migratory

activity in Atlantic salmon within the river is not

clear, and is likely to be complex. He further

emphasised that the stage of migration reached by

the fish must be considered; there will be times when

fish are susceptible to increases in flows and times

when they are not. The complexity of the relationship

between water discharge and migration can be

exemplified by the study of Erkinaro et al. (1999),

who found that an increase in discharge was generally

associated with increased migration speed once the

salmon were moving, but no differences in discharge

were detected between days when migration occurred

and days without migration.

Natural migration barriers

Few studies have examined the relationship between

water discharge and the passage of natural riffles and

waterfalls. To allow a successful passage by Atlantic

salmon of any particular riffle or waterfall, water

discharge may have to be within a certain and site

specific range. For example, a 10 m high waterfall

described by Jensen et al. (1986, 1998) was not

passable until the water discharge was below

300 m3 s-1, even though the fish ascended through

a fishway. Rivinoja et al. (2001) recorded Atlantic

salmon passing a riffle area at water discharges of up

to 150 m3 s-1, but also that the fish migrated

downstream from below the riffle area when the

water discharge increased to about 210 m3 s-1.

Increases in water discharge may also stimulate the

passage of riffles. Erkinaro et al. (1999) showed that

salmon were passing a riffle area faster during high

flows ([300 m3 s-1) than during low flows (B300

m3 s-1). Although there are few studies of the effects

of water discharge on passage of natural migration

barriers, these examples clearly show that the effect

of water discharge is highly site specific.

River sections with reduced water discharge

River-regulation sometimes reduces water discharge

on certain river sections, such as occurs between

power station or fish farm intakes and outlets, or

downstream of interbasin water transfers. River

sections with reduced water discharge may extend

over several to many kilometres, and the reduction in

water discharge may be considerable (Rivinoja et al.

2001; Thorstad et al. 2003b, 2005b). In three studied

rivers during the upstream migration season, up to

75%, 97% and 97% of the flow, respectively, could

be removed from the target river and through the

356 Rev Fish Biol Fisheries (2008) 18:345–371

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power station when the power station was operated at

maximum production (Thorstad et al. 2005b). Sal-

mon may have problems with finding and entering

such river sections (see above). Once they have

entered, they may be further delayed in passing this

river section, even though the water discharge is

sufficiently high not to physically hinder migration,

and even though the regulated discharge may be

comparable to the natural discharge in smaller

Atlantic salmon rivers (Rivinoja et al. 2001; Thorstad

et al. 2003b, 2005b). Thus, it may not only be the low

water discharge per se that causes the problem, but

also how much the water discharge is reduced

compared to the natural river sections below. The

main problem seems to be related to salmon entering

a large river and then suddenly facing a river section

with a much lower water discharge. Such reduced

water discharge may even lead to some individuals

turning and migrating downstream again after they

have entered the residual flow section (Thorstad et al.

2003b, 2005b). The reason for such behaviour may be

that the fish try to find alternative routes, or wait for

more preferred conditions.

On many river sections with reduced water

discharge, weirs are provided as mitigation measures

for the reduction in flow caused by water abstrac-

tions. They artificially raise water levels, keeping the

channel of the river flooded. Weirs are provided both

for aesthetic purposes, and to provide habitat for

juveniles and facilitate movements by other fish

species such as brown trout (Salmo trutta). Such

weirs may be migration barriers for salmon (Thorstad

et al. 2003b, 2005b, see below). Atlantic salmon

spent on average 42 days in passing a 6 km residual

flow section with 12 dams/weirs and water discharge

of 3 m3 s-1, corresponding to a migration speed of

0.15 km day-1 (Thorstad et al. 2005b). After passing

the reduced flow section, average migration speed for

the same individuals increased to 3.6 km day-1 on

the natural river sections above, in a river with mean

annual water discharge of 88 m3 s-1. The proportion

of salmon passing this river section increased when

the flow was increased from 0.25 m3 s-1 to 3 m3 s-1,

emphasising the importance of securing sufficient

levels of residual flow, even though 3 m3 s-1 is still

not satisfactory for Atlantic salmon upstream migra-

tion in this river. More moderate reductions in water

discharge may not affect upstream migration. This is

exemplified by a 20 km long residual flow section

past a power station with a water discharge of

10–20 m3 s-1 (20 m3 s-1 in the main season for

upstream migration), and up to 60 m3 s-1 diverted

through the power station (Thorstad et al. 2005b).

Migration speeds in this residual flow section did not

differ from the migration speeds on the natural river

sections above.

The examples above refer to regulated rivers with

a residual flow regulated by law. In river sections

without such regulations, the water discharge may in

some periods be even lower, or zero. Obviously, there

will be no upstream migration if the salmon are

physically stopped by dry river stretches.

Man-made migration barriers

The passage of power station outlets seemed to a

limited extent to be influenced by variation in water

discharge in the tunnel outlets and the bypass

stretches (based on Thorstad et al. 2003b, 2005b).

Fish passed power station outlets during a wide range

of discharges, and the water discharge during passage

was similar to discharges previously experienced by

the same individuals when holding at the power

station outlet. Thus, the salmon were not waiting at

the power station outlet for a specific, preferred water

discharge for passage (Thorstad et al. 2003b, 2005b).

An attempt to close a power station for a period of

some days did not stimulate the salmon to pass the

tunnel outlet and migrate further upstream (Thorstad

et al. 2005b). However, this happened during a

period with low water discharges and unusually

warm weather conditions. A positive effect of closing

the power station during periods with rain and higher

water discharge cannot be ruled out. In contrast to

these studies, in a much larger river, Lundqvist et al.

(2007) found that salmon positioned in the confluence

area between the power station outlet and bypass

channel responded strongly to increased water dis-

charge in the bypass channel in combination with low

flows from the power station outlet. Increases in

discharge from the power station caused fish to

migrate downstream from the power station outlet

(Rivinoja et al. 2001; Lundqvist et al. 2007). Scruton

et al. (2007) also found a relationship between

discharge from the power station and attraction of

salmon to the tailrace. This difference among studies

may be due to the much higher water discharges and

Rev Fish Biol Fisheries (2008) 18:345–371 357

123

the much larger variation in water discharge in the

bypass channel and from the power station in the

studies of Lundqvist et al. (2007) and Scruton et al.

(2007).

When fish must find the nearby entrance to a

fishway in order to pass a power station outlet, they

find themselves in an area with a complex hydraulic

situation. Water is supplied from the power station

turbines, an often much smaller amount of water

comes from the fishway, and sometimes water is also

supplied over the dam or in spillway channels.

Variation in water discharge through any of these

sources may affect fish behaviour below the dam and

power station outlet (Webb 1990; Chanseau and

Larinier 1998, 1999), and finally affect the success of

passage (Laine et al. 2002; Larinier et al. 2005).

Several studies have examined effects of water

discharge on the upstream migration past fish coun-

ters installed in weirs and fisways, with varying

conclusions. Alabaster (1970) concluded that the

median flows at which Atlantic salmon moved into

traps in both the upper and lower reaches of rivers

tended to be higher than the median flow available.

Dunkley and Shearer (1982) reported that the number

of fish movements increased during periods when

discharge was decreasing after a spate. Trepanier

et al. (1996) found that landlocked Atlantic salmon

favoured periods of decrease in water discharge for

river ascent. According to Jensen et al. (1986),

increases in water discharge stimulated upstream

migration, and most salmon ascended before the flow

had reached its maximum. However, the stimulating

effect of increases in water discharge on upstream

migration was larger in a small than in a large river

(Jensen et al. 1998). McKinnell et al. (1994) detected

no effect of river flow on the salmon run. Hellawell

et al. (1974) concluded that salmonids tended to

move at discharges lower than those generally

available. These studies clearly show the importance

of water discharge in stimulating Atlantic salmon to

pass obstacles such as weirs and fishways, but we are

far from understanding the complexity of how this

mechanism works, as was also noted by Trepanier

et al. (1996). The fish counter site (weir, fishway)

will in many cases act like a migration barrier, with

its own specific water discharge requirements. The

results of such studies, therefore, may be valid only

for the specific site and type of obstacle, explaining

the widely different results (also discussed by Banks

1969; Trepanier et al. 1996; Smith et al. 1997). The

lack of information about availability of fish below

the counting site also limits the interpretation and

value of such results (as discussed in the method

section above).

Artificial freshets

In regulated rivers, artificial freshets may be used to

stimulate Atlantic salmon to pass power station

outlets, river sections with reduced water flow and

other obstacles (Baxter 1961). However, clear and

successful results in using artificial freshets to stimu-

late within-river upstream migration have not yet been

demonstrated. Artificial freshets did not succeed in

moving Atlantic salmon past two different power

station outlets (Thorstad et al. 2005b). However,

artificial freshets stimulated the number of weirs

passed per hour in a 6 km residual flow section in

one of two study years, as number of weirs passed per

hour and distance moved during the freshets were

larger than during residual flow (Thorstad and Heggb-

erget 1998; Thorstad et al. 2005b). However, the

migration speeds were so low and the movements on

this stretch so erratic that the importance of the freshets

in stimulating the salmon to pass the entire residual

flow section was considered insignificant (Thorstad

et al. 2005b). It was concluded from these studies that

relatively short and small artificial freshets in large

regulated rivers may be a waste of water and money

(Thorstad and Heggberget 1998; Thorstad et al.

2005b). By contrast, Lundqvist et al. (2007) demon-

strated that larger artificial freshets may have more

successful effects on salmon passing a power station.

In general, studies of effects of larger and longer

lasting artificial freshets on upstream migration are

lacking. It has been suggested that the stimulus for

movement is not only associated with the increased

discharge, but also with the accompanying changes in

concentration of dissolved substances (Alabaster

1970), which may suggest that artificial freshets would

be more effective when the extra water is released in

periods with rain. This remains to be tested. Poten-

tially, inverse artificial freshets, which decrease the

water discharge over a 24 h period and then restore the

discharge to its original level for a few hours (Hayes

1953), could affect movement behaviour, but no

studies have documented this.

358 Rev Fish Biol Fisheries (2008) 18:345–371

123

Day and night

In UK rivers, within-river migration on stretches

without large migration barriers mainly takes place

during night, starting at dusk and ending at dawn, but

with some individual variation (Hawkins and Smith

1986; Hawkins 1989; Laughton 1989; Webb 1989,

1990; Laughton 1991; Solomon et al. 1999).

Increased daytime activity may occur during spate

conditions and turbid water (Laughton 1989, 1991;

Solomon et al. 1999), as occurred in a Swedish river

(Rivinoja et al. (2001). However, in a northern

Norwegian/Finnish river above the polar circle with

midnight sun conditions and only a negligible

difference in light between day and night, migration

was not associated with any particular time of the day

(Karppinen et al. 2004).

The diel pattern in passage of fish counters in

weirs and fishways seems site specific, with conflict-

ing results among studies. Webb (1990) recorded

increased activity at a fishway entrance during

morning and evening hours. Chanseau et al. (1999),

on the other hand, reported most passing of obstacles

during the day, except passage of natural bypass

channels during the night. Dunkley and Shearer

(1982) found that fish tended to move upstream just

after sunset, whereas both Kristinsson and Alexan-

dersdottir (1978) and Gowans et al. (1999b) found a

distinct diurnal migration pattern. Hellawell et al.

(1974) found that in clear water all movement

occurred at night, but in turbid water migration was

observed both day and night. There may be a conflict

between the need for light in order to ascend

obstacles, and a preference for darkness or turbid

water as an antipredator behaviour (Banks 1969). The

different results at different obstacles may, thus, be a

result of different requirements associated with visual

orientation in passing the obstacles.

Other environmental factors

Environmental factors other than water discharge,

such as water and air temperature, turbidity, atmo-

spheric pressure, cloud cover, and variations in

concentrations of many dissolved ions, may affect

the upstream migration in Atlantic salmon (Banks

1969). Many of these factors may be covariates with

changes in water discharge. It can be still be

concluded, as Banks concluded nearly 40 years ago,

that such covariates rarely have been acknowledged

in studies of upstream migration.

The only environmental factor in addition to

water discharge that has to some extent been

studied, and found to influence the upstream migra-

tion, is water temperature (reviewed by Banks 1969;

Jonsson 1991). Swimming capabilities are reduced

at lower and higher water temperatures (Beamish

1978; Booth et al. 1996), so that particularly phys-

ically demanding obstacles may be difficult to pass

at low and high temperatures. Even small obstacles

may be difficult to ascend at water temperatures

below 5–6�C (Jensen et al. 1986, 1998; Gowans

et al. 1999b), even though passage of a waterfall has

been recorded at water temperatures as low as 3�C

(own unpublished results). Both Johnsen et al.

(1998) and Gerlier and Roche (1998) reported

waterfalls that were not passed by upstream migrat-

ing Atlantic salmon when water temperatures

dropped below 10�C. Also high water temperatures

([20�C) may reduce the Atlantic salmon upstream

migration activity (Alabaster 1990). Both the lower

and upper limits for fish activity are to some extent

dependent on acclimatisation (Beamish 1978).

Water temperature limits for passage of different

obstacles will, therefore, depend on the physical

effort required for passage and the previous accli-

matisation period of the fish.

Low temperatures usually coincide with high

water discharges in the spring, which in combination

may delay fish at obstacles. Atlantic salmon did not

ascend a fishway in a 10 m high waterfall in spring

before the water temperature had reached 8�C and the

water discharge was below 300 m3 s-1 (Jensen et al.

1986). Increases in water temperature and water

discharge were both correlated to daily ascent in this

fishway (Jensen et al. 1998). Jensen et al. (1998)

speculated that fish attempting to overcome obstacles

during high flows may be doing so at an energetic

disadvantage compared to those waiting for more

optimal conditions. In this river, which is generally

difficult to ascend, Atlantic salmon may have adapted

to these conditions and respond to increasing water

temperatures as a cue for decreasing water discharge.

Few studies have correlated migration on river

sections without migration barriers with water tem-

perature, with the exceptions of Erkinaro et al. (1999)

and Karppinen et al. (2004). Erkinaro et al. (1999)

Rev Fish Biol Fisheries (2008) 18:345–371 359

123

found that migration speed tended to increase with

decreasing air temperatures. Similarly, Karppinen

et al. (2004) found that migration speeds were higher

at lower temperatures. These studies were conducted

at moderate water temperatures (8.5–17.0�C), which

means that this was not an effect of fish activity being

reduced at extremely high temperatures. Results from

fish counters in weirs have showed limited effect of

water temperature on the number of fish passing

(Trepanier et al. 1996).

Banks (1969) drew attention to a special thermal

problem at power stations when the turbine water is

taken from below the hypolimnion of the reservoir,

being colder than the surface water. When the

fishway spills water from the surface of the reservoir,

the fish experiences a temperature and water quality

difference between water from the turbine and the

fishway. The fish will become acclimatised to the

temperature of the mixed water from the two sources,

which will be closer to the turbine water due to the

larger volume. This may further add to the problem of

fish being attracted to the turbine water rather than

the fishway. A lower water temperature of the turbine

water can also have implications for the river

temperature many kilometres downstream and may,

therefore, affect the migration of fish a long way from

the power station.

Water pollution

Sublethal exposure to environmental stressors may

induce a behavioural response to avoid the stressor,

and the ability to avoid physical and environmental

stressors and find areas of more favourable conditions

may have significant effects on fish survival rates

(Gray 1983, 1990; Atchison et al. 1987). Laboratory

studies have shown that fish can detect and avoid

chemical and physical components such as gas

supersaturation, thermal effluents, metals, low pH

and aluminium (Gray 1983; Atchison 1987; Atland

1998), but such avoidance behaviour has rarely been

demonstrated in nature.

Accidental release of 1,000 m3 non-toxic waste

from decommissioned wood pulp industry into a

large river (125 m3 s-1 during the pollution event)

induced an avoidance response in Atlantic salmon

that were resting in fresh water before spawning

(Thorstad et al. 2005a). When the wooden fibres and

pulp were released, 16 of 32 (50%) radio tagged

salmon showed an immediate avoidance response

either by moving upstream (6 fish, 19%) or down-

stream (10 fish, 31%) (Thorstad et al. 2005a). Of the

salmon moving downstream, eight (25%) moved all

the way to sea (average 14.8 km) (Thorstad et al.

2005a). Four of these fish later re-entered the river,

two entered a neighbouring river and two were not

recorded again (Thorstad et al. 2005a). Fish displaced

downstream but without entering the sea, moved on

average 5.3 km during the episode, whereas those

moving upstream moved on average 6.7 km (Thors-

tad et al. 2005a). Interestingly, fish responding with

an upstream movement were distributed further

upriver during the accidental release than those

responding with a downstream movement (Thorstad

et al. 2005a). Upstream movement may be a suc-

cessful strategy to avoid a stressor just as well as

downstream movement, unless contaminants are

released in the upper part or above the accessible

stretches for the fish.

Atlantic salmon have been documented avoiding

other substances. Anglers observed Atlantic salmon

escaping a high pH and labile aluminium event by

downstream movement (Skogheim et al. 1984). Sim-

ilarly, downstream movement of Atlantic salmon

through a counting fence was recorded during events

of copper and zinc pollution (Saunders and Sprague

1967). It is possible that upstream avoidance reac-

tions occurred during the events described by

Skogheim et al. (1984) and Saunders and Sprague

(1967), but these author’s methods would reliably

have detected downstream rather than upstream

movements. The ability to detect and avoid a stressor

probably depends on concentrations. Atlantic salmon

did not avoid a power station tunnel outlet even

though the pH was reasonably lower and concentra-

tions of inorganic monomeric aluminium higher than

in the river residual flow (average pH 5.8 vs. 6.1,

average UM-Al 19 vs. 8 Al L-1) (Thorstad et al.

2003b).

When fishes are not able to avoid the stressor,

exposure to water pollution may affect the swimming

performance, and thereby affect the upstream migra-

tion and ability to pass strenuous river sections. For

instance, exposure to acid river water with elevated

aluminium concentrations reduced the swimming

performance of Atlantic salmon (Ytrestøyl et al.

2001).

360 Rev Fish Biol Fisheries (2008) 18:345–371

123

Catch-and-release angling

In order to reduce the mortality from recreational

angling on fish stocks, catch-and-release programmes

have been introduced in many Atlantic salmon rivers

in North America and Europe (ICES 2006; Arling-

haus et al. 2007). Catch-and-release of Atlantic

salmon has increased in popularity over the last

decade (ICES 2006). The mortality after catch-and-

release angling is generally low in Atlantic salmon,

but increases at high water temperatures (Dempson

et al. 2002; Thorstad et al. 2003a). However, catch-

and-release angling has been shown to alter the

upriver migration patterns and result in unusual

delays, downstream movements, erratic displace-

ments (Webb 1998; Makinen et al. 2000; Thorstad

et al. 2003a, 2007), and may even reduce the ultimate

distance a fish was willing to migrate (Tufts et al.

2000). The reasons for altered movement and migra-

tion patterns after catch-and-release angling are not

known, but this behaviour could signal deleterious

stress effects. Stress-related behaviour is driven in

part by the complex biochemical and physiological

changes that occur in response to stress (Schreck

et al. 1997). Downstream movements and delays

after release may result from a slow recovery from

handling stress, or a result of loss of orientation from

the capture process. However, downstream move-

ments may also simply be an avoidance response in

order to escape areas that are perceived to have

‘‘unfavourable conditions’’ (cf. Thorstad et al.

2005a). In one study, it was concluded that fish

behaviour seemed little altered after catch-and-

release angling (Whoriskey et al. 2000).

The behavioural reactions seem to differ between

Atlantic salmon caught and released during early

(\one week in freshwater) and late stages ([one

month in freshwater) of the upriver migration

(Thorstad et al. 2003a, 2007, both studies conducted

in a natural setting with ordinary anglers). For salmon

caught and released in the upper reaches of the river

after the migration had principally ended, catch-and-

release seemed to result in a more erratic movement

pattern than for undisturbed Atlantic salmon (i.e.

salmon captured and radio tagged in the sea before

entering the river), with up- and downstream move-

ments in the river towards spawning (Thorstad et al.

2003a). For salmon caught during their early

upstream migration, catch-and-release resulted in

downstream movements (31% of the fish) and a

temporary pause (average 34 days) in upstream

displacements (Thorstad et al. 2007). Downstream

movements are generally not seen in wild Atlantic

salmon during the upstream migration, except during

the search phase (see section on migration pattern in

pristine rivers without significant migration barriers,

above). The individuals with downstream movements

after being caught and released were recorded 4–

24 km further upstream during spawning, and their

downstream movements were, therefore, not likely

part of a search phase. However, a high proportion of

the caught and released fish were recorded in known

spawning areas during the spawning season (95%),

with no difference between fish caught during an

early and late stage of the upstream migration

(Thorstad et al. 2003a; 2007).

General discussion

Consequences of factors affecting the upstream

migration

Migration delays

A number of studies have documented that the

riverine upstream migration of Atlantic salmon may

be delayed up to many weeks at man-made installa-

tions such as power station outlets, dams, weirs,

fishways and other obstacles. The fish may also be

similarly delayed at natural migration barriers such as

waterfalls and beaver dams. Often, the magnitude of

delay is not predictable; fish may be considerably

delayed at barriers that appear easily passable, or they

may quickly pass barriers that appear difficult.

Stressful events like catch-and-release angling may

also cause considerable delays in upstream migration.

Prolonged delays that prevent salmon from reach-

ing suitable resting or spawning areas in time may

obviously reduce their reproductive success. This

may especially be of importance in rivers with many

migration barriers, which have a cumulative effect on

the total delay. The fish may either have to spawn in

unsuitable areas, or they may reach suitable spawning

areas too late. Overripening of gonads may have a

negative effect on egg viability, female spawning

behaviour and spawning capacity (de Gaudemar and

Beall 1998), so that delayed entry to the spawning

Rev Fish Biol Fisheries (2008) 18:345–371 361

123

areas may have a negative effect on reproductive

success. However, as long as the salmon reach

spawning areas in time before spawning, the popu-

lation consequences of delays during the upstream

migration are not known.

Atlantic salmon may enter the rivers several

months before spawning, and may hold position on

or near the spawning grounds for one to more than

two months before spawning—and there is no

satisfactory adaptive explanation for such early river

entry (see section on Atlantic salmon spawning

migration above). If the early river entry is associated

with advantages at the spawning grounds, delays

during the upstream migration may involve corre-

sponding disadvantages. Considering that Atlantic

salmon cease feeding during the upstream migration

phase and solely depend on utilising stored energy

reserves that otherwise could have contributed to

development of gonads and spawning activity, there

appears to be obvious disadvantages associated with

entering the river early compared with remaining in

the sea feeding. Atlantic salmon lose between 50 and

70% of their total energy content during river

migration and spawning (Jonsson et al. 1991c,

1997). Given these potential disadvantages, and the

fact that the timing of migration is both variable and

heritable (e.g. Klemetsen et al. 2003; Hansen and

Jonsson 1991), it is reasonable to expect that there

would have been a relatively strong natural selection

against early river entry if its benefits had not

exceeded the disadvantages. It is, therefore, not

likely that the early river migration is a latent genetic

relic of advantages in the past. Rather, it should

confer some kind of advantages under present

conditions.

There are several possible hypotheses that may

explain the early river entry of Atlantic salmon:

• Salmon may arrive early to avoid unfavourable,

and maybe unpredictable, environmental condi-

tions (e.g., low water discharge, high water

temperatures), that would prevent upstream

migration. This may for example be a good

strategy in small rivers. Salmon might have to use

the opportunity during spring spates and low

water temperatures to enter the river system and

arrive at pools or lakes where it is possible to hold

position until spawning, because the opportunities

for such conditions later in the season may be bad

or uncertain. In large rivers, water temperatures

may be too low or too high close to the spawning

period, again favouring an early river entry. Poor

estuary survival due to high water temperatures

and low dissolved oxygen in summer in the

southern part of the natural range of salmon may

favour spring and autumn runs of salmon (Solo-

mon and Sambrook 2004). Mills (1991) suggested

that constraints imposed by sea and river temper-

atures are one of the basic controls of salmon

migration. However, this cannot explain early

river entry in large rivers with suitable conditions

both in the estuary and river throughout the

season, including sufficient water discharge and

moderate water temperatures.

• Several studies have found that early ascending

Atlantic salmon migrate to spawning areas further

from the river mouth than later arriving individ-

uals (Saunders 1967; Laughton 1991; Laughton

and Smith 1992; Webb and McLay 1996; Økland

et al. 2001). However, some studies have not

found such a relationship (Thorstad et al. 1998). It

could be that the greater the distance to be

negotiated in fresh water, the more time is

required to reach the spawning grounds and, thus,

the earlier the migration must start. However, it

seems a weak argument that a lack of time to

reach spawning areas facilitates early arrival in

rivers without significant migration barriers,

given that fish can arrive in areas close to the

spawning area 1–2 months or more before

spawning.

• Early arrival at spawning grounds may provide

competitive advantages in acquiring spawning

sites and partners. However, studies demonstrat-

ing that salmon may not necessarily stay at the

spawning site during the long holding period (e.g.

Økland et al. 2001) to some extent contradicts

this explanation.

• There may be physiological or physical con-

straints, necessitating transition from salt- to

freshwater and a need to undergo the strenuous

migration before completing gonad development

and morphological changes before spawning. As

the salmon migrate from saltwater to freshwater

and start to undergo morphological changes and

gonadal maturation, only fuelled by stored energy

reserves, many physiological and morphological

processes are in progress (Kadri et al. 1995;

362 Rev Fish Biol Fisheries (2008) 18:345–371

123

Persson et al. 1998; Booth et al. 1999; Doucett

et al. 1999; Witten and Hall 2002; Kacem and

Meunier 2003). There may be advantages associ-

ated with finishing the upriver migration before

many of these processes advance, which may at

least explain why salmon migrate and enter areas

close to the spawning grounds 1–2 months before

spawning. However, no studies have been done to

test this hypothesis, except that it is partly

supported by the reduced swimming perfor-

mances recorded by Booth (1998) towards the

spawning period.

• There may be factors in the ocean along the return

migration route that need for some reason to be

passed during certain time periods, favouring an

early return migration. However, this explanation

cannot apply for UK salmon, which can return to

the rivers at any month of the year. For Baltic

salmon, however, Dahl et al. (2004) suggested

that salmon started migrating earlier in years with

higher sea temperatures to reduce migration

energy costs associated with high temperatures.

• The cues for the onset of return migration may be

inaccurate, or the time required to complete the

return migration may be unpredictable, so that an

early return migration may be a security precau-

tion in order to reach the spawning areas in time.

Thus, no universally valid explanation for the early

river entry seems available for all salmon rivers and

populations. The answer for any given river or

population may be a combination of different expla-

nations, and there may be different explanations for

the early river entry for different populations in

different rivers. This is supported by the variation in

the pattern of timing of river entry among different

areas in the range of the Atlantic salmon. The

consequences of migration delays that do not extend

beyond the spawning period cannot be fully under-

stood before we understand the reasons for, and

possible advantages of, early river entry.

In addition to population effects, migration delays

may cause conflicts in the sport fishery, with

increased catches in the lower parts of the rivers,

especially below migration barriers, and decreased

catches in the upper parts of the rivers. Further,

accumulation of a large number of fish below

migration barriers may increase a population’s

vulnerability of disease outbreak, such as for instance

furunculosis, especially at low water discharges and

when the water temperatures are high (Mills 1991;

Johnsen and Jensen 1994).

Altered migration patterns

Migration barriers may not only delay upstream

migration, but also alter the migration pattern,

causing downstream movements, erratic movement

patterns, or even result in some fish leaving the river

and entering neighbouring rivers. This means that the

absence of fish below an obstacle should not be taken

as proof that the obstacle does not act as a migration

barrier, as delayed fish may not always remain

immediately below the obstacle. Stressful events like

catch-and-release angling may also cause a more

erratic movement pattern. Similarly, water pollution

may induce an avoidance response with fish moving

up- or downstream to escape the unfavourable

conditions. This may also cause some fish to leave

the river and enter neighbouring rivers.

When fish leave the river and enter neighbouring

rivers, the size of the spawning population is reduced

and the straying among rivers is increased. As

demonstrated in Thorstad et al. (2005a), the spawn-

ing population was probably reduced by 13% as a

result of fish leaving the river as an avoidance

response to suspended solids. However, when the

pollution is lethal to the fish, leaving the river will

enhance survival.

The biological significance of altered behaviours

such as erratic movement patterns is not known, but it

is speculated that it may lead to a shift in the

distribution of the spawning population within the

river (see below) and disadvantages at spawning

grounds (if for instance competition or dominance

relationships are affected). Increased movement may

also lead to increased energy consumption at the cost

of gonad development and spawning activity, but no

studies have tried to quantify such an effect.

Increased energy consumption during upstream

migration may further reduce the survival of kelts

after spawning, thus reducing the number of times the

fish return for spawning, and thereby their lifetime

reproductive fitness (and the proportion of large

multi-sea-winter salmon in some rivers).

Rev Fish Biol Fisheries (2008) 18:345–371 363

123

Altered within-river distribution of the spawning

population

Migration barriers may also alter the distribution of

the spawning population within the river. Salmon

may give up migration and spawn in lower reaches

than intended. Similarly, unusual migration patterns

after stressful events such as catch-and-release

angling, or an avoidance response due to escape

from unfavourable conditions, may also alter the

distribution of the spawning population.

If precise homing within the river leads to genetic

variation among river sections (see section on

Atlantic salmon life history above), it follows that

an altered spawning distribution could reduce this

genetic variation within a river. Thus, there should be

less genetic variation among river sections in rivers

with many large migration barriers compared to

rivers without significant migration barriers. Simi-

larly, there should be less genetic variation among

river sections in small river systems where in some

years there is insufficient water discharge for spaw-

ners to reach the upper sections of the river, and

spawners intending to home to upper reaches instead

spawn in lower reaches.

In a relatively small river in South West England,

it was concluded that in two dry autumns, the

geographical distribution of spawning activity was

severely truncated due to low flows (Solomon et al.

1999). Fish appeared to be particularly held-up by

specific mills and weirs. The proportion of tagged fish

located at spawning time upstream a mill 42 km from

the river mouth averaged almost 70% in the ‘‘normal

autumns’’, but less than 19% in the two very low-flow

autumns.

Survival of Atlantic salmon juveniles is strongly

density-dependent during the first months following

emergence, resulting from the spatial distribution of

nests (Einum and Nislow 2005). Thus, the number of

juveniles surviving through the first summer depends

on the spatial distribution of utilised breeding hab-

itats, so that a truncated or patchier distribution of

spawners caused by migration barriers may cause a

reduced total salmon production. Consequently, the

maximum production potential of the river is not

realized.

Migration barriers may impact the migration

ability of different sizes/ages of fish in different

ways. This might result in a different within-river

distribution between groups of fish. Some fishways

may, for instance, facilitate passage of grilse more

than of multi-sea-winter salmon, such that the salmon

population above the fishway will be dominated by

grilse. Other migration barriers may be too physically

tough for small fish, only allowing large multi-sea-

winter salmon to pass.

Escaped farmed salmon seem to be less able than

wild salmon to pass difficult waterfalls, and if

fishways make it easier for fish to ascend, the

numbers of farmed fish ascending the river may

increase when fishways are constructed (Johnsen

et al. 1998). Escaped farmed salmon without previ-

ous river experience tend to distribute themselves in

the upper parts of rivers when not stopped by any

migration barriers. This means that the negative

effects of escaped farmed salmon upon wild salmon

will be most pronounced in the upper parts of the

rivers. Negative effects can include reduced repro-

ductive success and reduced genetic diversity of the

wild population, or destruction of the wild fish redds

as late-spawning farmed fish dig them up as they cut

their own redds (Heggberget et al. 1993b; McGinnity

et al. 1997; Fleming et al. 2000; McGinnity et al.

2003).

Effects of water discharge

The effects of water discharge on migration are

complex. There is no particular median flow or flow

pattern that is preferable for salmon in all rivers, at

all sites and migration stages, or for all years, or at

all times of the year in a given river. There is also a

large individual variation among salmon in response

to variation in water discharge at any given site and

any given time. Water discharge seems to affect

migration in a complex synergy with other factors.

This complexity often makes it impossible to predict

the effects of water discharge on upstream migration,

and to define threshold values. The apparently

stronger influence of water discharge on river entry

from the sea (see the Atlantic salmon spawning

migration section above) than on upstream migration

in many large rivers where water discharge is not

physically limiting, might indicate that freshwater

supply to fjords and coastal areas is important for

salmon orientation and recognition of the home

river.

364 Rev Fish Biol Fisheries (2008) 18:345–371

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Consequences for management

Recognising the effects of various factors influencing

the within-river upstream migration of Atlantic

salmon has consequences for the management of

salmon rivers:

• Hatchery-reared fish tend to return to the release

site in the river, and thus releases should be

spread along the spawning areas of the river to

avoid future reduced production due to density-

dependent competition among juveniles.

• With some exceptions, man-made constructions

like power stations, dams, weirs and fishways

disturb the upstream migration and result in some

potential loss to the population. Care must be

taken when designing such constructions to avoid

such impacts.

• The number of man-made migration barriers,

even those seemingly not difficult to pass, should

be kept to a minimum in salmon rivers.

• Handling of fish may cause delays and down-

stream movements, and unnecessary handling at

counting fences, dams etc. should be avoided.

• There is a need for separating between the

different stages of upstream migration and results

should not uncritically be extrapolated from one

stage and site to another. The effects of migration

barriers and water discharge are to a large extent

site specific, and must therefore be considered for

each site specifically.

• Upstream migration appears to generally occur

during the night, but during the day at some

migration barriers. This knowledge can be used

for site-specific application of mitigation mea-

sures to facilitate migration, such as artificial

freshets or closing of power stations.

• Small and short artificial freshets seem to be of

little benefit to salmon in large rivers, as it has a

negligible impact on within-river upstream migra-

tion in such rivers.

• The relationship between water discharge and

upstream migration is complex. There is no single

‘‘formula’’ for applying variations in water dis-

charge that will predictably facilitate salmon

upstream migration in problem areas in regulated

rivers.

• Large reductions in water discharge affect

upstream migration negatively, whereas moderate

reductions may not cause any effects. The mag-

nitude of the reduction seems more important

than the absolute value of water discharge.

• Effective residual flow regulations are important

in regulated rivers, especially in rivers where the

water discharges are considerably reduced com-

pared to the natural flow.

• Entrances to fishways around obstacles should be

placed where their flow is easily detected by the

fish. One should also be aware of possible

differences in temperature of the different sources

of water in areas like hydropower stations with

reservoirs, which may affect the fish migration

negatively.

• Many beliefs exist regarding factors affecting the

upstream migration of salmon, and it is important

that management is based on quantitative studies

with proper statistical analyses.

• As a precautionary principle, a variable water

discharge throughout the season is recommended

for regulated rivers rather than a static water

discharge. Variations in water discharge increase

the likelihood of meeting the requirements of

different individuals in different phases of the

upstream migration, and which use different sites

in the river. However, the variation must not be so

fast and large that stress levels and occurrence of

fish stranding increases.

Future need for research

It can be concluded from this summary that the

upstream migration of Atlantic salmon is vulnerable to

disturbances by migration barriers, changes in water

discharge and other factors. However, the understand-

ing of the general mechanisms that stimulate fish to

commence within-river migration is lacking. It can

still not be reliably predicted under which conditions a

fish will pass a certain migration barrier or which water

discharge conditions are needed to stimulate migration

at different sites. The factors known to affect migration

in different river sections and sites are previous

experience of the fish, water discharge, water temper-

ature, water velocity, jumping height required, fish

size, fish acclimatisation, light, water chemistry/pol-

lution, time of the season and handling. How these

factors affect upstream migration is to a varying extent

Rev Fish Biol Fisheries (2008) 18:345–371 365

123

understood, and for many factors the knowledge is

limited. There are likely to be a number of additional

factors that are important, such as maturation stage,

physiological processes, energetic status, social fac-

tors, rainfall and air pressure, and the relationship

among different factors is complex.

Many previous studies have suffered from either

using methods not suitable for studying factors

affecting migration pattern, or from too small sample

sizes. To gain information on general mechanisms,

and to be able to identify the important factors, more

studies with a high resolution in time and space and

with large sample sizes are needed, together with

detailed information on hydraulic conditions and

environmental factors. Controlled experimental stud-

ies in a laboratory setting are also needed, for

example on cruising, sustained and burst swimming

performances at different stages. Further, many

migration studies are conducted in regulated rivers

and at migration barriers, but relatively few in

pristine rivers. Information on natural resting stops,

stops at natural migration barriers, migration patterns

and factors affecting the migration in natural rivers

are lacking. Such studies would provide information

on general mechanisms, and would also help define

what a ‘‘delay’’ really means, and when the term

‘‘delay’’ actually should be used in regulated rivers

and at migration barriers. If a migration stop at for

instance a power station outlet, or a dam, is within the

time frame of a resting stop in a similar pristine river

without any migration barriers, can this be termed a

‘‘delay’’? Areas below obstacles may be offering

preferential habitat for the salmon to hold in, and

duration of stop, habitat type, time of season and

distance from spawning grounds should be consid-

ered before a migration stop is regarded as a ‘‘delay’’.

Upstream movement of Atlantic salmon has a long

history of proponents arguing for control mediated by

freshets or associated conditions (reviewed by Banks

1969; Jonsson 1991). The strong focus on the effects

of water discharge through decades, in our opinion, to

some extent has hampered the focus on other factors,

and a stronger focus also on other factors in future

studies, will lead to a better understanding of what

controls the Atlantic salmon upstream migration

pattern.

Acknowledgements The Norwegian Institute for Nature

Research (NINA) and the Danish Institute of Fisheries

Research (DFU), Department of Inland Fisheries, Silkeborg,

are thanked for providing financial support. We would like to

thank Odd Terje Sandlund for reading through and

commenting on an earlier version of the manuscript. We

would also like to thank Fred Whoriskey and an anonymous

reviewer for constructive comments and suggestions that

helped improving the manuscript.

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