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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 Parameters

Md. Rashed-Un-Nabi
Institute of Marine Sciences and Fisheries, University of Chittagong, Bangladesh,

Md. Abdulla Al-Mamun
Institute of Marine Sciences and Fisheries, University of Chittagong, Bangladesh,

Md. Hadayet Ullah
1Institute of Marine Sciences and Fisheries, University of Chittagong, Bangladesh,

M. Golam Mustafa
The World Fish Center, Bangladeshand South Asia Office, Dhaka, Bangladesh

Abstract/ Executive Summary:

Fish and shrimp species, together with water quality data, were collected from two different stations located inside the Bakkhali river estuary of Bangladesh during winter, premonsoon and monsoon periods. Significant temporal differences were observed for water temperature, salinity and dissolved oxygen. The average catch of fish and shrimps per net between stations 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 in the 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 abundances were 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 Shannon Wiener 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 fish and shrimp distribution in the Bakkhali river estuary.

Keywords: Ambassis dussumieri, Analysis of similarity, Bakkhali river estuary, Metapenaeus lysianassa, Species assemblage

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End Date:

Program Area: Conservation and Biodiversity

Commodity: Fish, Shrimp

Objectives:

The present study has been designed to provide anextensive report on the fish and shrimp assemblage structure of Bakkhali river estuary in relation to water quality parameters.

Methodology:

Study area The Bakkhali river estuary is located at the south-eastern coast of the Bay of Bengal in Bangladesh. A number of small streams originating from the south-eastern hills of Mizoram (India) meet at the Naikhongchhari of Bandarban districtand form the river Bakkhali. It flows through Naikhongchhari and Ramu of Cox’s Bazar district and falls into the Moheshkhali channel of the Bay of Bengal. This river is relatively wide compared to other rivers of the Cox’s 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 Lin2001). Salt intrusion extends up to 6 km upstream where a rubber dam was constructed for irrigation purposes. 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.), naturalullo grassImperata cylindrica, cord grass Spartinasp.and sea grass Halophila beccariiare present (Henaet al. 2007). The lower part of this estuary is heavily influenced by anthropogenic and industrial activities including 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 producing toxic substances, which ultimately affect the biodiversity. Sampling took place at two stations, 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 local fishermen 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’. In Cox’s Bazar region, Char jal are used to catch various aquatic species from river banks inundated during high tide. Net fencing is made from bamboo poles which are submerged during high tide. Samples 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 chosen the winter (December, January and February), pre-monsoon (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 November) 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 & Whitehead1974; 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 refractrometer (NewS-100, TANAKA, Japan), a pen pH meter (s327535, HANNA Instruments), a thermometer in centigrade and a DO meter (HI9142, HANNA Instruments), respectively. A Secchidisc (20 cm diameter) was used to measure the water transparency. Diversity of the species assemblage was expressed by the Shannon Wiener index (H) (Shannon 1949;Shannon & Weaver 1963; Ramos et al. 2006) using formula.

Source of Information: Marine Biology Research, 2011; 7: 436452

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Achievement/Findings:

In the Bakkhali river estuary, environmental influence 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 defineds patial 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 through out the seasons except for Mystus gulio. The presence of this species seems to be more related to the spawning season during monsoons. In the Bakkhali river estuary, the four common fish species Ambassisdussumieri, Terapon jarbua, Gerres filamentosus, Lizatade were present for most of the sampling time,which is possibly due to their higher salinity tolerance. Favorable 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.

Knowledge Management: Journal