Environmental Factors Structuring the Fish Assemblage Distribution and Production Potential in Vembanad Estuarine System, India  

C. V. Asha , R. I. Cleetus , P. S. Suson , S. Bijoy Nandan
Department of Marine Biology, Microbiology & Biochemistry, School of Marine Sciences, Cochin University of Science and Technology, Fine Arts Avenue, Kochi-682016, Kerala, India
Author    Correspondence author
International Journal of Marine Science, 2015, Vol. 5, No. 23   doi: 10.5376/ijms.2015.05.0023
Received: 03 Jan., 2015    Accepted: 10 Mar., 2015    Published: 30 Mar., 2015
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Asha et al., 2015, Environmental factors structuring the fish assemblage distribution and production potential in Vembanad estuarine system, India, International Journal of Marine Science, Vol.5, No.23 1-13 (doi: 10.5376/ijms.2015.05.0023)


Vembanad wetland system, a Ramsar site on the West coast of India is the largest estuarine system, renowned for its rich fishery diversity and endemicity. In 1976, to regulate salinity intrusion into the Vembanad estuarine system, the Thannermukkom barrage was constructed across the estuary dividing it into fresh water dominated southern and a northern region dominated with brackish water, which has grossly altered the eco-biology of the region. In this context, the influence of environmental factors on fish assemblage structure has been poorly recognised in Vembanad estuarine system mainly for effectively implementing proper management and conservation measures. Information on fish assemblage in relation to environmental parameters collected from February 2012 to January 2013 is reported in this contribution. Estimates of annual average fishery production indicated a catch of 4774.46 t in the estuary, in which 10.1% and 89.9% was contributed by southern and northern zones respectively. Seventy four species of finfishes, eleven species of shell fishes were identified for the study period.Multidimensional scaling and canonical correspondence analysis significantly demarcated the spatio – temporal variation of fish species distribution and observed a strong correspondence between fish assemblage and environmental variables. Depth, temperature, pH and salinity were the most important parameters explaining the variation in fish assemblage composition and abundance in Vembanad estuarine system. The aging Thaneermukkom barrage and its faulty operation reduced the tidal flushing altering the circulation and mixing process, especially in the southern part of estuary. This influenced the hydrological characteristics leading to a decline in estuarine fishery and its production potential.

Environmental factors; Fish assemblage; Ramsar site; Vembanad wetland system

In an aquatic environment, understanding the dynamics of fish population and species distribution in response to environmental factors are essential. Variation in the abundance and composition of fish species in an ecosystem provide the basic idea for implementing the ecosystem – based fisheries management. For the evaluation of environmental variables on organisms, fishes are important because they participate in multiple trophic levels in aquatic community and are having long life span and are easily sampled. Estimation of fish assemblage structure has high value in estuarine quality assessment. Species assemblage is an important ecological unit that interact trophically and respond to environmental and habitat conditions (Craig and Bosman, 2013). Fish communities of coastal estuarine environment possess a mixture of euryhaline species adapted to brackish environments, along with true marine and fresh water species. Therefore, the diversity of fish species in estuaries are very high and are composed of marine, estuarine, freshwater and migrating species (Henderson, 1988; Lobry et al., 2003). Biological condition of aquatic environment is important because fishes react with external aquatic environmental parameters. It leads to shift in the assemblage composition of fish species. Fish assemblages are recognized as sensitive indicators of habitat degradation, environmental contamination and overall ecosystem productivity. The fish assemblages in the estuarine systems are typically dynamic, reflecting the changing environmental conditions in which they are exposed (Tremain and Adams, 1995; Able and Fahay, 1998; Idelberger and Greenwood, 2005). Our attempt was to gather all information on the distribution of fish species to provide a spatial context for understanding the relationship of environmental factors on fish assemblage in Vembanad estuarine system.

The backwaters in Kerala form a habitat for over 200 resident and migratory fish and shellfish species and fishing activities in these water bodies provide the livelihood to about 2,00,000 fishers and provide full time employment to over 50,000 fishermen (Bijoy Nandan, 2008). There is limited information on the fishery catch and its production trends from the coastal backwaters of the west coast of India. But, scattered information is available on the fishery composition, abundance and catch from the Vembanad backwater (Shetty, 1965; Kurup., 1982; Kurup et al., 1993; Bijoy Nandan, 2008; Harikrishnan et al., 2011; Bijoy Nandan et al., 2012; Asha et al., 2014). Jayachandran et al., (2013) observed the influence of environmental factors on fish assemblage in Kodungaloor – Azhikode estuary, south west coast of India. The study on physical habitat characteristics structuring the fish assemblage in the Vembanad estuary is also limited. In view of this, the study is significantly aimed to observe the environmental factors structuring the fish assemblage of an estuarine system on the west coast of India.
1 Materials and Methods
1.1 Study area
Vembanad estuarine system is one of the largest estuaries on the south west coast of India and an important Ramsar site. It is bordered by Alappuzha, Kottayam and Ernakulam districts of Kerala covering an area of about 200 sq. km and extending 80 km in a NW-SE direction from Munambam in the north to Alleppuzha in the south (09˚00’ -10˚40’N and 76˚00’-77˚30’E). The width of the estuary varies from 500m to 4km and the depth from <1m to 9m. The backwater has two permanent openings into the Arabian sea – one at Cochin and the other at Azhikode. Manimala, Meenachil, Pamba and Achenkovil rivers flows into the estuary south of Thanneermukkom and Muvattupuzha river flows into north of Thanneermukkom barrage. The Vembanad estuarine system has a freshwater dominant southern zone and a salt water dominant northern zone, both separated by the Thanneermukkom barrage (1,400 metres) where the estuary has its minimum width. The barrage, a bridge cum - regulator was constructed in 1976, to prevent salt water intrusion and to promote double cropping of paddy in about 55,000 ha. of low lying fields in the area (Padasekharams). The barrage remains closed during summer months (January – May) and with no proper flushing in the upper reaches of the estuary. The faulty condition of the barrage and the reclaimed areas that is filled with red earth forming road along the Thaneermukkom barrage permanently prevented the connectivity of the south and northern parts of the estuary. The unscientific operation of the sluice gates of the barrage, their rusted condition failed to safeguard the fishery as well as agricultural activity that have also blocked the connectivity of the wetland to the Lakshadweep Sea (Arabian Sea) for a major part of the year. This has severely affected the ecological characteristics of the estuarine ecosystem and restrained the seasonal intermixing of fresh and saline water and thereby interfering with the natural cleansing mechanism of the estuary, threatening accelerated loss of habitats and biodiversity (Anon, 2007). Construction of the new shutters have been initiated in the 470m long reclaimed portion in the middle part of the barrage (Anon, 2014).
1.2 Data collection
Ten sampling stations were selected in the estuary, (90 30’ 069’’N & 760 21’ 268’’ E - 90 53’ 519’’N & 760 18’ 139’’E) for sampling and analysis of various parameters (Figure 1). Catch composition, diversity of fish species and environmental parameters were collected on a monthly basis from February 2012 to January 2013 period. Observations were made seasonally viz., pre monsoon (March – May), monsoon (June – September) and post monsoon (October –January) periods. Landing centre based direct data collection method was adopted for the fish landing estimation and the major landing centres; Alappuzha, Kumarakam, Muhamma, Thaneermukkom, Vaikkom, Chembu, South Paravoor, Aroor, Arookutty and Thevara were spread around the backwater system (FAO, 2002; FAO. 2003; Sparre and Venema, 1992). Gill net was widely used in Vembanad estuary along with stake net, seines, Chinese dip net, cast net and hook and line. Gill net size varied from 30 - 150m and mesh size in the range 22 – 150mm in different designs. Finfishes from the gill net were used for the fish catch composition, fish diversity and assemblage study.Catch per unit effort (CPUE) is defined as one operation of the net which occurred once per site or the number of fish collected per operation. CPUE was expressed as No. 100 m-net hr-1 and used as index of relative abundance (FAO, 2002; Sparre and Venema, 1992). Total catch was collected and sorted into finfish and shell fish. After sorting and counting, representative finfish samples were preserved in 10% formalin for taxonomic studies. Species level identification was madewith help of standard references (Talwar and Jhingran, 1991; Bijoy Nandan, 2012; Fish Base (www.fishbase.org).

Figure 1 Location of the sampling sites in the Vembanad estuary (Source: IRS.IA LISSLANSAT IMAGE 1990)

Water temperature was measured at the time of sampling using mercury thermometer; pH with
Systronics pH meter (No. 317; accuracy ± 0.01); transparency using Secchi Disc (Strickland and Parsons, 1972); dissolved oxygen by modified Winklers method (Strickland and Parsons, 1972) and salinity by Systronics water analyser (No. 317; accuracy ± 0.01) calibrated with standard seawater.
1.3 Data Analysis
Two-way analysis of variance (ANOVA) was used for hydrological parameters (temperature, pH, transparency, dissolve oxygen and salinity) to calculate any existence of difference among the stations, months and seasons. Species diversity, richness and evenness index of fin fishes were calculated for each gill net sample based on species numerical abundance. Diversity indices such as, Shannon-Wiener diversity (H’), Margalef’s richness index (d) and Pielou’s evenness index (J’) were computed using PRIMER Vs. 6.0 (Clarke and Gorley, 2006). Multidimensional scaling (MDS) was used to graphically display the two-dimensional ordination plots showing differences of organization of the fish abundance. A stress value of <0.2 gives a useful representation of results (Clarke and Warwick, 2001). Species numeric abundance in relation to environmental variables was analysed using the canonical correspondence analysis (CCA). It is an example of direct gradient analysis, where the gradient in environmental variables is known a prior and the species abundances (or presence/absences) are considered to be a response to this gradient. This ordination method was used to detect patterns of species association directly related to environmental variables. A perpendicular line is traced between the species and environmental vector, that represents the relation between the species and the factor (Ter Braak and Verdonschot, 1995).
2 Results
2.1 Physico -Chemical characteristics
The depth of the estuary ranged between1.4 to 8.5m with an average of 3.93 ± 1.93m. Seasonal average values of major physico-chemical parameters are given in Table 1. The lowest temperature of 28ºC was observed in Station 1, Station 2, Station3 and Station 4 and maximum of 33ºC in Station 9 and Station 10. The pre monsoon season (31.03 ± 1.14 ºC) showed the maximum temperature as compared to post monsoon (31.4 ± 0.68 ºC) and monsoon (30.25 ± 1 ºC). The average minimum temperature of 29.
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