temporal and spatial distribution of fish and shrimp of bakkhali estury of bangladesh

8/3/2019 Temporal and Spatial Distribution of Fish and Shrimp of Bakkhali Estury of Bangladesh

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Temporal and spatial distribution of fish and

shrimp assemblage in the Bakkhali river estuary

of Bangladesh in relation to some water quality

parametersMd. Rashed-Un-Nabi

a, Md. Abdulla Al-Mamun

a, Md. Hadayet Ullah

a& M. Golam

Mustafab

aInstitute of Marine Sciences and Fisheries, University of Chittagong, Bangladesh

bThe World Fish Center, Bangladesh and South Asia Office, Dhaka, Bangladesh

Available online: 04 Jul 2011

To cite this article: Md. Rashed-Un-Nabi, Md. Abdulla Al-Mamun, Md. Hadayet Ullah & M. Golam Mustafa (2011): Temporal

and spatial distribution of fish and shrimp assemblage in the Bakkhali river estuary of Bangladesh in relation to some

water quality parameters, Marine Biology Research, 7:5, 436-452

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ORIGINAL ARTICLE

Temporal and spatial distribution of fish and shrimp assemblage in the

Bakkhali river estuary of Bangladesh in relation to some water qualityparameters

MD. RASHED-UN-NABI1*, MD. ABDULLA AL-MAMUN1, MD. HADAYET ULLAH1 &

M. GOLAM MUSTAFA2

1Institute of Marine Sciences and Fisheries, University of Chittagong, Bangladesh, and

2The World Fish Center, Bangladesh

and South Asia Office, Dhaka, Bangladesh

Abstract

Fish and shrimp species, together with water quality data, were collected from two different stations located inside theBakkhali river estuary of Bangladesh during winter, premonsoon and monsoon periods. Significant temporal differenceswere observed for water temperature, salinity and dissolved oxygen. The average catch of fish and shrimps per net betweenstations varied between 1.8990.36 kg at station 1 and 7.5494.39 kg at station 2, while the average catch in winter,premonsoon and monsoon periods was found to be 2.7991.08 kg/net, 6.3191.03 kg/net and 5.0692.89 kg/net,respectively, with a significant difference in catch per net between stations although no significant difference in catch pernet was observed between seasons. A total of 18,467 individuals of fish (35 species) and shrimp (10 species) were found inthe present study. Three species of shrimps were observed to be dominant (10.0%) and these were Metapenaeus lysianassa(17.07%), Ambassis dussumieri(14.54%) and Macrobrachium villosimanus (12.13%). Clear differences in faunal abundanceswere observed between seasons and stations with higher mean abundances during winter (1747.839421.99 individuals/5 kg)and at Station 1 (14449866.74 individuals/5 kg). Similarly, the diversity indices, both ShannonWiener and Margalef,showed significant differences between stations and seasons (except Shannon for stations). Analyses of similarity (ANOSIM)results confirm both spatial and temporal differences in species community structure with a highly diverse assemblage.Canonical Correspondence Analysis results indicated that salinity and transparency were the main variables influencing fishand shrimp distribution in the Bakkhali river estuary.

Key words: Ambassis dussumieri , analysis of similarity, Bakkhali river estuary, Metapenaeus lysianassa , species

assemblage

Introduction

Estuaries are transition zones between sea and fresh-

water; they are occupied by a combination of fresh-

water andmarine species as well as juveniles (Claridge

et al. 1986). They serve important economic func-

tions including transportation, industry and tourism,

but also drainage of waste from domestic, industrial

andagriculture activities (Heip & Herman 1995; Raz-Guzman & Huidobro 2002). Simultaneously, these

ecosystems offer protection, not only for resident

species, but also for a wide range of marine and

freshwater species which migrate there at certain

stages of their life cycle (Weinstein 1985; Weisberg

et al. 1996; Cowley and Whitfield 2002; McLusky

and Elliott 2004; Blaber 2000). Fish assemblage

structure of estuaries is characterized by high diversity

and high abundance, especially for juveniles (Whit-

field 1999). An examination of the ecological factors

is important in defining habitats for fishes and has

been the main focus of many previous studies (Able

1999; Martino & Able 2003). Most estuaries arecharacterized by high biological productivity asso-

ciated with relatively extreme and varying environ-

mental conditions (Day et al. 1989; Kennish 1990;

Whitfield 1999). As boundary systems between

*Correspondence: Md. Rashed-Un-Nabi, Institute of Marine Sciences and Fisheries, University of Chittagong, Chittagong 4331,

Bangladesh. E-mail: [email protected]

Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway, and the Marine Biological Laboratory,

University of Copenhagen, Denmark

Marine Biology Research, 2011; 7: 436452

(Accepted 19 August 2010; Published online 4 July 2011; Printed 11 July 2011)

ISSN 1745-1000 print/ISSN 1745-1019 online # 2011 Taylor & Francis

DOI: 10.1080/17451000.2010.527988


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watersheds and the sea, estuaries exhibit environ-

mental gradients that favour the recruitment of a

variety of species with diverse physical and trophic

structures (Sanchez & Raz-Guzman 1997; Harris

et al. 2001; Kimmerer et al. 2001). Since estuaries

serve as nurseries for many commercially important

fish and crustaceans (Shenker & Dean 1979; Wein-

stein 1979; Rakocinski et al. 1996; Blaber 2000;

Elliott & Hemingway 2002; Akin et al. 2003), it isnecessary to examine the environmental factors that

shape the species assemblage structure. Fishes play an

important role in estuaries as they constitute perma-

nent and temporary community components, with

marine species visiting these habitats for feeding,

reproduction, growth and protection (Raz-Guzman

& Huidobro 2002). The distributions of fish within

biologically and physically complex estuarine systems

may be influenced by many mechanisms. Several

estuarine ecologists have pointed out that biotic

processes, such as competition and predation, may

be important in driving the occurrence of spatial andtemporal patterns of fish abundance and assemblage

in estuaries (Holbrook & Schmitt 1989; Ogburn-

Matthews & Allen 1993; Lankford & Targett 1994;

Barry et al. 1996).

By nature, estuarine habitats are highly productive

(Nixon et al. 1986; Day et al. 1989) and their role as

nursery grounds for fishes is well documented for

temperate (Powles et al. 1984; Elliott et al. 1990;

Kennish 1990; Drake & Arias 1991; Szedlmayer &

Able 1996; Whitfield 1999; Blaber 2000; Shackell &

Frank 2000; Elliott & Hemingway 2002) and tropical

regions (Raynie & Shaw 1994; Sanvicente-Anorve

et al. 2000; Harris et al. 2001; Cowley & Whitfield

2002; Franco-Gordo et al. 2003). Several biological

and abiotic factors affect the occurrence and habitat

of fish and shrimp within estuaries. These factors

include salinity, temperature, turbidity, dissolved

oxygen (DO), freshwater inflow, structural attributes

of habitat, depth, geographic distance from the

estuary mouth, and hydrography (Gunter 1961;

Blaber & Blaber 1980; Weinstein et al. 1980; Rogers

et al. 1984; Zimmerman & Minello 1984; Thorman

1986; Peterson & Ross 1991; Sogard & Able 1991;

Cyrus & Blaber 1992; Rakocinski et al. 1992; Cowen

et al. 1993; Everett & Ruiz 1993; Szedlmayer & Able1996; Fraser 1997; Maes et al. 1998; Marshall &

Elliott 1998; Araujo et al. 1999; Wagner & Austin

1999; Whitfield 1999; Hagan & Able 2003;

Jaureguizar et al. 2003; Martino & Able 2003).

The assemblages of fish in estuaries are variable

both in terms of species composition and distribution

patterns (Harris et al. 1999). Changes in species

assemblage are continuous, according to reproductive

seasons of the species and the environmental fluctua-

tions (Whitfield 1994; Harris & Cyrus 1995; Hettler

& Hare 1998; Garcia et al. 2003). However, through

discussion with local fishermen from the Bay of

Bengal, Bangladesh, during the present study, there

seems to be a general tendency for estuarine fish

larvae to peak in abundance during the monsoon in

this region. Furthermore, Hossain et al. (2007) also

reported a similar trend for juvenile fish species at

Naaf river estuary. Fish and shrimp assemblage

structure in the estuaries of Bangladesh has not beenwell studied; although there are some scattered works

on different biological aspects of the coastal estuarine

system of Bangladesh (Hossain et al. 2007), none of

them examined the species assemblage structure.

The Bakkhali river estuary located at the south-

eastern part of Bangladesh is heavily supported

by small scale and multigear fisheries. The coastal

areas show a typical tropical multi-species fisheries

ecosystem. There are about 490 species of fishes

(Hossain 1971) and 19 species of shrimps/prawn

(Chowdhury & Sanaullah 1991) available in this

area. These fisheries are characterized by fishinghouseholds rather than commercial organizations

and play a greater role in sustaining the livelihoods

and ensuring the food security of large numbers

of rural people throughout the developing world

(Whitmarsh et al. 2003). The future of these poten-

tially huge resources has not been well documented.

However, for the sustainability of this fishery resource

proper scientific study is an urgent task. Hence, the

present study has been designed to provide an

extensive report on the fish and shrimp assemblage

structure of Bakkhali river estuary in relation to water

quality parameters.

Materials and methods

Study area

The Bakkhali river estuary is located at the south-

eastern coast of the Bay of Bengal in Bangladesh

(Figure 1). A number of small streams originating

from the south-eastern hills of Mizoram (India) meet

at the Naikhongchhari of Bandarban district

and form the river Bakkhali. It flows through

Naikhongchhari and Ramu of Coxs Bazar district

and falls into the Moheshkhali channel of the Bay ofBengal. This river is relatively wide compared to

other rivers of the Coxs Bazar district and has a

length of about 67 km. The Bakkhali river estuary

has a semidiurnal tidal regime. Its hydrology is also

heavily influenced by monsoon wind. The tidal

range varied between 0.07 m and 4.42 m during

neap and spring tide respectively (Hossain and Lin

2001). Salt intrusion extends up to 6 km upstream

where a rubber dam was constructed for irrigation

purposes.

Bakkhali estuary fish and shrimp assemblage 437


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The bottom of this river consists mainly of mud

and sand particles. The estuarine zone is also

characterized by long intertidal mudflats where

mangrove vegetation (mainly Avicennia sp.), natural

ullo grass Imperata cylindrica, cord grass Spartina sp.

and sea grass Halophila beccarii are present (Hena

et al. 2007). The lower part of this estuary is heavily

influenced by anthropogenic and industrial activitiesincluding fish harbours, fish processing plants and a

large number of fish and shrimp farms. The large

amount of organic and inorganic waste changes the

chemical characteristics of the water body by produ-

cing toxic substances, which ultimately affect the

biodiversity. Sampling took place at two stations

(Figure 1), one (St1) about 5 km upstream from the

estuary, which is protected from the sewage and

anthropogenic intervention, and another (St2) at the

lower stream near the mouth of the estuary, heavily

influenced by domestic and industrial activities.

Apart from these two stations, nets were not set on

a regular basis in the other areas of the Bakkhali river

estuary. Net setting and collection of samples was

largely dependent on the local fishermen who have

used these areas for generations. Therefore, these

areas are allowed for fishing only by the local

fishermen. Hence, through negotiation with the localfishermen these two stations were considered for the

present study.

Sampling gear

The fish and shrimp samples were collected using

barrier nets known locally as Char jal (Figure 2). In

Coxs Bazar region, Char jal are used to catch

various aquatic species from river banks inundated

during high tide. Net fencing is made from bamboo

Figure 1. Map of Bakkhali river estuary and location of sampling stations (St1, St2).

438 Md. Rashed-Un-Nabi et al.


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poles which are submerged during high tide. Sam-

ples are collected during low tide. The net frame is

around 2.5 m in height and 150 m in length, forming

part of a circle so that there is approximately 120 m

distance between two ends of the net. The nylon net

has a mesh size of 0.8 cm. Bamboo poles are secured

on the shore of the river during low tide. During high

tide the water is allowed to enter and after 22.5 h of

the high tide the fishermen secure the upper portion

of the net and create a barrier. At low tide all the

animals inside the fence become trapped and the

fishermen harvest the fish, shrimp and crab. Finally,

the net is released from the bamboo and is ready for

the next high tide.

Sampling periodicity

Samples were collected each month between

December 2007 and August 2008. Of the four

seasons specified by Mahmood et al. (1994), three

seasons were chosenthe winter (December, January

and February), premonsoon (March, April and May)

and monsoon (June, July and August) to conduct

the sampling. Sampling was done during the full

moon and new moon, as during these periods higher

abundance of fish and shrimps were reported by the

fishermen. No samples were collected during the

post-monsoon period (September, October and No-

vember) as the fishermen become engaged in Hilsha

fishery, which is the single largest commercial fishery

of Bangladesh (Mazid 2002). Fishermen are engaged

by Hilsha boat owners to leave this less-profitable

Char jal fishing during the post monsoon season.

Sample collection

Sample catches from Char jal were taken directly

from the nets. In the laboratory, samples were sorted

and identified to species level (Fischer & Whitehead

1974; Shafi & Kuddus 1982a, 1982b; Talwar &

Jhingran 1991; DeBruin et al. 1995). The total

numbers of each species and their wet weight from

each net were also recorded. During sampling, in situ

water quality parameters were measured at each

sampling site. The salinity, pH, temperature and

dissolved oxygen were determined by using a re-

fractrometer (NewS-100, TANAKA, Japan), a pen

pH meter (s327535, HANNA Instruments), a

thermometer in centigrade and a DO meter (HI

9142, HANNA Instruments), respectively. A Secchi

disc (20 cm diameter) was used to measure the water

transparency.

Data analysis

Diversity of the species assemblage was expressed by

the ShannonWiener index (H?) (Shannon 1949;

Shannon & Weaver 1963; Ramos et al. 2006) using

the following formula:

H?0XS

i01

Pi1log P

i

where S is the total number of species and Pi is the

relative cover of ith species.

Richness was measured by Margalef index (d)(Margalef 1968) using the following formula:

d0 (S(1)=log(N);

where S is total species and N is total individuals.

For environmental parameters (temperature, sali-

nity, DO, pH and water transparency) one-way

analysis of variance (ANOVA) was used to calculate

if there is any difference between two stations. The

same procedure was followed for seasons. Prior to

ANOVA tests, all data were checked for normality

Figure 2. A Char jal in the Bakkhali river estuary.



6/18

using the KolmogorovSmirnov test and homogene-

ity of variances using Levene test (Sokal & Rohlf

1998). Furthermore, in the event of significance, a

post hoc Tukey HSD test was used to determine

which meansweresignificantly different at a 0.05 level

of probability (Spjotvoll & Stoline 1973). The

KruskalWallis test (Akin et al. 2005) was performed

on data which did not satisfy the assumptions of

normality and homogeneity, after performing diversedata transformations (Clarke & Warwick 1994).

Except for salinity, all other water quality parameters

met the criteria of normal distribution and homo-

geneity of variances. One-way analysis of similarity

(ANOSIM) (Clarke & Warwick 1994) was used to

conclude the significance of spatial and temporal

variation in the fish and shrimp assemblage structure.

This test is based on a BrayCurtis rank similarity

matrix and was calculated using log-transformed

data. Similarity percentages analysis (SIMPER)

(Clarke 1993) was used to observe the percentage

contribution of each species to the average dissim-ilarity between samples of the various seasons and

station-pair combinations. Hierarchical agglomera-

tive clustering with group average linking and

non-metric multi-dimensional scaling (nMDS) were

performed to investigate similarities among stations

and seasons (Clarke & Warwick 1994). This analysis

was based on the BrayCurtis similarity measure

(Bray & Curtis 1957). Only species with more than

1% of the total species were included in the analysis to

avoid any unusual effects of rare species. All the

multivariate analyses were performed using the soft-

ware PRIMER V6 (Plymouth Routines Multivariate

Ecological Research) (Clarke & Warwick 1994). Asso-

ciations between species and environmental variables

were examined with the canonical correspondence

analysis (CCA) using the ECOM 1.32 version (En-

vironmental Community Analysis 2000) software. To

reduce the effects of rare species, only species con-

tributing 1% of the total based on all species and

samples were included in CCA after log transforma-

tion (Log10(x'1)). CCA was proposed to constrain

the axes in classical Correspondence Analysis (CA)

to be linear functions ofa-prioridefined or measured

variables associated with species records. The ordi-

nation axes of CA are termed Eigenvectors. EachEigenvector has a corresponding Eigenvalue, often

denoted by l. The Eigenvalue is actually equal to

the (maximized) dispersion of the species scores on

the ordination axis, and is thus a measure of

importance of the ordination axis. The first ordina-

tion axis has the largest Eigenvalue (l1), the second

axis the second largest Eigenvalue (l2), and so on.

The Eigenvalues of CA all lie between 0 and 1.

Values over 0.5 often denote a good separation of the

species along axis (Jongman et al. 1995).

Results

Environmental parameters

The measured environmental parameters are sum-

marized in Table I and illustrated in Figure 3. Water

temperature ranged between 218C (in winter; Jan-

uary 2008 at St1) and 318C (in premonsoon and

monsoon; March and July 2008, respectively, at St2)

with a mean of 27.1693.338C. No significantdifference was observed in temperature between

stations (F1,12 0 2.13, P 0 0.17). However, winter

season showed a significant difference from mon-

soon and premonsoon (F2,12066.65, PB0.001)

although there was no significant difference between

premonsoon and monsoon.

Salinity values (mean 20.00911.94) ranged from

2.00 ppt (during monsoon season; August 2008) to

31.00 ppt (during winter season; December 2007

and February 2008). No significant differences were

found in salinity between the stations (H00.788,

P0

0.375), while significant differences were ob-served among the seasons (H015.316, PB0.001)

with very low salinity during monsoon (Figure 3).

Oxygen concentration (mean04.2190.58) at-

tained a maximum in March (5.02 mg/l at St1)

and a minimum in June (3.24 mg/l at St1). No

significant differences were found in oxygen concen-

tration throughout stations (F1,1200.30, P00.59);

in contrast, significantly higher dissolved oxygen

concentration was observed during premonsoon

season (4.7990.24 mg/l) compared to winter

(4.1290.40 mg/l) and monsoon (4.4090.49 mg/l)

seasons (F2,1209.22, PB0.001).

Water transparency varied from 26 cm (duringmonsoon; July 2008 at St2) to 70 cm (during winter

season; January 2008 at St1) with a mean of 42.839

14.86 cm. Significant differences were observed in

water transparency between stations (F1,120212.08,

PB0.001). Similarly, water transparency exhibited a

strong seasonal gradient (F2,1209.06, P00.01).

Mean value in winter season (62.0094.69 cm) was

noticeably higher than the premonsoon (37.839

3.63 cm) and monsoon season (28.6692.16 cm).

The highest pH value (7.7) was observed during

the premonsoon season at St1, while the lowest pH

value (6.3) was observed during the winter season,also at St1. Mean pH value was observed to be

7.2090.34. No significant differences were found

for pH between stations (F1,1201.20, P00.29) and

among the seasons (F2,1201.28, P00.31).

Species community composition by weight

The catch per net at St1 ranged between 1.31 kg

(during winter season) to 2.52 kg during the pre-

monsoon period with an average of 1.8990.36 kg.



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The average catch per net at St1 during winter,

premonsoon and monsoon seasons was found to be

1.6490.29 kg, 1.9490.57 kg and 2.1090.07 kg,

respectively. Of the fish species, Liza tade was found

to be the most abundant (23.6%) followed by Mystus

gulio (11.96%), Gerres filamentosus (9.34%) and

Terapon jarbua (8.76%). Among the shrimps, Macro-

brachium villosimanus was found to be moderately

higher (7.98%) in species composition by weight.Only 12 species contribute about 90% of the total

catch. No significant difference was observed in

average catch per net between winter, premonsoon

and monsoon (F2,601.37, P00.32).

The catch per net at St2 ranged between 2.71 and

14.77 kg with an average of 7.5494.39 kg. The

average catch per net at St2 during winter, premon-

soon and monsoon seasons was found to be 3.949

1.94 kg, 10.6792.12 kg and 8.0195.86 kg, respec-

tively. The highest percentage (27.40%) was found

for the fish Mystus gulio followed by Acanthopagrus

latus (10.25%), Cynoglossus cynoglossus (8.12%) and

Macrobrachium villosimanus (7.47%). Only 13 spe-

cies including those reported above contributed

about 87.11% of the total catch (Table II). No

significant difference was observed in average catch

per net between winter, premonsoon and monsoon

(F2,602.42, P00.17).

However, a significant difference was obtained in

average catch per net between St1 and St2 (F1,160

14.74, P00.001), but no significant difference was

observed among the seasons (F2,1501.09, P00.35).

Species community composition by number

During the study period a total 18,467 fish, shrimp

and crab were collected from the Char jal with a

mean abundance of 10269803 ind/5 kg of species

(Table II). The maximum species abundance (2869

ind/5 kg of species) was observed during the winter

at St1 while the minimum (241 ind/5 kg of species)

was observed during the monsoon period at St2.

The species abundance per net in St1 ranged

between 412 ind/5 kg (during monsoon season) to

2869 ind/5 kg during the winter period, with an

average of 1444.009886.74 ind/5 kg. The average

abundance during winter, premonsoon and mon-

soon seasons were found to be 2304.669489.24 ind/

5 kg, 1585.669341.70 ind/5 kg and 441.66925.73

ind/5 kg, respectively. Ambassis dussumieri was found

highest (16.83%) followed by Metapenaeus lysianassa

(15.27%), Gerres filamentosus (12.45%) and Terapon

jarbua (11.10%) in species abundance. Only 18

species contributed about 97.25% of the total catch.A significant difference was observed in average

species abundance between winter, premonsoon

and monsoon (F2,6022.26, P00.002).

The species abundance per net in St2 ranged

between 241 ind/5 kg to 1601 ind/5 kg with an

average of 607.889477.25 ind/5 kg. The average

abundance during winter, premonsoon and mon-

soon seasons was found to be 1191.009357.31 ind/

5 kg, 368.66998.77 ind/5 kg and 264919.92 ind/

5 kg, respectively. The highest percentage (23.49%)

was found for Macrobrachium villosimanus followed

by Metapenaeus lysianassa (21.35%), Mystus gulio(9.16%) and Ambassis dussumieri (9.10%). Only 13

species contribute about 92.56% of the total abun-

dance (Table II). A significant difference was ob-

served between winter, premonsoon and monsoon

(F2,6016.83, P00.003). A significant difference in

average species abundance was also observed be-

tween stations (F1,16 06.42, P00.02) and seasons

(F2,1508.58, P00.003).

Species diversity

The ShannonWiener diversity index ranged be-

tween 0.95 (at St2 during monsoon) and 2.62 (at

St1 during premonsoon) with a mean diversity value

of 1.9190.46 (Figure 4A). No significant difference

was observed (F1,1600.915, P00.353) in the mean

ShannonWiener diversity values between the sta-

tions. However, this difference was found significant

between the seasons (F1,16014.264, PB0.001) with

higher mean diversity value (2.41690.16) during

the premonsoon period.

Table I. Mean of the different environmental parameters in different seasons and stations during the study period.

Season Sampling Temperature (8C) Salinity (ppt) Transparency (cm) pH DO(mg l(1

)

Winter St1W1 22.3391.15 29.6690.57 65.3394.04 7.2390.2082 4.1590.48

St2W1 23.3390.57 30.6690.57 58.6692.30 6.8690.5132 4.0990.42

Premonsoon St1P1 29.3391.15 25.6691.15 4092.00 7.3390.4726 4.8590.14

St2P1 3091.00 26.3390.57 35.6693.78 7.0390.05774 4.7390.34

Monsoon St1M1 28.6691.15 2.3390.57 29.3392.51 7.390.3606 3.8190.58

St2M1 29.3391.52 5.3390.57 2892.00 7.4390.2082 3.6590.49

Station 1 26.7793.49 19.22912.80 44.88916.22 7.2890.31 4.2790.60

Station 2 27.5593.32 20.77911.74 40.77914.03 7.1190.37 4.1690.59



8/18

The minimum Margalef richness value (1.14) was

observed at St1 during monsoon while the maximum

value (4.50) was found in station St2 during pre-

monsoon (Figure 4B) with a mean richness value of

2.5291.08. The mean species richness values at St1

and St2 was found to be 1.8590.49 and 3.2091.09,

respectively. In the case of seasons, the highest mean

richness value (3.3091.22) was observed in

Figure 3. Temporal and spatial variations in mean environmental parameters at the study area (s, St, st, station).



9/18

premonsoon while the lowest mean value (1.569

0.40) was observed during monsoon. Significant

differences were found for Margalefs index for

both stations (F1,16011.34, P00.004) and seasons

(F1,1606.757, P00.008).

Species assemblage

The analysis of similarity (ANOSIM) showed sig-

nificant difference in assemblage structure between

stations (Global R00.365; P00.008) (Table III).

Species assemblage at each station was found to be

Table II. Fish and shrimp species recorded in the Bakkhali river estuary from December 2007 to August 2008 showing relative contribution

(%) to the total abundance by stations and seasons.

Station Season

Species name Code Total ind. (%) St1 (%) St2 (%) Win (%) Premon (%) Mon (%)

Metapenaeus lysianassa (De Man, 1888) Mly 3152 17.07 15.27 21.35 27.57 4.45 0.00

Ambassis dussumieriCuvier, 1828 Adu 2685 14.54 16.83 9.10 20.64 8.89 0.00

Macrobrachium villosimanus (Tiwari, 1949) Mvi 2240 12.13 7.35 23.49 13.38 14.28 0.00

Terapon jarbua (Forsskal, 1775) Tjr 1687 9.14 11.10 4.46 9.83 6.00 14.36Gerres filamentosus Cuvier, 1829 Gft 1684 9.12 12.45 1.21 10.90 7.73 4.16

Liza tade (Forsskal, 1775) Ltd 1130 6.12 6.96 4.13 4.57 7.52 9.92

Mystus gulio (Hamilton, 1822) Mgl 861 4.66 2.77 9.16 0.10 3.10 31.55

Metapenaeus monoceros (Fabricius, 1798) Mmc 810 4.39 3.85 5.67 0.22 4.50 24.70

Butis butis (Hamilton, 1822) Bbt 769 4.16 5.56 0.84 0.10 12.74 0.57

Acanthopagrus latus (Houttuyn, 1782) Alt 432 2.34 2.35 2.30 2.43 1.89 3.12

Eleutheronema tetradactylum (Shaw, 1804) Etd 415 2.25 3.17 0.05 0.00 7.08 0.00

Glossogobius giuris (Hamilton, 1822) Ggr 396 2.14 1.82 2.91 1.43 2.80 3.87

Valamugil speigleri (Bleeker, 1858-59) Vsg 395 2.14 1.96 2.56 0.30 5.22 2.74

Cynoglossus cynoglossus (Hamilton, 1822) Ccg 194 1.05 0.00 3.55 0.56 0.73 4.35

Pseudapocryptes elongatus (Cuvier 1816) Plc 183 0.99 1.38 0.07 0.00 3.12 0.00

Penaeus semisulcatus de Haan, 1844 Pss 164 0.89 1.26 0.00 1.56 0.00 0.00

Macrobrachium rude (Heller, 1862) Mrd 154 0.83 1.15 0.09 1.42 0.09 0.00

Mugil cephalus Linnaeus, 1758 Mcp 150 0.81 1.02 0.31 1.43 0.00 0.00

Johnius belangerii(Cuvier, 1830) Jbg 147 0.80 0.00 2.69 0.00 2.51 0.00Exopalaemon stylifera (H. Milne-Edwards, 1840) Esf 130 0.70 1.00 0.00 1.24 0.00 0.00

Sardinella fimbriata (Valenciennes, 1847) Sfb 124 0.67 0.79 0.38 0.00 2.11 0.00

Escualosa thoracata (Valenciennes, 1847) Etc 99 0.54 0.76 0.00 0.00 1.69 0.00

Penaeus indicus H. Milne Edwards, 1837 Pic 65 0.35 0.50 0.00 0.00 1.11 0.00

Metapenaeus brevicornis (H. Milne Edwards, 1837) Mbc 48 0.26 0.00 0.88 0.46 0.00 0.00

Portunus pelagicus (Linnaeus, 1758) Ppg 47 0.25 0.00 0.86 0.32 0.22 0.00

Scylla spp. Scy 37 0.20 0.00 0.68 0.13 0.39 0.00

Sillago sihama (Forsskal, 1775) Ssh 36 0.19 0.22 0.15 0.34 0.00 0.00

Platycephalus indicus (Linnaeus, 1758) Pid 31 0.17 0.09 0.35 0.09 0.38 0.00

Setipinna taty (Valenciennes, 1848) Stt 31 0 .17 0.06 0.42 0.00 0.39 0.38

Apocryptes bato (Hamilton, 1822) Abt 24 0 .13 0.09 0.22 0.11 0.20 0.00

Scatophagus argus (Linnaeus, 1766) Sag 23 0.12 0.10 0.18 0.22 0.00 0.00

Cynoglossus lingua Hamilton, 1822 Clg 22 0.12 0.00 0.40 0.15 0.10 0.00

Ilisha megaloptera (Swainson, 1839) Imp 21 0.11 0.10 0.15 0.20 0.00 0.00

Penaeus japonicus Bate, 1888 Pjp 18 0.10 0.00 0.33 0.00 0.31 0.00Eleotris fusca (Forster, 1801) Efc 15 0.08 0.00 0.27 0.14 0.00 0.00

Penaeus monodon Fabricius, 1798 Pmd 11 0 .06 0.00 0.20 0.06 0.07 0.05

Lutjanus johnii (Bloch, 1792) Ljn 10 0.05 0.00 0.18 0.00 0.17 0.00

Siganus javus (Linnaeus, 1766) Sjv 8 0.04 0.03 0.07 0.08 0.00 0.00

Nuchequula blochii(Valenciennes, 1835) Lbc 6 0.03 0.00 0.11 0.00 0.10 0.00

Coilia dussumieri Valenciennes, 1848 Cdm 5 0.03 0.00 0.09 0.00 0.09 0.00

Macrobrachium rosenbergii(De Man, 1879) Mrb 3 0.02 0.00 0.05 0.02 0.00 0.05

Labeo rohita (Hamilton, 1822) Lrh 2 0.01 0.00 0.04 0.00 0.00 0.09

Plotosus canius Hamilton, 1822 Pcn 1 0.01 0.00 0.02 0.00 0.02 0.00

Oreochromis niloticus niloticus (Linnaeus, 1758) Tnt 1 0.01 0.00 0.02 0.00 0.00 0.05

Hyporhamphus limbatus (Valenciennes, 1847) Hlb 1 0.01 0.00 0.02 0.00 0.00 0.05

Total number of taxa 45 28 4 1 30 32 16

Total number of individuals 18,467 12,996 5471 10,487 5863 2117

Mean 1026 1444 607.89 1747.83 977.17 352.83

Stdev 803 866.75 477.25 421.99 207.77 22.81Shannon 2.01 1.81 1.69 2.42 1.62

Stdev 0.33 0.56 0.24 0.13 0.23

Margalef 2.17 3.84 2.71 3.30 1.56

Stdev 0.13 0.66 0.24 1.23 0.40



10/18

highly diverse and at St1 Liza tade (15.24%), Terapon

jarbua (15.10%), Gerres filamentosus (13.45%) and

Acanthopagrus latus (10.86%) were found to be the

most dominant (10%) species, while at St 2

Terapon jarbua (12.48%), Cynoglossus cynoglossus

(11.57%) and Mystus gulio (11.16%) were found to

be the dominant (10%) species (Table IV). Ac-

cording to SIMPER results, other contributory

species to the assemblage structure of the studied

area were Glossogobius giurus, Metapenaeus monoceros,

Metapenaeus lysianassa, Valamugil speigleri, Ambassis

dussumieri and Butis butis (Table IV).

Figure 4. Temporal and spatial variations of (A) ShannonWiener index and Abundance and (B) Margalef diversity index and Abundance

of the Bakkhali fish and shrimp assemblage. St, station.

Table III. Result of one-way ANOSIM (R value and significant levels) and SIMPER analysis of fish and shrimp abundance between stations

and different seasons.

ANOSIM SIMPER

Groups R P

Average

dissimilarity (%)

Most discriminating

species

Contribution

(%)

St1 vs. St2 0.036 0.08 40.46 Metapenaeus lysianassa 10.23

Analysis of similarity results among different seasons

Global R00.726 P00.001

Winter vs. Premonsoon 0.491 0.04 34.70 Butis butis 10.62

Winter vs. Monsoon 1 0.02 48.29 Metapenaeus lysianassa 18.36

Premonsoon vs. Monsoon 0.594 0.02 39.95 Macrobrachium villosimanus 11.42



11/18

A highly diverse species assemblage was alsoobserved among all seasons through SIMPER ana-

lysis (Table III). Significant difference were observed

for temporal community structure of the studied

area (Global R00.726; P00.001) with a clear

separation of different seasons. The pair-wise com-

parison of seasons also showed distinct separation

(Table V). Metapenaeus lysianassa (17.19%) and

Ambassis dussumieri (13.43%) were found to be the

most contributory species during winter, while

during the premonsoon season it was Velamugil

spaglari (11.23%) and Mystus gulio (10.38%). On

the other hand, Mystas gulio (21.21%) and Terapon

jarbua (17.12%) were found the most contributory

species during the monsoon season (Table V).

At the similarity level of 65%, no marked separa-

tion, either for the stations or for the seasons, was

observed by cluster analysis. Two clusters were

identified the first consists of St2 during monsoon

and premonsoon period along with St1 during

monsoon, and the second group consists of St1

during premonsoon and winter along with St2

during winter (Figure 5).

Canonical correspondence analysisCCA eigenvalues of the first four axes were 0.36

(CCA1), 0.34 (CCA2), 0.11 (CCA3), and 0.10

(CCA4). Speciesenvironment Pearson correlation

coefficients for the first four axes were 0.96, 0.92,

0.82, and 0.79, respectively. The cumulative percen-

tage variance of species for the first four axes (CCA

14) was 49.18. The first and second axes modelled

19.4 and 18.1% of species data, respectively. There-

fore, the results obtained from the first two axes were

plotted (Figure 6). However, the vector length of a

given variable indicates the importance of that

variable in CCA analysis. Salinity (0.92), which hasthe longest vector along the first axis, was signifi-

cantly correlated with premonsoon at St1 (Figure 6).

Furthermore, transparency was also significantly

associated (0.81) with winter season of St1

and St2. As shown in CCA ordination (Figure 6),

high values of salinity concentration are the most

significant water parameters for Butis butis and

Eleutheronema tetradactylum. High values of water

transparency was associated with occurrence of

Ambassis dussumieri and Gerres filamentosus. High

values of pH are associated with the occurrence of

Valamugil speigleri, Glossogobius giurus and Metape-

naeus monoceros. However, Acanthopagrus latus

showed the highest association with dissolved oxygen

(DO), while no species was found to be closely

associated with temperature.

Table IV. Average similarity and discriminating fish and shrimp in

each station using SIMPER analysis.

Average similarity (%)

Stati on 1 (6 9.63%) Station 2 (69 .53 %)

Contributory species Contributory species

Species (%) Species (%)

Liza tade 15.24 Terapon jarbua 12.48

Terapon jarbua 15.10 Cynoglossus cynoglossus 11.57

Gerres filamentosus 13.45 Mystus gulio 11.16

Acanthopagrus latus 10.86 Liza tade 9.66

Glossogobius giurus 9.76 Glossogobius giurus 9.36

Valamugil speigleri 8.17 Metapenaeus monoceros 8.58

Metapenaeus lysianassa 5.69 Acanthopagrus latus 7.67

Mystus gulio 5.61 Valamugil speigleri 6.86

Ambassis dussumieri 5.47 Gerres filamentosus 5.65

Macrobrachium 4.26 Butis butis 5.59

villosimanus Metapenaeus lysianassa 4.27

Table V. Average similarity and discriminating fish and shrimp in each season using SIMPER analysis.

Average similarity (%)

Winter (79.07%) Premonsoon (72.65%) Monsoon (78.87%)

Contributory species Contributory species Contributory species

Species (%) Species (%) Species (%)

Metapenaeus lysianassa 17.19 Valamugil speigleri 11.23 Mystus gulio 21.21

Ambassis dussumieri 13.43 Mystus gulio 10.38 Terapon jarbua 17.12

Liza tade 10.44 Terapon jarbua 9.76 Metapenaeus monoceros 13.68

Terapon jarbua 10.20 Liza tade 9.52 Liza tade 11.49

Macrobrachium villosimanus 9.68 Butis butis 8.5 Glossogobius giurus 9.65

Glossogobius giurus 9.49 Acanthopagrus latus 7.61 Valamugil speigleri 8.01

Acanthopagrus latus 9.32 Glossogobius giurus 7.21 Acanthopagrus latus 7.71

Gerres filamentosus 8.90 Metapenaeus lysianassa 7.16 Gerres filamentosus 7.16

Metapenaeus monoceros 4.33 Ambassis dussumieri 7.07

Macrobrachium villosimanus 6.69

Gerres filamentosus 5.97



12/18

Discussion

Environmental parameters

During the present study, no significant spatial

variation was observed for temperature, salinity,

dissolved oxygen and pH. This is most probably

due to the presence of a rubber dam upstream of thisriver, which prevents freshwater influx in this area

during winter and premonsoon and ultimately the

whole area, i.e. both St1 and St2, is similarly

governed by the brackish water entering through

tidal influence. In the same way, during monsoon,

the rubber dam remains open and huge amount of

freshwater is discharged through the dam. As a

consequence, the water parameters in both stations

remain the same. However, a significant difference

in water transparency was observed between St1 and

St2, where lower transparency was observed in St2

compared to St1. This may be due to the higher

water turbulence downstream (St2) due to strong

tidal fluctuation and anthropogenic causes such as

the presence of the fish harbour, jetty, etc., in the

mouth of the river (St2). Comparatively lower

dissolved oxygen was observed at both stations.

Hena & Khan (2009) also reported a lower level of

DO in the same estuary (1.895.37 mg/l). This maybe due to the nearby domestic, agricultural and

industrial waste water discharges which affect the

water and sediment quality and lead to a hypoxic

condition, as stated by Van Eck et al. (1991).

Fluctuations in water transparency influence the

primary productivity which ultimately affects the fish

distribution (Arthington & Welcome 1995; McAllis-

ter et al. 2001). Rozengurt & Hedgepeth (1989)

reported changes in natural recruitment and species

abundance in the Caspian Sea due to an increase in

salinity. McAllister et al. (2001) also reported changes

in species abundance due to a salinity increase.According to Maes et al. (2004), dissolved oxygen is

one of the most important factors for fish abundance

and distribution. Fish communities are highly af-

fected by temperature within estuaries (Cyrus &

Mclean 1996). A sudden increase or decrease in

water temperature may cause fish mortality (Blaber

2000). Environmental parameters such as tempera-

ture, salinity, dissolved oxygen, water transparency

and pH play an important role for species abundance

and diversity (Whitfield 1999), especially for the

tropical regions where the fluctuation of these para-

meters are frequently due to seasonal changes (Blaber

2000). The Bakkhali river estuary is no exception.

Significant temporal differences were observed for

temperature, salinity, water transparency and dis-

solved oxygen during the present study which may

almost certainly affect the assemblage structure.

Species composition

The average catch per net at St2 (7.5494.39 kg)

was found to be significantly higher compared to St1

(1.8990.36 kg). St2 is located in the river mouth

and possesses a highly diverse type of larger-sized

fish compared to St1. On the other hand, at St1 asmall area of salt marsh is situated nearby, which acts

as the nursery ground for small and juvenile fishes.

As fish grew, their daily food requirement also

increased and fish began to migrate to the river

mouth/estuary in the search of food (Davies & Day

1986). Hence, larger fish were caught and the total

weight of the catch per net was higher at St2.

A total of 45 fish and shrimp species were recorded

during the study. Among them are Metape-

naeus lysianassa, Ambassis dussumieri, Macrobrachium

Figure 5. Dendrogram (A) showing cluster based on BrayCurtis

similarity matrix of catch composition, and the ordination in 2D

(B) using MDS on the same similarity matrix. St, station; W,

winter; p, pre-monsoon; m, monsoon.



13/18

villosimanus, Terapon jarbua, Gerres filamentosus, Liza

tade, Mystus gulio, Metapenaeus monoceros, Butis butis,

Acanthopagrus latus, Eleutheronema tetradactylum,

Glossogobius giur us, Valamugil speigleri, and Cynoglos-

sus cynoglossus, each contributing more than 1% of

the composition. Islam et al. (1992) reported about

185 species from the coastal waters of Bangladesh

collected from the estuarine set bagnet. On the other

hand Hossain et al. (2007) reported about 161species collected by different types of net from Naaf

river estuary located around 50 km from the present

study site. A smaller number of species observed in

the present investigation is most probably due to the

use of only one type of net, i.e. Char jal which catches

only species living very near to the shore and closer to

the bottom. Another reason is the controlled envir-

onment of the Bakkhali estuary by the rubber dam

limiting the species abundance. Also, the dam-

induced changes in water characteristics may have

profound effects on species numbers in the river

(McAllister et al. 2001).During the study period, only one exclusively

freshwater fish (Labeo rohita) was collected. The

rubber dam, constructed at the upper stream of

the river, creates an unusual environment. During

the monsoon season huge amounts of freshwater flow

into the river and create a freshwater-influenced

estuarine environment where salinity ranges were

found to vary from 2.3390.57 to 5.3390.57. This

supports the findings of the presence of a freshwater

species in the investigated area. Freshwater fishes are

usually incapable of osmoregulating in saltwater and

consequently tend to be found in estuaries only when

salinities decline to very low levels during periods of

heavy freshwater discharge (Potter & Hyndes 1999).

Species abundance

The species abundance found in the Bakkhali river

estuary is composed of small numbers of specieswith high contribution and a large number of species

whose contributions are very negligible, a common

feature of estuarine faunal populations (Gaughan

et al. 1990; Harrison & Whitfield 1990; Drake &

Arias 1991; Harris & Cyrus 1995; Whitfield 1999).

The number of taxa in this study (45 species) was

found to be lower than in the Naaf river estuary,

another estuary close to the study area (Hossain

et al. 2007). However, these types of judgements

must be based on differences in sampling gear,

sampling period and most importantly habitat char-

acteristics. Moreover, each estuarine system mayhave a different abiotic environment (Blaber 1997),

resulting from the tidal range, freshwater input,

geomorphology and human pressure (Dyer 1997;

McLusky & Elliott 2004) which also affects the

species abundance. So a difference in species abun-

dance is not likely to be the exception.

A remarkably lower number of species was

observed in the upstream area (St1) of the Bakkhali

river estuary, but the diversity index H? was much

higher. Out of 45 total species, 17 were relatively

Figure 6. The CCA ordination of species abundance and environmental parameters (code for each species is given in Table II; St, station;

p, premonsoon; m, monsoon; w, winter).



14/18

common in the upper stream which is characterized

by juveniles of large-sized species and adults of

small-sized species. The main reason for the high

abundance of juvenile fishes is the presence of a salt

marsh and seagrass bed at St1. These observations

agree with the comments of Hena et al. (2007)

seagrass and salt marsh habitats are among the most

productive ecosystems in the world in terms of the

quantity of vegetation production closely linked tothe high production rates of associated fisheries.

The lower stream (St2) has a wider zone and is

characterized by strong tidal influence and man-

groves. As a consequence, relatively higher num-

bers of species with brackish to marine origin were

captured in this zone. Downstream, water trans-

parency showed higher values (28.0065.33 cm)

when compared to other estuaries of Bangladesh

(Mahmood 1986). This is because freshwater flows

are scarce here for a major part of the year leading

to the presence of marine species which have been

described as being related to clearer waters (Blaberet al. 1997). The exception occurred during

monsoon months (June, July) when the rubber

dam is opened causing a flushing effect by fresh-

water flows from the upper, hilly areas.

An estuarine water body along with mangrove

plants is the most productive region for zooplankton

especially for shrimps and prawns (Hena & Khan

2009). The Bakkhali river estuary is influenced by

mangrove plants including Avicennia alba, A. mar-

ina, and Acanthus ilicifolius, which have created a

huge potential habitat for phytoplankton, zooplank-

ton, shellfish and fish larvae. Plankton communities

living in mangrove waters are well adapted to the

water motion (Alongi 2009). Whether a source of

food, shade, or refuge, mangrove forests are an

important habitat for coastal organisms that either

float or swim on the ebb and flood of the tide

(Alongi 2009). Following this pattern, the Bakkhali

river estuary also supports a huge abundance of

shellfish of both marine and brackish origin.

Although mangroves support fisheries by playing a

significant role as nursery ground for shrimps

including the giant tiger shrimp (Penaeus monodon)

which is the major species of the industrial bottom

trawl fishery of Bangladesh (Islam & Haque 2004),the present study did not show the same result for

the penaeid group where P. monodon, P. indicus, P.

semisulcatus and P. japonicus were found to be 0.06,

0.35, 0.89 and 0.10%, respectively. The same was

encountered for Macrobrachium rosenbergii (0.02%),

although it is the major species found in different

estuarine studies of Bangladesh. On the other hand,

Metapenaeus lysianassa (bird shrimp), Macrobra-

chium villosimanus and Metapenaeus monoceros con-

tributed 17.07, 12.13 and 4.39% of total catch,

respectively. This may be due to the use of Char jal,

a different gear compared with the previous study,

and a higher average salinity gradient for most of the

sampling period.

Species diversity

Seasonal variation in species diversity is a very

common phenomenon in tropical estuaries and the

estuary studied here is not different. However, no

significant difference for the diversity values (both

ShannonWiener and Margalef) between the sta-

tions indicated that the ecosystem for both stations

was unique. However, the diversity values were

lower than the value reported by Hossain et al.

(2007) where the Shanon value was found to be 2.6.

This difference may be due to the use of multigear

fishing materials in the Naaf river estuary, the

controlled environment of the Bakkhali estuary

which limits the species interaction from the up-

stream communities and scarcity of marine species

which normally come to the estuary for breeding.

Although several studies have reported the

dominance of the resident species in the estuaries

(Thompson 1966; Hotos & Vlahos 1998), in the

case of the Bakkhali estuary no species was found

to be dominant. Rather than a single species, five

to six species were found to be dominant at

different stations and different seasons. Blaber

(2000) also stated that the estuarine resident

species are a relatively insignificant proportion of

the fish fauna available in an estuary and are

generally all relatively small-sized fish.

Metapenaeus lysianassa was found most abundant

overall (17.07%). However, the species showed its

maximum abundance during the winter season

(27.57%) and at St2 (21.35%). As M. lysianassa is

a marine-dominant species, it only appears during

the winter season at St2 where the salinity is very

high. Ambassis dussumierialso showed the same trend

except of higher abundance (16.83%) at St1. This is

most probably due to the presence of salt marshes at

St1. A similar trend was observed for Macrobrachium

villosimanus except for the fact that they were

dominant during premonsoon (14.28%) and winter

(13.38%). As this species is a brackish species, it

shows a higher abundance during premonsoon and

winter seasons. Mystus gulio and M. monoceros were

more abundant during the monsoon season at St2,

while M. monoceros is a brackish to marine habitat

species. However, their higher abundance at St2

during the monsoon period may be due to spawning.

The same is also probably true for freshwater to

brackish-water species such as Mystus gulio.



15/18

Species assemblage

Regarding spatial and temporal fish and shrimp

assemblage structure, two major groups were indi-

cated by cluster analysis in the Bakkhali river

estuary. Group 1 comprises the sample of the

monsoon season from St1 and St2 along with the

premonsoon season from St2 within a 65% similarity

level. The capture of abundant large-sized species in

the premonsoon and monsoon seasons and the

absence of the major contributing species like

Metapaeneus lysianassa, M. villosimanus and Ambassis

dussumieri substantiates this. The sample taken from

the lower stream during the premonsoon season at

St2 included large numbers of adult fish species.

The abundant presence of A. dussumieri, Terapon

jarbua, Gerres filamentosus, Liza tade and Butis butis

also supports this grouping.

Group 2 comprises the samples from St1 and St2

during the winter season and samples from St1

during the premonsoon period. In general, samples

from St1 and St2 during winter showed the sametrend on the basis of catchability.

Seasonality is the most important feature among

different studied parameters affecting the fish and

shrimp assemblage, similar to results from other

estuaries (Whitfield 1989; Loneragan & Potter 1990;

Drake & Arias 1991; Barletta-Bergan et al. 2002;

Young & Potter 2003). In general, differences

between seasons were observed to be more pro-

nounced in this study. According to Lam (1983), the

seasonal water variability in the spawning area has an

important influence on the spawning activity, and on

nursery areas (McErlean et al. 1973).

Canonical correspondence analysis

In CCA, species plotted closer to the vector, have

stronger relationships with them. Species located

near the origin either do not show a strong relation-

ship to any of the variables or are found at average

values of environmental variables (Marshall &

Elliott, 1998). In this study, salinity, water transpar-

ency and pH were found to be three signifi-

cant variables affecting species composition in the

studied estuary. However, temperature and DO

were not found to be significant. Half of the speciesin the estuary had average values in relation to

environmental variables. Only two species, Eleuther-

onema tetradactylum and B. butis, indicated a strong

response to the longitudinal salinity gradient. The

five variables measured in this study explained

species distributions well compared with most other

estuarine studies where salinity and transparency

were found to be the most influential factors for fish

distribution patterns. For example, Marshall &

Elliott (1998) found that five environmental

variables accounted for 18.4% of the total species

variation even though they included bottom, mid and

surface values of each variable in CCA. Rakocinski et

al. (1996) used 11 environmental variables that

together explained only 21.9% of the total species

variations in CCA. On the other hand, Martino &

Able (2003) explained 29.9% of the total species

variation in Mullica River Estuary, New Jersey, using

five environmental variables that included salinityand geographic distance. However, during this study,

environmental variables accounted for 49.18% of the

total species variation. These results are also in

agreement with Akin et al. (2005) for the Koycegiz

Lagoon Estuary, Turkey.

Conclusion

In the Bakkhali river estuary, environmental influ-

ence was apparently more extensive during pre-

monsoon and winter, when water transparency and

salinity fluctuation leads to an increase in diversity.

In this study, seasonality of the environmental

conditions explained the major variations of the

fish and shrimp assemblage. Seasonal variations

occurred not only in total abundance and diversity,

but also in the structure of the species assemblage

of the Bakkhali river estuary. Besides seasonal

variations, the assemblage also exhibited a defined

spatial pattern. The migrating marine species

Metapenaeus lysianassa was more abundant in the

shallow salt marsh zones, while estuarine residents

showed more or less equal distribution throughout

the seasons except for Mystus gulio. The presence of

this species seems to be more related to thespawning season during monsoons. In the Bakkhali

river estuary, the four common fish species Ambassis

dussumieri, Terapon jarbua, Gerres filamentosus, Liza

tade were present for most of the sampling time,

which is possibly due to their higher salinity

tolerance. Favourable environmental conditions,

mainly salinity, enable these fish species to spawn.

Therefore, it can be said that these species use the

Bakkhali estuary as a spawning ground.

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Akin S, Buhan E, Winemiller KO, Yilmaz H. 2005. Fish

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