Impact of Aquatic Macrophytes on Diversity and Production of Fish in Large Subtropical Reservoir

Egypt. J. Bot., Vol. 50, 137-158 (2010)

e-mail: [email protected], phone +20108771224/fax: +20973480450

Impact of Aquatic Macrophytes on Diversity

and Production of Fish in Large Subtropical

Reservoir Magdi M. Ali

1, Hussein A. Adam

2, Alaa El-Haweet

3

1Department of Botany, Faculty of Science at Aswan, South Valley University,

Aswan 81528, Egypt 2Fishery Management Centre, High Dam Lake Development Authority, Sahari,

Aswan, Egypt 3National Institute of Oceanography and Fisheries, Academy of Scientific

Research and Technology, Alexandria, Egypt.

QUATIC macrophytes are important to managers of captive

fisheries in inland waters. Construction of the Aswan High

Dam created a lake, which extends for 480 km; 300 km within the

Egyptian borders (Lake Nasser). We investigated the impact of

composition, structure, and distribution of aquatic macrophytes on

the production and diversity of fish in Lake Nasser. Macrophytes,

fish, and water were sampled from four stations in Lake Nasser

(Kalabsha, Garf Hussein, Kurusku and Tushka) representing

different bottom habitats, either in texture or in steepness, of the

lake, which are important to submerged plants distribution and

abundance, and hence their associated fish. At each site, submerged

macrophytes were collected from shallow and deep water, down to

the depth limit of plants colonisation. The plants were identified and

the mean dry weight standing crop was calculated for each species.

Fish were sampled using both trammel and floating-gill nets. For

each fishing method, total weight of the catch, total weight of each

species and number of individuals per species were determined. A

one-litre surface-water sample was collected from each station and

temperature, dissolved oxygen, pH, electrical conductivity, Secchi

disc transparency, carbonate, and bicarbonate were measured. Three

macrophyte communities were detected: a) Myriophyllum spicatum;

b) Myriophyllum spicatum - Potamogeton schweinfurthii; and c)

Myriophyllum spicatum - Najas horrida - Najas marina. Thirteen

fish species were recorded. Oreochromis niloticus dominated the

trammel-net catch in all regions and the floating gill-net catch at

Garf Hussein. O. niloticus dominated the littoral semi-pelagic zone

(>10 m depth), while Lates niloticus was co-dominant. The lateral

pelagic zone (<10 m depth) is dominated by L. niloticus and O.

niloticus in the northern sector and Hydrocynus forskalii,

Sarotherodon galilaeus and Tilapia zillii in the southern sector.

CCA indicates that temperature, dissolved oxygen, transparency,

carbonates and bicarbonates are the most important water variables

influencing the distribution of macrophytes and their associated in

fish in the littoral semi-pelagic, while water pH, bicarbonate, and

carbonates are the most influential variables in the lateral pelagic

zone. PCA indicates that the three submerged macrophyte

communities recognized and fish characterizing each of them are

A

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

well separated into consolidated groups. The study indicates that

habitat complexity exerts a significant influence on ecological

communities and in particular, composition and structure of

submerged macrophytes had a pronounced impact on fish

production and diversity in Lake Nasser.

Key words: Lake Nasser, Myriophyllum spicatum, Najas horrida,

Najas marina, Potamogeton schweinfurthii, Periphyton, Nile perch,

Tilapia.

Aquatic macrophytes are among those factors, which a fishery manager will try

to understand and include in his strategies for optimizing capture fisheries in

inland waters (Petr, 2000). Aquatic macrophytes can influence the lake

ecosystem in a number of ways, which fall into three main categories: (1)

limnological effects related to changes in physical and chemical conditions in the

water and sediments; (2) metabolic effects related to production and processing

of organic matter and nutrient cycling; and (3) effect on biotic interactions and

community structure related to the role of macrophytes in providing a structured

habitat (Gasith and Hoyer, 1998). Some plants or their combinations seem to be

better fish habitats than other (Opuszynski and Shireman, 1995; Fernandez et al.,

1998). Composition, distribution and percentage cover of aquatic macrophytes

may determine fish species composition, individual fish species production,

access to fish stocks by fishermen, fishing gear and sometimes also boat access

and transport possibilities for getting the fishery product to the markets (Petr,

2000; Padial et al., 2009). Plant density is of importance, and the trophic state of

the water body also plays an important role in determining what species and

what biomass of fish will be present (Weaver et al., 1996).

As result of the construction of the Aswan High Dam in the1960s, the High

Dam Lake (Lake Nasser and Lake Nubia) was created. In Egypt, Lake Nasser

has created considerable possibilities for fisheries, navigation, agriculture and

tourism (BISHAI et al., 2000). Dams have a range of possible effects depending

on the reservoir size and operation regime. In general, dams reduce the seasonal

and inter-annual variation in stream flow, with potentially serious consequences

for riparian and aquatic ecology. Moreover, they reduce incidence of extreme

floods and droughts, so reducing the occurrence of harsh environmental

conditions (Kondolf, 2001).

Regulated water bodies have distinct features associated with them. In

particular, hydroelectric reservoirs are characterised by an array of

geomorphologic features, soil types and environmental factors and tend to be

much steeper-sided than other lakes (Rørslett, 1988). Impoundment and

regulation of Lake Nasser for hydroelectric power production has altered the

environmental habitats and composition of communities, which existed before

regulation (Ali, 1992). The part of the Nile Valley, which became Lake Nasser,

had a relatively rich submerged and emergent flora (57 species) (Boulos, 1967;

El-Hadidi, 1976). After the construction of the Aswan High Dam, some species

disappeared and another were discovered (Ali, 2000). Previous studies (Ali,

1992, 2000; Ali et al., 1995; Ali and Soltan, 2006) have indicated that changes in

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

the submerged macrophytes communities were mainly due to water level

fluctuations and the transition from a lentic water system to one of the world

largest man-made lakes. Also, with the gradual increase of the impounded lake's

area, fish landing increased with the growth in fishing activity. It was the highest

34,206 tonnes in 1981 (Craig, 2000), the minimum 8,000 tonnes in 2000, just

after the lowest water level experienced in 1988, during the drought period (Ali

et al., 1995) and became 30,571 tonnes in 2005. The main gears used are gill and

trammel nets. The predominant fish caught by gill net are Alestes dentex L. and

Hydrocynus forskalii Cuv. Tilapias (O. niloticus and S. galilaeus) and Nile perch

(L. niloticus) form the major bulk of the trammel net catch (Khalifa et al., 2000).

The intense disturbance caused by the very large Lake Nasser water level

fluctuation has prevented any major expansion of emergent plant growth around

the shoreline, and prevents growth of floating macrophytes. Therefore, only

submerged macrophytes currently structure the aquatic habitat along the shores

of the Lake. Communities of submerged aquatic plants are particularly dynamic,

often because of high disturbance rates (Ali and Soltan, 2006). M. spicatum,

which invaded the lake in 1993, now develops massively over shoreline zone,

throughout Lake Nasser, and has had a major impact on its ecosystem (Ali and

Soltan, 2006).

Habitat complexity is thought to exert a significant influence on ecological

communities, but its operation under variable natural conditions is not well

understood, particularly in freshwater. To elucidate the role of habitat complexity

in the Lake, in particular the impact of submerged macrophyte communities on

fish diversity, an intensive field survey of four large side branches (locally

known as 'Khors') of the lake was carried out in the period form 13-21 March

2006.

Study area

The High Dam Lake stretches for 480 km from the High Dam (HD) in Egypt to

the Dal Cataract in Sudan (Fig. 1). It extends for 300 km within the Egyptian

borders (as Lake Nasser; 22° 00’ - 23° 58’ N and 31° 19’ - 33° 19’ E), and 180

km within the Sudanese borders (as Lake Nubia; 20° 27’ - 22° 00’ N and 30°

35’–31° 14’ E). Lake Nasser lies in extremely arid subtropical arid region with a

very hot summer climate and extends in a desert area between rocky hills

interrupted by vast sandy or stony plains of varying heights and nearly free from

vegetation. Lake Nasser's shoreline is very irregular, with numerous inundated

valleys (locally called 'Khors'). The total number of Khors is 85 (48 on the

eastern shore and 37 on the western shore).

Materials and Methods

Sites and sampling

Macrophytes and water were sampled from four stations (Khors) in Lake Nasser:

Kalabsha (44 km south of the HD); Garf Hussein (75 km south of the HD);

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

Figure 1. Location map of sites surveyed in each of the four main stations and in Lake Nasser;

Only macrophyte samples; Fish and macrophyte samples

Kurusku (177 km south of the HD); and Tushka (245 km south of the HD).

These Khors are representing different bottom habitats, either in texture or in

steepness, of the lake, which is important to submerged plants distribution and

abundance (Ali, 1992), and hence their associated fish. Kalabsha and Tushka are

wide Khors of sandy bottom and gently slope, while Khor Kurusku is steep, and

relatively narrow with a rocky bottom. Garf Hussein has a moderate width and

sandy bottom (Bishai et al., 2000). In total 22 sampling sites were surveyed. Six

sites () were sampled in each station (Khor) (Fig. 1); three from each of the

northern and the southern sides of each Khor, except in Garf Hussein only four

sites were sampled two from each side. Fish were only sampled from four sites

(), the northern site in each station for logistic reasons.

Macrophytes

At each site, submerged macrophytes were sampled using a grapnel method

to collect relative standing crop samples (five grapnel-hauls per sampling area)

from shallow and deep-water zones, down to the depth limit of plant

colonisation. This method gives good comparative values of abundance, but does

not estimate absolute standing crop per unit area of substrate (Murphy and Eaton,

1983). In the laboratory, plants were identified, separated into different species

and the mean dry weight standing crop (DWSC g sample-1

) was calculated for

each species per grapnel haul (sample) after air-drying (ambient temperature 45

C).

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

Fish

Fish were sampled using two net types, floating-gill and trammel nets. A

floating-gill net (locally known as Sakarota) is targeting pelagic fishes. A

number of net units (ranging from 20 to 40 units) are strung together. Each unit

varies in length from 20 to 50 m and from 2.5 to 5 m height. The mesh size

varies from 3.0 to 5.0 cm. The fishing process is operated overnight. The

trammel net (known as Ghazl Mesagin or Duk) is, the dominant fishing gear

used, targeting bottom and semi-pelagic fishes. The net length ranges from 20 to

30 m and 1.2 to 1.5 m height. Its mesh size of the inner layer ranges usually from

4.0 to 7.0 cm and of the outer layer is from 9.0 to 12 cm. Two to five units of

nets joint together, sometimes up to 20 units. Three layered net fishes in a

stationary manner over night or for half an hour using sound noise. A fisherman

hits the surface of the water using a pole and drums the deck with his feet.

Regarding over night operation, the net is fixed after sunset and collected before

dawn. Total catch from each fishing method was identified and separated into

different species. For each fishing method, total weight of the catch, total weight

of each species (tW), and total number of individuals per each species (tN) were

determined.

Water

A one-litre surface-water sample was collected from each location and

temperature, dissolved oxygen, pH, electrical conductivity, carbonate, and

bicarbonate were measured using standard methods (APHA, 1985). Also,

transparency was measured using a Secchi disc.

Data analysis

Water physico-chemical variables, macrophytes dry weight standing crop

(DWSC g sample-1

) and fish variables measured were loge transformed to

standardise the data sets (Flower et al., 2000). For each fishing net type, the data

were drawn up in the form of two matrices. One matrix of fish total weight of

each species and macrophytes DWSC (species) x sampling sites and another

matrix of water (environmental) variables x sampling sites. The constrained

ordination canonical correspondence analysis (CCA), CANOCO for Windows

version 4.0 (Ter Braak and Smilauer, 1998) was carried out for the paired

matrices in order to search and define the best explanatory water variables

characterising the fishing zones in Lake Nasser habitats and influencing the

distribution of submerged macrophytes and fish associated with them. Principal

component analysis (PCA) was used to test variability in fish communities in

relationship to macrophytes dominating the aquatic habitat in the four stations

studied, away from influence of water variables measured.

Results

Macrophytes

Four macrophytes are recorded: M. spicatum, P. schweinfurthii, N. horrida

and N. marina. M. spicatum is the commonest aquatic macrophyte in the four

stations (Fig. 2). It dominates the submerged plant communities in Garf Hussein

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

0

10

20

30

40

50

Kalabsha Garf Hussein Kurusku Tushka

Mean

DW

SC

g/s

am

ple

M. spicatum P. schweinfurthii N. horrida N. marina

Figure 2. Mean dry weight standing crop (DWSC g sample-1) of submerged macropohytes

recorded in each of the four stations in Lake Nasser

and Kurusku and forms pure stands in Tushka. P. schweinfurthii dominates the

submerged vegetation in Kalabsha and is abundant in Garf Hussein. N. horrida is

recorded only in Kalabsha and Garf Hussein with low DWSC and N. marina is

observed only in Kalabsha and Kurusku with very low DWSC.

Composition and structure of aquatic macrophytes communities

Three aquatic macrophyte communities are recognized in the northern site in

each of the four sites where fish was only sampled. Morphological and

architectural characteristics of each of these communities have made them differ

in their composition and structure. These communities are:

a) Myriophyllum spicatum Community

M. spicatum is a perennial that creates dense compact monospecific stands of

high biomass, especially in the littoral zones. The stems are produced in clusters

from a rooted base and float upward to just below the surface; they branch

several times near the water surface. Leaves are finely dissected feather-like in

whorls of four on the stem. M. spicatum is an opportunistic species that invades

the disturbed lakebeds. M. spicatum community solitary dominates the

submerged vegetation in sites from Kurusku and Tushka stations (mean DWSC

14.9 and 17.8 g sample-1

, respectively).

b) Myriophyllum spicatum - Potamogeton schweinfurthii Community

It usually inhabits the deeper pelagic zone. P. schweinfurthii has robust

unbranched or sparingly branched stem and submerged sessile to shortly

petiolate lanceolate to oblong-elliptical leaves of membranous lamina. M.

spicatum - P. schweinfurthii distinguishes the submerged macrophytes in Garf

Hussein Station (mean DWSC 9.4 and 2.0 g sample-1

, respectively).

c) Myriophyllum spicatum - Najas horrida - Najas marina Community

M. spicatum forms the main portion. N. horrida has an unarmed stems,

which have bushy upper parts because of the curved leaves. Leaves are linear-

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

lanceolate with conspicuous spiny teeth on both sides. N. marina has spiny stems

that branch toward the upward portion of the plant. Leaves are opposite or in

whorls of three and have triangular teeth along the leaf margins and along the

midrib on the underside of the leaf. Leaf apex is acute with one tooth. Leaves

are spreading to ascending and stiff. M. spicatum - N. horrida - N. marina is

represented the submerged macrophytes in Kalabsha Station (mean DWSC 9.1,

0.1 and 0.8 g sample-1

, respectively).

Fish diversity

Thirteen fish species are recorded (Table 1). These can be divided into three

categories: a) economic edible fish: this category includes eight species

Oreochromis niloticus L., Sarotherodon galilaeus Art., Tilapia zillii Gerv., Lates

niloticus L., Hydrocynus forskalii Cuv., Alestes dentex L., Bagrus bajad Forssk.

and Bagrus docmak Forssk.; b) uneconomic edible fish: this category includes

four species Mormyrus spp., Synodontis spp., Barbus spp. and Clarias spp.

which are of small total weight, therefore, they are gathered together under a

group named 'Other Species'; and c) uneconomic inedible fish: this category

includes one species Tetraodon lineatus L. which is of well detectable total

weight.

The lateral pelagic zone is more diverse (13 fish species) than the littoral

semi-pelagic zone, where, only 11 species are caught by trammel net. Also,

number of species of well detectable total weight (nine species) varied between

stations. In the littoral semi-pelagic zone, number of species varies between one

species in Kurusku and six species in Tushka. However, in the lateral pelagic

zone, it ranges between five species (in Kalabsha, Garf Hussein and Tushka) and

six species (in Kurusku).

The littoral semi-pelagic zone is dominated by O. niloticus, while L.

niloticus is co-dominated (Table 1). One fish of tiger fish (H. forskalii) is caught

from Garf Hussein Station and one fish of B. bajad is caught from Tushka. The

group of the uneconomic edible species 'Other' (Mormyrus spp., Synodontis spp.,

Barbus spp. and Clarias spp.) is found in Kalabsha, Kurusku and Tushka. The

uneconomic inedible fish T. lineatus is recorded only in Tushka (Table 1).

Number of the economic edible species is varying between one species in

Kurusku and five species in Tushka. Total number of individuals varies between

3 ind. in Kurusku and 61 ind. in Tushka.

In the lateral pelagic zone, is also dominated by O. niloticus and H. forskalii

and L. niloticus are co-dominated (Table 1). The group of the uneconomic edible

species 'Other' is found in all stations (Khors). The uneconomic inedible fish T.

lineatus is recorded only in Kalabsha. Number of the economic edible species is

varying between four species in Kalabsha and Tushka, and six species in

Kurusku. Total number of individuals ranges between 23 ind. in Tushka and 64

ind. in Garf Hussein.

T. zillii is only caught from the littoral semi-pelagic zone in the southern

sector of the lake in Tushka Station and B. docmak is only caught from the

lateral pelagic zone in Kurusku Station. The dominat Tilapia (O. niloticus) has

higher total biomass and number in the littoral semi-pelagic zone than in the

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

lateral pelagic zone. Nile perch (L. niloticus) individuals of higher biomass is

found in the littoral semi-pelagic zone, while greater individual number of lower

biomass is found in the lateral pelagic zone (Table 1).

Table 1. Total weights (kg) and numbers of fish caught by trammel net in the littoral semi-

pelagic zone and floating-gill in the lateral pelagic zone nets from the northern site in the four

stations (Khors) surveyed.

Fish

Trammel nets

littoral semi-

pelagic zone

Floating-gill nets

lateral pelagic

zone

Total

weight

(kg)

Total

number

(ind.)

Total

weight

(kg)

Total

number

(ind.)

a) Economic edible fish

O. niloticus 27.5 52 19.6 22 S. galilaeus 0.6 2 5.2 31

T. zillii 1.2 9 0.7 3

L. niloticus 15.8 8 7.1 24 H. forskalii 1.2 1 10.9 30

A. dentex 0 0 5.4 17

B. bajad 1.2 1 0.9 6 B. docmak 0 0 1.1 7

TOTAL 47.5 73 50.9 140

b) Uneconomic edible fish Mormyrus spp., Synodontis spp.,

Barbus spp., Clarias spp. 4.8 10 9.1 17

c) Uneconomic inedible fish

T. lineatus 5.1 10 0.6 2

Stations (Khors) Kalabsha West 29.5 21 13.2 32

Garf Hussein West 5.1 8 29.4 64

Kurusku East 3.6 3 13.5 40 Tushka West 19.1 61 4.7 23

TOTAL 57.6 93 60.8 159

Water

Water transparency and dissolved oxygen are the lowest in Tushka Station

and the highest in Garf Hussein Station. Other water variables measured vary

slightly between the four stations and characterise by being of warm temperature,

alkaline pH, well-oxygenated, moderate electrical conductivity, carbonate and

bicarbonate (Table 2).

Effect of water variables on composition and distribution of submerged

macrophyte and fish communities

The CCA diagram (Fig. 3a) indicates that water temperature, dissolved

oxygen, transparency, carbonates and bicarbonates, the longest vectors, are the

most important water variables influencing the distribution of submerged

macrophytes and their associated fish caught by trammel nets in the littoral semi-

pelagic zones (>10 m depth) of the four stations studied in Lake Nasser. They

explained 80.4% of the total macrophytes and fish distribution that are contained

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

Table 2. Minimum, maximum and mean values of the water characteristics in each of the four

stations surveyed in Lake Nasser.

in the first two axes. The situation of M. spicatum and O. niloticus points close

to the centre of the CCA diagram indicates that they are common in all stations

surveyed. In the CCA diagram, the closer position of Kalabsha and Kurusku

Stations points indicates their similarity in both submerged plants and fish

compositions. These two stations are located at the high gradient of water

carbonates, dissolved oxygen and electric conductivity, and low water

temperature, bicarbonates and pH (Fig. 3a). Less fish species are caught from

these two stations, however, Tushka station is the most fish diverse station,

where six species were caught. It is characterised by high water temperature and

pH, and low values of water electrical conductivity, transparency, and dissolved

oxygen (Fig. 3a). This habitat is favoured by fish that are never caught by

trammel nets elsewhere, such as T. zillii, S. galilaeus, B. bajad and the

uneconomic inedible fish T. lineatus.

The CCA ordination diagram (Fig. 3b) illustrates that water pH, bicarbonate,

and carbonates, the longest vectors, are the most influential variables that dictate

the distribution of macrophytes and their associated fish caught by floating-gill

nets in the lateral pelagic zone (<10 m depth) of the four main stations in Lake

Nasser. M. spicatum, situated in the centre of the CCA diagram, is the most

common aquatic macrophyte in the four stations. Kalabsha Station, the northern

most station, is characterised by having the highest water temperature, electric

conductivity, carbonates and pH, and the lowest values of water transparency,

bicarbonate and dissolved oxygen. This aquatic habitat is dominated by the

submerged macrophytes N. horrida and N. marina subsp. armata and much

preferred by the fish T. lineatus (uneconomic inedible fish). Garf Hussein

Station, northwest of the lake, is characterised by the highest water transparency

and bicarbonate, moderate values of dissolved oxygen, electric conductivity and

pH, and the lowest values of temperature and carbonates, where P.

schweinfurthii dominates the submerged vegetation and O. niloticus, S. galilaeus

and L. niloticus characterise the fish community. Kurusku Station, in the

southeast part of the lake, is characterised by having the highest values of

dissolved oxygen, transparency, carbonate and temperature, but it has the lowest

electric conductivity, pH and bicarbonates. This habitat is preferred by the fish

H. forskalii, A. dentex and B. docmak. Tushka station, the southern-most station,

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

is characterised by having the highest electric conductivity, pH and bicarbonates.

Aquatic vegetation is dominated by pure stands of M. spicatum, which is

preferred by T. zillii, which was only caught from this station, and S. galilaeus.

(a)

(b)

Figure 3. Canonical Correspondence Analysis (CCA) triplot of relationship between submerged

macrophytes dry weight standing crop () and total weight of fish () caught either by

trammel net in the littoral semi-pelagic zone (>10 m depth; Figure 3a) or floating-gill net in the

lateral pelagic zone (<10 m depth; Figure 3b) and water variables (arrows) in four stations ().

-1.5 1.0

-1.5

1.0

O. niloticus

L. nilotica

S. galilaeus

T. lineatus

H. froskalii

A.

dentex

B. docmak

B. bajad

T. zillii

Other

M. spicatum

P. schweinfurthii

N.

horrida

N. marina

Garf

Hussein

Kurusku

Tushk

a

Transparency

Temperature

pH Dissolved

Oxygen

Electric

Conductivity

Carbonates

Bicarbonates

Kalabsh

a

-1.5 1.0

O. niloticus

L. nilotica

S. galilaeus

T. lineatus

H. froskalii B. bajad T. zillii

Other

M. spicatum

P. schweinfurttii

N. horrida N. marina

Kalabsh

a

Kurusk

u

Tushka Transparency

Temperature

pH

Dissolved

Oxygen

Electric

Conductivity

Carbonates

Bicarbonates

Garf

Hussein

1.0

-1.0

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

Effects of composition and structure of submerged macrophyte communities on

spatial distribution of fish

The PCA ordination diagram elucidates the relationship between the

submerged macrophyte composition and the diversity of fish communities in the

four stations caught by trammel nets in the littoral semi-pelagic zones (Fig. 4a)

and in the lateral pelagic zone (Fig. 4b). PCA indicates that the aquatic habitat of

the littoral semi-pelagic zones may be divided into three ecological meaningful

groups (Fig. 4a). Group I contains species mainly from Kalabsha and Garf

Hussein. These stations are distinguished by having submerged macrophytes N.

horrida and N. marina (in Kalabsha), which is favoured by tilapia (O. niloticus)

and P. schweinfurthii (in Garf Hussein), where the piscivores tiger fish (H.

forskalii) is only recorded. Also, Nile perch (L. niloticus) was associated with

both macrophytes. Group II comprises species mostly from Kurusku Station,

where M. spicatum solitary dominates the submerged vegetation and 'Other'

uneconomic edible fish (Clarias spp., Barbus spp., Mormyrus spp., and

Synodontis spp.) are the most abundant. Group III embraces species largely from

Tushka Station, where submerged vegetation consists of pure stands of M.

spicatum. Group III is distinguished by four fish species (T. zillii, S. galilaeus, B.

bajad and the uneconomic inedible fish T. lineatus) that are not recorded in any

of the other stations. Each station forms a separate group, with slight interference

between group II and III, as M. spicatum dominates the submerged vegetation in

many sampling sites.

PCA diagram (Fig. 4b) indicates that the lateral pelagic zone is clearly

separated into four consolidate groups. Group I encompasses species from

Kalabsha, where submerged vegetation is dominated by N. horrida and N.

marina that is favoured by abundant of 'Other' uneconomic edible fish (Clarias

spp., Barbus spp., Mormyrus spp., and Synodontis spp.) and the uneconomic

inedible fish T. lineatus, and/or M. spicatum that is preferred by L. niloticus and

B. bajad. Group II contains species from Kurusku (southern east station of the

lake). This group characterised by M. spicatum that dominants the submerged

macrophyte and by the bottom living fish B. docmak that is only recorded there.

Group III comprises species from the most southern station (Tushka), where M.

spicatum solitary forms dense vegetation stands and T. zillii is only recorded.

Group IV encompasses species from Garf Hussein, where P. schweinfurthii

characterises the submerged macrophytes, which are favoured by O. niloticus, S.

galilaeus, H. forskalii and A. dentex.

Discussion

Morphological traits possessed by M. spicatum give rise to a compact

community structure with less inter-specific spaces between ramets, which

gradually decrease going from deep (lateral pelagic) water zone to shallow

(littoral semi-pelagic) water zone, as the volume of water column decreases. The

composition and structure of M. spicatum community, not only provide shelter

and refuge, but also, these features are likely to create a particular substrate for

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

(a)

(b)

Figure 4. Principal component analysis (PCA) ordination diagram of the relationship between

submerged macrophytes dry weight standing crop () and total weight of fish () caught

either by trammel net in the littoral semi-pelagic zone (Figure 4a) or floating-gill net in the

lateral pelagic zone (Figure 4b) in four stations (). Groups I, II, III or IV is indicated by lines

in each diagram.

-1.0 1.5

-1.0

1.0

O. niloticus

L. niloticus

S. galilaeus

T. lineatus

H. froskalii

A. dentex

B. docmak

B. bajad T. zillii

Other

M. spicatum

P. schweinfurthii

N. horrida N.marina

Garf

Hussein

Kalabsha

Kurusku

Tushka

Group I

Group II

Group III

Group IV

-1.0 1.0

-1.5

1.0

T. niloticus

S. galilaeus

T. lineatus

H. froskalii

B. bajad

T. zillii

P. schweinfurthii

N. horrida N. marina

Garf

Hussein Kalabsha

Kurusk

u Tushka

Group I

Group II

Group III

Other

O. niloticus

M. spicatum

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

algae and invertebrates, which are accomplished nutrients for fish. Chase and

Knight (2006) found that the abundance of both algae and M. spicatum

significantly increased with experimentally increased nutrient loading, while the

abundance of native submerged macrophytes declined. Similar findings were

noticed by Ali and Soltan (2006) due to high nutrient inputs resulted from

fertiliser usage in cultivation of Lake Nasser shorelines and treated sewage

effluents release by tourist ships, which cruise between Aswan and Abu Simbel,

into the Lake. Ali et al. (2007) indicate that Annelida were abundant in M.

spicatum in the shallow littoral semi-pelagic zone and Copepoda in the deeper

lateral pelagic zone. This study indicates that dense compact structure

monospecific stands of M. spicatum is preferred by periphyton-plankton feeders

such as O. nilotucus and omnivorous feeders such as the uneconomic edible fish

species (Mormyrus spp., Synodontis spp., Barbus spp. and Clarias spp. in

Kurusku Station. Similar findings on food preference of the fish caught were

obtained by pervious studies (Willoughby, 1974; Moor and Bruton, 1988;

Bailey, 1994; Bishai et al., 2000). Also, in Tushka Station, M. spicatum habitat is

favoured by periphyton-plankton feeders such as T. zillii and S. galilaeus

(Bailey, 1994; Bishai et al., 2000), exclusively piscivorous such as B. bajad

(Reed et al., 1967; Olaosebikan and Raji, 1998, Bishai et al., 2000), and the

uneconomic inedible fish T. lineatus that feeds on fry.

In the present study, fish associated with M. spicatum community are few

such as O. nilotucus; juveniles of low to medium weight fish, such as T. zillii and

S. galilaeus; or the seldom seen bottom feeders such as Mormyrus spp.,

Synodontis spp., Barbus spp. and Clarias spp. Similar fish behaviour was

documented by Lewis (1974). The fish composition and behaviour observed may

be due to the phenolic compound (Tellimagrandin) that is slowly released by M.

spicatum under natural circumstances (Gross et al., 1996). Although,

morphological architecture of M. spicatum provides suitable substrate for algal

growth; tellimagrandin released may prevent development of thick cover of

epiphytes on it and subsequently suppressed the availability of food materials.

In the present study, Tushka that is characterised by extreme values of water

pH and temperature, and low dissolved oxygen is preferred by T. zillii, which

was only fish recorded there. Similarly, other studies indicate that T. zillii can

tolerate aquatic habitats of high pH up to 9.0 (Eccles, 1992) and high

temperature up to 36 C (Philippart and Ruwet, 1982). This may be attributed to

the dense M. spicatum, which is known to alter water quality by raising pH,

decreasing oxygen under the mats, and increasing temperature (Smith and Barko,

1990). Also, Pelicice et al. (2005) stated that critical values of oxygen,

temperature and pH could be found in the inner-most areas of extensive

vegetation beds, due to limited water exchange and reduced atmospheric contact.

The present study indicates that fish are more diverse in the lateral pelagic

zone than in littoral semi-pelagic zone (e.g. in Kurusku). This may be attributed

to the less compact open structure of M. spicatum and dilution of the phenol

compound (Tellimagrandin) released by increasing the volume of the water

column with depth. These physical changes in M. spicatum community and the

high transparency characterising Kurusku waters, allow better environmental

M. M. Ali et al.

Egypt. J. Bot. 50, 137-158 (2010)

conditions for flourishing periphyton-plankton species indicated by the high

dissolved oxygen in the CCA diagram (Fig. 3b), and subsequently plentiful

invertebrates feeding on them (Ali et al., 2007). Pervious studies (e.g. Mageed,

1995; Bishai et al., 2000) stated that areas with high chlorophyll a concentrations

(i.e. rich phytoplankton) in the middle and southern regions of the Lake (e.g.

Kurusku) contain the highest number of zooplankton (90 ind. l-1

). This aquatic

habitat is preferred by the omnivorous pelagic A. dentex (Bailey, 1994) and the

benthopelagic potamodromous T. zillii that are usually found at depth of 1–7 m

(Eccles, 1992). Also, high water transparency and the less compact open

structure of M. spicatum in the pelagic zone makes it more visible for the pelagic

piscivores, particularly A. dentex (Reed et al., 1967), L. niloticus, H. forskalii

and B. bajad. This is agree with the finding of Priyadarshana et al. (2001) who

demonstrated that high macrophyte density can decrease predator efficiency by

reducing visual contact with prey and hindering movement. On the other hand, in

the pelagic zone of Tushka Station, O. nilotucus and T. lineatus were not

recorded. This may be because O. nilotucus usually inhabits the shallow littoral

dense vegetated zone (<10 m; Agaypi, 2000) during spawning season (March to

May), which coincidently is the sampling period (March, 16 - 21) of the preset

study. However, the usual depth ranges from 5 m (Bailey, 1994) to 20 m (van

Oijen, 1995). Also, T. lineatus is confined to the same zone, because they feed

on molluscs (Bailey, 1994) inhabiting the littoral submerged vegetation. Also,

they may prey on the spawning stock of O. nilotucus.

The present study indicates that the littoral semi-pelagic and lateral pelagic

zones of Garf Hussein are discriminated from other stations surveyed by having

M. spicatum - P. schweinfurthii community. Ali and Soltan (2006) found that P.

schweinfurthii usually abundantly inhabits the deeper pelagic zone in Lake

Nasser. The two-species composition of this macrophyte community, in

particular with the morphological traits possessed by P. schweinfurthii, makes it

of very open structure than macrophyte community solitary composes of M.

spicatum. This open structure, in addition to high water transparency

characterising Garf Hussein, is favoured by the visual predators such as the Nile

perch (L. niloticus) and tiger fish (H. forskalii) that feed on small tilapia (O.

nilotucus). Ali et al. (2007) indicated that M. spicatum - P. schweinfurthii

community is characterising by high counts of invertebrate (e.g. Copepoda) in

deeper water, which is considered an excellent food for the omnivorous pelagic

A. dentex (Bailey, 1994). In turn, small-sized A. dentex increases the nutrition

value of the deeper water zone characterised by M. spicatum - P. schweinfurthii

for the pelagic piscivorous L. niloticus and H. forskalii (Reed et al., 1967).

In the present study, CCA indicates that submerged plant community

composed of M. spicatum - N. horrida - N. marina distinguishes Kalabash from

other stations. The composition of this community confers a structure of an inter-

specific character, which is intermediate between the dense compact structure of

community solitary composed of M. spicatum and the open structure of

community composed of M. spicatum - P. schweinfurthii. Several studies have

demonstrated that highest values of fish density and species richness are detected

in areas of intermediate macrophyte density (Killgore et al., 1989; Dibble et al.,

IMPACT OF AQUATIC MACROPHYTES ON FISH

Egypt. J. Bot. 50, 137-158 (2010)

1997), or in the open-water/macrophyte interface (Agostinho et al., 2002). The

composition and structure of this three-species community contained high

invertebrate density (423 ind. g-1

plant dry weight) (Ali et al., 2007), are quite

preferred by O. nilotucus and other edible uneconomic fish, which include

feeders on benthic animals (Bailey, 1994), crustaceans and insects nymph, larvae

and eggs (Willoughby, 1974), in the littoral semi-pelagic zone. However, in

deeper zone, this macrophyte community is favoured by higher fish diversity

(e.g. O. nilotucus, S. galilaeus, L. niloticus, the uneconomic inedible fish T.

lineatus and other edible uneconomic fish). This may be because the submerged

plant community gradually becomes sparse with vegetation-free upper stratum

going deeper, which allow more space for fish mobility. These conditions

prevailing in the pelagic zone make it more transparent than the littoral semi-

pelagic zone, which partially explain the presence of the piscivorous Nile perch

(L. niloticus) (Reed et al., 1967).

This study indicates that each of the submerged communities recognized in

Lake Nasser possesses distinctive morphological and architectural features that

make each of them of distinguishable composition, which supports readily

distinguishable fish communities. The present study indicates that the submerged

macrophyte community solely composed of M. spicatum supports a lower

biomass of fish than those composed of a mixture of M. spicatum and other

macrophytes. This may be due to M. spicatum beds supporting significantly

fewer benthic and foliar invertebrates than did mixed beds of pondweeds

(Potamogeton spp.) (Keast, 1984). Likewise, fish abundance in the Potamogeton

mixed community during daytime feeding periods was 3 to 4 times greater than

in M. spicatum beds. A dense cover of M. spicatum (such as in Kurusku and

Tushka Stations) allows high survival rates of young fish. In addition, larger

predator fish lose foraging space and are less efficient at obtaining their prey

(Lillie and Budd, 1992; Engel, 1995). Madsen et al. (1995) found growth and

vigour of a warm-water fishery reduced by dense M. spicatum cover. These are

in general agreement with the findings of the present study.

M. spicatum is an opportunistic nuisance submerged weed that invaded Lake

Nasser in 1993 (Ali, 2000). Since that date, Lake Nasser submerged macrophytes

communities have changed greatly. M. spicatum has a pronounced influence on

the community composition of submerged macrophytes, since it competes

aggressively to displace and reduce the diversity of the originally present aquatic

plants (Ali and Soltan, 2006). Conclusively, the present study suggests that

increasing trend and invasion capacity of the nuisance M. spicatum into Lake

Nasser have diverse impact on the fisheries of the shallow littoral zone of the

Lake and will lead to severe reduction in diversity and production of its

economic fish, especially tilapia (O. niloticus) and Nile perch (L. niloticus) in the

future.

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تأثير النباتات المائية على تنوع وأنتاج األسماك في خزان شبه أستوائي

على. مجدى م

1آدم. و حسين ع

2و عالء الحويط

3

1 ، مصر81518قسم النبات، كلية العلوم بأسوان، جامعة جنوب الوادى، صحارى

مركز أدارة صيد األسماك، هيئة تنمية بحيرة السد العالي، ، صحارى، أسوان، مصر2 المعهد القومي لعلوم البحار والمصايد، أكاديمية البحث العلمي والتكنولوجيا، أسكندرية، مصر3

لااأ أاءاال السااد العاالي ب سااوان . النباتاا الماييااة مهماة لمااديرو مصاايد المياااة الدا لياة

، فحصاانا تاا تير تكااوي (بحياارة ااصاار)كاا بالحاادود المصاارية 344كاا 084بحياارة تمتااد

جمااا تاا. يااا النباتااا الماييااة ولااأ تنااوأل وأاتاااب األسااماك فااي بحياارة ااصااروتركيااو وتو

كالبءااة وجااري سااي )النباتااا واألسااماك والمياااة ماا اربااا محيااا فبااي بحياارة ااصاار

تمثااب بيئااا مةتلنااة ماا ياااأل البحياارة ماا يااث الملماا أو األاحاادار، ( وكرسااكو وتوىااكأ

فاي كاب موياا، . م تا األساماك المرتبياة بهااالمهمة في تو يا ووفرة النباتا المغمورة، و

جمعت النباتا المغمورة م المياة الضحلة والعميقة، اازو تاأ العماأ النهاايي لمساتعمرة

جمعات . تا تعرياا النباتاا وتا سااز الاو ن القااي القااي للمحصاو لكاب ااوأل. النباتا

يد، تاا تقاادير الااو ن الكلااي لكااب يريقااة صاا". السااكاروت "و" الااد "األسااماك كااب ماا ىااباك

جماا وا اد لتار ما ماال . للصيد السمكي و الو ن الكلي لكب اوأل وودد األفراد فاي كاب ااوأل

الساااين مااا كاااب محياااة، وتااا يياااا الحااارارة واألكساااقي الااا ايو والاااري الهيااادروجيني

.والتوصيب الكهربي والءنافية والكربواا والهيدروكربواا

( ز Myriophyllum spicatumذو األلا ورية ( ا: تيةت تحديد تالث مقتمعا ابا

Myriophyllum spicatum - Potamogeton schweinfurthiiالحريش -ذو األلا ورية

Myriophyllum spicatum - Najas horrida - Najasالءلبيكة – ذو األلا ورية( و جـ

marina . ساد البليي . اوأل سمكي 33ت تسقيبOreochromis niloticus الصيد

جري في "السكاروت " الصيد السمكي بءبكة في كب المنايأ و" الد "السمكي بءبكة

، بينما ىارك يءر البياض (م34<ومأ )البحرية -ساد البليي المنيقة السا لية ىبة. سي

Lates niloticus ساد يءر البياض (م34>ومأ )المنيقة القاابية البحرية في . السيادة ،

والبليي القليلي Hydrocynus forskaliiي القياأل الءمالي وساد كلو السمك والبلي

Sarotherodon galilaeus والبليي األ ضرTilapia zillii أوضن . القياأل القنوبي

والءنافية والكربواا الحرارة المال واألكسقي ال ايوأن CCAتحليب العاليا المتوافقة

واألسماك المرتبية تو يا النباتا الماييةكاات أه العوامب المؤترة في والهيدروكربواا

والكربواا البحرية، بينما كان الري الهيدروجيني للمال -في المنيقة السا لية ىبة بها

أوضن تحليب المكواا . المنيقة القاابية البحرية فيأكثر العوامب ت تيرا والهيدروكربواا

قتمعا الثالث للنباتا المايية المغمورة المتعاري وليها واألسماك أن الم PCA الرييسية

توضن الدراسة أن تركيو البيئة المعقد . التي تميزها منصولة جيدا الي مقمووا تابتة

تكوي وتركيو النباتا الماييةيمار ت تير يير ولأ المقتمعا البيئية بها و صوصا

.وأاتاب األسماك في بحيرة ااصرتنوأل المغمورة ل بالغ األتر ولأ