The effect of high air and water temperature on juvenile Mytilus edulis in Prince Edward Island, Canada

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Aquaculture 243 (

The effect of high air and water temperature on juvenile

Mytilus edulis in Prince Edward Island, Canada

Neil LeBlanca,*, Thomas Landryb, Henrik Stryhna, Rejean Tremblayc,

Mary McNivena, Jeff Davidsona

aAtlantic Veterinary College, Department of Health Management, University of Prince Edward Island, 550 University Avenue,

Charlottetown, Prince Edward Island, Canada C1A 4P3bDepartment of Fisheries and Oceans, Science Branch, Mollusc Section, Gulf Fisheries Centre, P.O. Box 5030, Moncton,

New Brunswick, Canada E1C 9B6cInstitut des sciences de la mer, Universite du Quebec a Rimouski, 310 allee des Ursulines, Rimouski, QC, Canada G5L 3A1

Received 24 February 2004; received in revised form 8 July 2004; accepted 28 September 2004

Abstract

Mussel aquaculture on Prince Edward Island (PEI), Canada, is an important but relatively new industry. Although seed

manipulation using hatcheries for mussel culture occurs on the west coast of North America, seed supply on the east coast of

Canada, including Prince Edward Island (PEI), is based solely on wild collection. Two techniques for culling seed (b10 mm)

were studied in this experiment to look at the effect on productivity, measured as size, growth and survival. The separate effects

of air exposure or high water temperature treatments on a sample of Mytilus edulis spat were examined in the lab and in the

field. The in vitro treatments resulted in a ~50% mortality from an air exposure of 11 h at 27 8C and ~75% mortality from a 6-h

exposure to 33 8C water. Survivors of each treatment (n=1152) were measured, along with controls (n=2304) and randomly

placed in compartmentalized (condo) cages. Cages were deployed on a mussel farm in each of three bays on Prince Edward

Island, Canada. Size, growth and survival were monitored over a 10-month period. After the initial treatment, survivors of the

air exposure treatment were significantly ( pb0.01) larger than the control. Survivors of the high water temperature treatment

were smaller than the control ( pb0.01). Results from the field study showed that the treatments had a significant effect on size,

growth and survival. These results suggest that relatively simple husbandry practices of weeding out weaker mussel seed can

affect productivity. Given the limited availability for lease expansion in PEI bays, new husbandry practices are an important

avenue to investigate, in order for the mussel industry to maximize production capability.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Mytilus edulis; Cull; Size; Mortality; Temperature; Air exposure

0044-8486/$ - s

doi:10.1016/j.aq

* Correspon

E-mail addr

2005) 185–194

ee front matter D 2004 Elsevier B.V. All rights reserved.

uaculture.2004.09.035

ding author. Tel.: +1 902 566 0995; fax: +1 902 566 0823.

ess: [email protected] (N. LeBlanc).

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194186

1. Introduction

Over the past decade, the mussel aquaculture

industry has become an important part of the economy

in Prince Edward Island (PEI), Canada. Seed supply is

a crucial part of this industry, which depends solely on

natural spat collection. Traditionally, it was believed

that the main broodstock contributing to the seed

production in a bay was the wild population. How-

ever, a recent study in Tracadie Bay, PEI, suggested

that the earliest and largest pulse of larvae comes from

the cultured stock (T. Landry, DFO, personal commu-

nication). This study, therefore, suggests that from a

quantitative standpoint, cultured stocks are self-

sufficient. The experience in Japan with the cultured

scallop, Patinopecten yessoensis indicates that cul-

tured stocks of bivalves can indeed be self-sustaining

(Gosling, 2003).

Historically, in some areas, large-scale mass

mortality has been a problem for growers in the

mussel aquaculture industry (Incze et al., 1980; Mallet

et al., 1990; Myrand and Gaudreault, 1995). Studies

suggest that the seed quality or genetics may be a

cause of these mortalities (Dickie et al., 1984; Mallet

et al., 1990; Myrand and Gaudreault, 1995). Seed

source along with other factors such as stocking

density have been studied in areas of large-scale

mussel culture. These studies have found that the seed

used has a significant effect on production (Fuentes et

al., 1992, 1994; Molares and Fuentes, 1995). Culling

practices are commonly used in hatcheries with the

objective of achieving a more productive crop by

selecting stronger individuals to use as stock (Lestor,

1983). Air exposure and high water temperature are

stresses that cultured mussels are often exposed to

during the farming process, particularly in PEI.

During the summer, waters in the shallow bays of

PEI often reach temperatures of 25 8C (Bernard,

1999). These temperatures come close to reaching a

lethal temperature for Mytilus edulis, which has been

reported in Australia to be 28.2 8C (Wallis, 1975). It

should be noted that since publication of the article by

Wallis (1975), Australia has been reported to have

Mytilus galloprovincialis, rather than M. edulis

(Gosling, 1992). Field experiments in Rhode Island

showed that continuous temperatures above 27 8Chave been found to be lethal to M. edulis (Gonzalez

and Yevich, 1976). Read and Cumming (1967) found

that at 27 8C, 50% mortality occurred in about 3 days.

Since temperature stress is a reality on mussel farms in

PEI, a process that removes mussels susceptible to

high water temperature was investigated. The farming

of mussels requires that mussels be exposed to air for

periods of time throughout the harvest cycle.

Although mussels are an intertidal species, they will

eventually die from air exposure, especially at high

temperatures (Tsuchiya, 1983). For this reason,

selecting for mussels that can better cope during

emersion was the other culling technique investigated

in this study.

The objective of this study was to examine whether

the culling practices employed can result in a higher

quality mussel seed for the grow-out phase. The study

examined the productivity measures, size and sur-

vival, to assess the effects the stress treatments had on

mussel seed immediately and over time in the field.

2. Materials and methods

2.1. Specimen manipulation

2.1.1. Sampling

Wild seed was manually collected from aquacul-

ture gear in St. Peter’s Bay, PEI. Approximately

30,000 individual spat were retained. Seed was

transported to a hatchery in Prince Edward Island.

The sample was divided in half and placed into two

upwellers (40 cm diameter) in a 1.2 m3 tank. Mortality

in these upwellers remained low (b3%) throughout the

course of the experiments. Flow rate of unfiltered tidal

river water varied from 0.1 to 0.5 L/s, with an average

temperature of 24.3 8C and average salinity of 28.2

ppt. In keeping with other literature, a minimum level

of mortality of 50% was targeted (Wallis, 1975; Cotter

et al., 1982; Tremblay et al., 1998), for the purpose of

detecting any effect of treatments.

2.1.2. Air exposure

An air exposure trial was conducted with approx-

imately 5000 individuals taken from the upwellers and

placed on a layer of brown paper towel and spread

over a counter top in the lab. The individuals were left

exposed for 11 h at an average temperature of

27.2F0.5 8C and average relative humidity of

55.6F10%. They were then placed in a recovery tank

Table 2

Mortality of mussels from a 6-h exposure to high water temperature

with SE for overall estimates

Treatment Time

(h)

Live

specimens

Dead

specimens

%

Mortality

S.E.

Sub-samples

High temperature 20 57 35 38.0

High temperature 20 41 59 59.0

High temperature 20 41 56 57.7

Total 20 139 150 51.9

High temperature 50 39 42 51.9

High temperature 50 58 60 50.8

High temperature 50 37 32 46.4

Total 50 134 134 50.0

Overall 70 76.0 2.1

Control 70 146 5 3.3

Control 70 158 8 4.8

Control 70 97 2 2.0

Overall 70 401 15 3.6 0.9

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194 187

(average water temperature 25.8 8C, salinity 27.8 ppt),except for three samples of 100 specimens each,

which were placed in separate identical tanks (with

water flow, average temperature 26.1 8C, salinity 27.7

ppt). Three other tanks were set up with 100 control

specimens each. The control specimens were

untreated mussels taken from the two control upw-

ellers. The three samples of 100 specimens were used

to calculate the mortality level after 10 h in the

recovery tanks (Table 1). Mortality was determined by

response to mantle stimulation. After 10 h, the live

specimens from the large sample were placed in an

upweller identical to the upwellers holding the

original sample. Survivors of the three air exposure

samples used for calculating mortality and the

controls were placed in two identical tanks for an

additional 3 days. No further mortality was observed

in either group. Standard errors for proportions were

calculated from the binomial distribution.

2.1.3. High water temperature tolerance

Approximately 8000 individuals were exposed to a

constant water temperature of 32.6 8C and salinity of

27.6 ppt for 6 h. Mortality was calculated in two

stages due to the slow progressive nature of the

population to succumb to the treatment. Three

samples of approximately 100 specimens each of the

treatment and control were placed in tanks with water

flow (average temperature 23.2 8C and salinity 28.3

ppt). Mortality was calculated from the samples after

20 h. The sample tanks were then stocked with

mussels that were still alive after 20 h, then mortality

was calculated after another 50 h at 23 8C. Combining

the two mortality calculations provided the overall

mortality (Table 2). The mortality was calculated as

Table 1

Mortality of mussels from an 11-h air exposure with SE for overall

estimates

Treatment Live

specimens

Dead

specimens

% Mortality S.E.

Air exposure 51 49 49.0

Air exposure 52 48 48.0

Air exposure 53 46 46.5

Overall 156 143 47.8 2.9

Control 97 3 3.0

Control 100 0 0.0

Control 99 1 1.0

Overall 296 4 1.3 0.7

1�p1p2, where p1 and p2 are the proportions of

survivors after 20 and 50 h, respectively. Standard

errors (S.E.) for proportions were calculated from the

binomial distribution, and the SE for the overall

mortality was computed by the formula, (SE)2=(Var

p1+p12)(Var p2+p2

2)�p12p2

2.

2.2. Field study

Seventy-two condo cages (with 32 individual

compartments in each cage) were filled with individ-

ual mussels from the air exposure treatment, high

water temperature treatment and control treatment.

Individual mussels were measured (umbo to furthest

point on the posterior edge) and placed in a fine mesh

sock divided into two sections, with one individual

per section. Each sock was placed into a compartment

of a condo to which it had been randomly assigned.

Therefore, the set-up consisted of two mussels per

compartment, however, the placement of each mussel

in a section of sock kept them separated as individ-

uals. Eight air exposure socks, eight high water

temperature and sixteen control socks were placed in

each condo. Condos were filled one at a time then

placed in one of the three holding tanks. Condo cages

were left in holding tanks for 2 weeks so all the

specimens would have a chance to acclimate to a

similar state before being deployed in the field.

Temperature and salinity conditions in the holding

tanks averaged 22.8 8C and 28.5 ppt. Flow rate in the

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194188

holding tanks was approximately 0.2 L/s. Dimensions

of the holding tanks were 2.13 m long�1.22 m

wide�0.91 m deep.

Six cages (containing four condos) were placed

into each of St. Peter’s, New London and Tracadie

Bay (Fig. 1); all three bays are used extensively for

mussel aquaculture. Although the bays are similar in

depth, temperature and salinity (Fisheries and Aqua-

culture Division, 2001; Crane, 2003), three different

field sites were used to detect any interaction between

the treatments and the environment. All were attached

to mussel long lines in the bays at a point where the

depth was 4.6–4.96 m (15–16 ft.). The cages were

sunk to a depth of 1.8–2.1 m (6–7 ft.). Water tem-

perature in all three bays were slightly lower (20.5–

21.5 8C) but similar to the temperature in the

experimental tanks, so no temperature acclimation

was necessary (Fisheries and Aquaculture Division,

2001).

After 4 months, mussels in each of the bays were

measured for length and mortality was counted.

Final measurements and mortality counts were done

at the 10-month period. The experiment was

initiated approximately 6 weeks prior to the normal

seed transfer time on PEI. This was necessary for

practical purposes, such as the availability of

facilities and technical support, including industry

partners.

Fig. 1. Map of Prince Edward Island, Canada, indicating the three bays u

follows: New London Bay (46829.756 N, 63828.185 W), Tracadie Ba

62841.599 W).

2.3. Species determination

A random sample of one hundred and ten mussels

was taken for species characterization as M. edulis or

Mytilus trossulus according to the PCR technique

developed by Rawson et al. (1996).

2.4. Statistical analyses

The effect of the treatments on initial mussel size

was assessed by one-way ANOVA using the length

measurements from the specimens placed in condos

for the field study.

The models used for analyzing the field data used

treatment and bay as fixed factors with condo as a

random factor to account for potential clustering

within condo cages. Mixed ANOVA models were

used for length data (software SAS 8.2; Littell et al.,

1996). Random effects binary logistic regression was

used to analyze mortality (software MLwiN 1.2;

Rasbash et al., 2000). In the analyses, missing

specimens were treated as dead. The mesh socks kept

the mussels separated within in the condo compart-

ments and prevented them from falling out when they

were small enough to fit through spaces in the condo

cage. A small percentage of specimens (b3%) which

showed negative growth were not used, although their

inclusion in the analyses did not affect the signifi-

sed as field sites. Locations of the study sites in the bays were as

y (46823.355 N, 62859.440 W), St. Peter’s Bay (46826.231 N,

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194 189

cance of the results. Shell deposition is permanent,

thus negative growth indicates an error in measuring,

recording, sampling, etc.; therefore, removal of such

data is appropriate. All statistical tests were inter-

preted using a 5% error level, and multiple pairwise

comparisons were carried out by the Bonferroni

method.

Table 3

Adjusted treatment and bay mean mussel sizes (mm; with S.E.),

initially and after 4 and 10 months

Size (mm)

Initial

(n=4608)

Four months

(n=843)

Ten months*

(n=394)

Treatment

Air exposure 8.07(0.05)c 26.55(0.41)b 35.06(0.60)b

High water

temperature

6.49(0.05)a 24.54(0.41)a 32.96(0.63)a

Control 6.81(0.03)b 25.93(0.33)b 31.69(0.56)a

Bay

Tracadie 7.14(0.04)b 24.69(0.37)b 30.91(0.57)a

New London 6.90(0.05)a 25.86(0.52)ab 35.60(0.76)b

St. Peter’s 7.10(0.04)b 26.47(0.42)a 33.20(0.85)ab

Statistical differences within each time period for treatment and bay

are indicated using superscripted letters.a,b,cCategories indicated by the same symbol are not statistically

significant at the 5% level.

* Comparisons for treatment and bay refer to average effects,

due to significant interaction (Fig. 2).

3. Results

3.1. Laboratory culling experiment

3.1.1. Eleven-hour air exposure

The mortality level was 47.8% from a sample size

of 299 specimens (Table 1). The vast majority of the

dead specimens were floating after 10 h in the

recovery tanks. Thirty floating specimens were physi-

cally examined first by exposing to the air to observe

gaping, then gently prying to test valve function and

finally opening the animal to inspect the soft tissue.

None of the mussels that were floating after 10 h in

the recovery tank were found to be alive. Only a few

dead specimens (b10%) were found on the bottom of

the tanks with the live individuals. Mortality in the

control groups was low (Table 1).

3.1.2. Six-hour high water temperature immersion

Mortality determination for the high water temper-

ature was more complicated because death occurred

over a longer period of time compared with the air

exposure (50+ h compared with 10 h) (Table 2). Also,

the vast majority of the dead specimens in the air

exposure treatment floated, facilitating separation, this

was not the case for this treatment. Due to the time

lag, mortality determination for this treatment was

done in two steps, so that dead specimens could be

removed from the upwellers and decomposing mus-

sels did not contribute to mortality.

Mortality from the treatment was 76% while the

mortality in the control groups remained low

(Table 2).

3.1.3. Size of mussels after treatments

Significant differences in mean size were present

between all three treatments (Pb0.01). The stress

treatments had opposite effects with the smaller

mussels surviving the temperature treatment and

larger ones surviving the air exposure (Table 3).

Although selection for mussels placed in each bay

was random, the mussels that went into New London

Bay were significantly smaller than mussels in the

other two bays ( pb0.05). This effect may have arisen

from New London Bay being prepared last. With the

initial sizes being quite small, there may have been an

unconscious tendency to select the larger mussels for

measurement.

3.2. Field study

3.2.1. Size and growth

Analysis was performed to determine the effect of

the treatments on the size of the mussels at each time

period and the growth over that time period. Size was

preferred over growth in the analysis because there

was no significant correlation between the initial size

of the mussels and the size at each time period. In

other words, differential growth of the mussels during

the experiment eliminated any size differences at the

outset.

Data showed a substantial variation among the

condo cages, indicating that the set-up procedure of

randomly placing treatment and control (s) within

each condo was prudent.

Table 4

Mortality of mussels after 10 months in the PEI bays with SE

N % Mortality

Treatment

Air exposure 234 56.8(3.2)

33 8C H2O 228 58.3(3.3)

Control 451 71.8(2.1)

Bay

Tracadie 529 69.4(2.0)

New London 148 55.4(4.1)

St. Peter’s 236 59.7(3.2)

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194190

3.2.2. Size after 4 months

The ordering of the treatments with respect to size

did not change from the initial measurements: The air-

exposed mussels were the largest and the high

temperature exposed mussels the smallest, however,

air-exposed mussels were no longer significantly

larger than control mussels (Table 3).

Mussels in St. Peter’s Bay were significantly larger

than mussels in Tracadie Bay, whereas, mussels in

New London Bay were not different from either of the

other two bays in size (Table 3).

3.2.3. Size after 10 months

Size differences were seen between treatments and

between bays, as well as in a significant interaction

( p=0.03, Fig. 2). The air exposure treated mussels

were significantly larger than the mussels in the high

water temperature treatment and control groups,

averaged across bays (Table 3). Among the bays,

mean mussel size in New London was significantly

larger than mussels in Tracadie Bay, averaged across

treatments ( pb0.05).

Pairwise comparisons for size between the treat-

ments in specific bays showed that air-exposed

mussels in New London Bay were larger than each

of the treatments in Tracadie Bay including controls,

as well as control mussels in New London and St.

Peter’s Bay. Also, the high water temperature treated

mussels in New London Bay were larger than the high

water temperature treated mussels in Tracadie Bay.

Fig. 2. Mean size of mussels after ten months in PEI bays with SE.

Table 5

Random effects logistic regression analysis of mussel mortality after

10 months with respect to bay and treatments

Variable Comparison Estimate S.E. Odds

ratio

p Value

Intercept Reference

(Tracadie, Control)

1.369 0.304

Bay 0.25

St. Peter’s vs.

Tracadie

�0.557 0.535 0.573 0.30

New London

vs. Tracadie

�0.844 0.545 0.430 0.12

Treatment b0.001

Air exposure

vs. Control

�0.762 0.179 0.467 b0.001

High water

temperature

vs. Control

�0.690 0.181 0.502 b0.001

Variance

(condo)

Reference

(Tracadie, Control)

0.724 0.281

3.2.4. Mortality

Mortality was analyzed over the 10-month period

of the experiment (Table 4). Significant differences

among the treatments were found (Table 5). The air

and temperature exposed mussels had significantly

lower mortality than the control mussels at the 5%

level but were not significantly different from each

other.

Although there are larger differences in mortality

among the bays than in the treatments (Table 4), the

random variability of the condo cages prevents any

statistically significant results. Treatments were nested

within the condo cages so the variability within the

condos did not affect the findings for the treatments as

it did for the bays.

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194 191

3.3. Species determination

The Gulf of St. Lawrence is an area where the

distributions of M. edulis and M. trossulus are known

to overlap (Gosling, 1992), however, the level of M.

trossulus in Prince Edward Island has been found to

be quite low (Mallet and Carver, 1995). The results

from the specimens tested in this study found that this

population was 100% M. edulis.

4. Discussion

The laboratory experiment was designed to cull

mussels through selective mortality resulting from

exposure to the stressful conditions of high water

temperature or air exposure. The selective effect of the

treatments was determined by measuring the size and

mortality of treated and control mussels over a 10-

month period in the field. The important factors for

the culling treatments are: The ability to selectively

cull out weaker mussels, the surviving mussels must

be able to fully recover from the treatment, and the

treatments must be feasible on a large scale. By

choosing relatively simple natural factors that affect

mussel populations, all the above criteria were met.

Mytilus is an intertidal species. The most important

factors in determining the upper limits for Mytilus on

rocky intertidal shores are physiological intolerance to

temperature extremes and desiccation (Suchanek,

1985). Given that Mytilus has evolved to be a

successful intertidal species (Lewis, 1997), often to

avoid predation (Ebling et al., 1964; Paine, 1974), it is

more likely that desiccation would not permanently

harm the animals. As for high water temperature, in

this region, water temperatures can reach and even

exceed 25 8C (Bernard, 1999; Myrand and Gau-

dreault, 1995). The hope was that exposure to

temperatures less than 6 8C above lethal temperatures

for a short time period (6 h) would not permanently

harm the mussels that survived the cull. An effective

cull needs to be practical in terms of time to complete

the procedure and it must have a minimal long-term

effect on the productivity of the survivors.

In both cases, the stress treatment did not appear to

permanently harm the surviving mussels, although

growth may have been stunted for some period of

time. With less mortality and comparable, if not

greater, size than the controls, the treated mussels at

the end of the 10-month field experiment appeared to

suffer no permanent damage. However, it still makes

sense to perform a culling treatment when the effect

on mussel growth would be minimal, perhaps in late

fall when growth is already retarded by natural

conditions of declining food and temperature.

The major observation on the immediate effect of

the treatments besides the level of mortality was the

selection for size. Clearly, smaller mussels better

survived the high water temperature treatment and

larger mussels were more resilient to air exposure. A

previous study by Bayne (1984) found that although

small mussels (M. edulis) have a higher maintenance

energy requirement and lose more weight during

starvation, small individuals were at an energetic

advantage in responding to high temperature. The

energetic advantage was calculated using scope for

growth or the energy available for growth and

reproduction. As temperature was increased, smaller

mussels (b0.6 g dry weight) maintained positive scope

for growth while larger mussels (up to 2.0 g dry

weight) did not. The results here indicate that smaller

mussels also had an advantage during acute exposure

to lethal high water temperature.

The survival from the air exposure treatment of the

larger mussels immediately seems to make it a more

favourable cull, as they immediately seemed to have

an advantage in getting to market size. The survival in

air of certain shellfish species, including Mytilus(De

Zwaan, 1977; Livingstone and Bayne, 1977; Widd-

ows and Shick, 1985), has been examined for differ-

ent reasons (Matthews and Mcmahon, 1995; Paukstis

et al., 1999; Bartsch et al., 2000) including long line

culture (Guderley et al., 1994). Survival times in air

for bivalves, including M. edulis, vary greatly depend-

ing on acclimation and various environmental factors

such as temperature and humidity (Tsuchiya, 1983;

Paukstis et al., 1999). These studies on Mytilus

dealing with air exposure and mortality (Tsuchiya,

1983; Eertman et al., 1993; Guderley et al., 1994) did

not mention a physiological advantage larger mussels

had in surviving air exposure. However, this advant-

age was seen in the freshwater bivalve, Dreissena

polymorpha, commonly known as the zebra mussel

(Matthews and Mcmahon, 1995; Paukstis et al.,

1999). Paukstis et al. (1999) stated that larger zebra

mussels (shell length N16 mm) were significantly

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194192

more likely to survive exposure for 36 or 60 h at 20

8C than were smaller individuals.

The field study was set-up to monitor the long-term

effects the treatments had on the mussels’ size and

mortality. Statistically, the treatments had an effect on

size, but there was an interaction between treatments

and the bays. In New London Bay, the air-exposed

mussels had significantly larger size than all other

bay/treatment combinations while in Tracadie Bay, the

high temperature exposed mussels were smaller than

elsewhere. The results suggest that the environment

will in part determine the effect of treatments. The air

exposure treatment had an advantage in size in some

circumstances and never had a negative interaction

with location, suggesting it may be the better

candidate as a culling technique.

There was no correlation between the initial size of

the mussels and the subsequent size measurements.

However measurements from the 4- and 10-month

periods were highly correlated. This raises the

question: Why does the size of the mussel at the start

of the experiment have no bearing on its size later on?

The genotypic factors related to fitness in mussels are

thought to be minor compared with environmental

factors (Seed and Suchanek, 1992). Under heavy

settlement, growth in individuals has been found to

vary by as much as 10-fold (Trevelyan, 1991). It is

hypothesized that the initial sizes were based mainly

on environmental factors (position, crowding, food

availability) and that once placed in similar con-

ditions, genotypic factors played more of a role in

growth.

The treated mussels showed lower mortality than

the controls. This indicates that the mussels that

survived the cull were more fit than those that were

not exposed to any stress treatment. There is an

important issue regarding the nature of the fitness

advantage that needs consideration. Did the differ-

ences seen in the treated mussels result from weaker

specimens being eliminated from the population or

did the treatments condition the survivors to better

withstand environmental stress? Currently, the answer

to this question is unknown. The long-term modifi-

cation of mussel populations to different thermal

regimes has been suggested (Newell and Branch,

1980), and fits the basic premise of evolution. What

about individual modification? It appears there is

some evidence that Mytilus can adjust itself after

experiencing a thermal shock. Buckley et al. (2001)

found that the expression of heat shock proteins

changes in Mytilus based on the previous thermal

history. This study by Buckley puts forward the

possibility that husbandry practices being developed

should investigate conditioning as well as culling as a

potential tool for improving the growth and survival

of farmed mussels.

5. Conclusions

This study found that in mussel seed from St.

Peter’s Bay, PEI:

(1) larger seed survived air exposure and smaller seed

survived exposure to high water temperature.

(2) exposing M. edulis to the air or high water

temperatures affected growth but there was an

interaction between the treatments and the

environment.

(3) mussel samples exposed to the air or high water

temperature had lower mortality than mussels

that had not undergone such treatments.

(4) the experimental husbandry practices examined

in this study affect the productivity of mussels

and this type of work has the potential to benefit

mussel aquaculture; this is especially true in a

place like Prince Edward Island where additional

leasing areas are not available to increase

production.

(5) this study showed an interaction between treat-

ments and the environment therefore further

research on mussel seed husbandry should take

into account as many environmental parameters

as possible.

Acknowledgements

We would like to thank Garth Arsenault for his

technical assistance and the three anonymous

reviewers for their helpful comments. We are also

grateful to Stephen Stewart (Stewart Mussels farms),

Bob Fortune (United Mussel Farms) and Russell

Dockendorff Jr. (PEI Mussel King) for their partic-

ipation in the field study. This work was supported by

the Prince Edward Island Aquaculture Alliance with

N. LeBlanc et al. / Aquaculture 243 (2005) 185–194 193

funds from NRC-IRAP (National Research Council-

Industrial Research Assistance Program), AFRI

(Aquaculture and Fisheries Research Initiative),

PEIAFAF (Prince Edward Island Department of

Agriculture, Fisheries, Aquaculture and Forestry)

and DFO (Department of Fisheries and Oceans).

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