Influences of Environmental Factors on Fish Assemblage in the Tropical Estuary of South West Coast of India, A Case Study of Kodungallur-Azhikode Estuary  

P.R. Jayachandran , S. Bijoy Nandan , O.K. Sreedevi , V.F. Sanu
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, 2013, Vol. 3, No. 2   doi: 10.5376/ijms.2013.03.0002
Received: 20 Nov., 2012    Accepted: 21 Dec., 2012    Published: 26 Dec., 2012
© 2013 BioPublisher Publishing Platform
Preferred citation for this article:

Jayachandran et al., 2013, Influences of Environmental Factors on Fish Assemblage in the Tropical Estuary of South West Coast of India, A Case Study of Kodungallur-Azhikode Estuary, International Journal of Marine Science, Vol.3, No.2 4-16 (doi: 10.5376/ijms.2013.03.0002)

Abstract

A proper monitoring of aquatic environment is crucial to appropriate management of the fisheries that rely on harvests from the environment and attempts of present study have been made to demonstrate links with environmental variability and fish abundance in the Kodungallur-Azhikode estuary (KAE). Annual average fish production in the estuary was declined considerably to 908.6 tons during 2009-2010, where 2747 tons was reported. Sixty three species of fin fishes, six species of penaeid shrimps, one species of Palaemonid prawns, two species of crabs, four species of bivalves and two species of edible oysters were observed in this study. Present study revealed that, Salinity is consistently the most important parameter explaining variation in assemblage composition and abundance of KAE; the availability of fish for recruitment into an estuary depends primarily upon the distributional range of euryhaline marine and estuarine species. The direct gradient analysis, first CCA axis, which explained most of the variation (45%) in the species data, was related to salinity, transparency and pH, and first two CCA axes together explained 72% of the cumulative percent variance of species-environment relationship. The importance of monitoring the estuarine condition in relation to fish assemblage was discussed, with significance on the potential use of estuarine fish assemblages and their monitoring and surveillance in management programs.

Keywords
Kodungallur-Azhikode estuary; Fish productivity; Water quality; Eutrophication

1 Introduction 

The structure and function of estuarine ecosystems are sustained by synergistic feedbacks between organisms and their environment. While many investigations aimed at detecting environmental and ecological changes within estuaries had focused primarily on water quality and the associated biota, there are relatively few studies based exclusively on fishes (Whitfield and Elliott, 2002). Several investigators had suggested that biotic processes, such as competition and predation, might be influential in controlling the spatial and temporal patterns of occurrence of fish in estuaries. In addition, a various abiotic factors have been associated with the structure of fish assemblages including salinity, temperature, turbidity, dissolved oxygen, freshwater inflow, structural attributes of habitat, depth, and hydrography (Martino and Able, 2003). 


Worldwide, nitrogen loadings resulting from human activities, as well as the number of estuaries and coastal seas reporting low dissolved oxygen concentrations had dramatically increased since the 1950s (Diaz, 2001; Boesch, 2002; Seitzinger et al., 2002; Diaz and Rosenberg, 1995). In many of the estuaries, illegal fishing had contributed to declining abundances of species that spend all or part of their life cycle in estuaries (Secor and Waldman, 1999; Lotze et al., 2006) and this recruitment of fishes from estuaries were strongly drive marine population dynamics (Elliott and Taylor, 1989). It was believed that large-scale patterns in the distribution of organisms result primarily from species responses to their physical environment, because dominant abiotic variables were thought to act like physiological sieve, thereby playing a vital role in the structuring of a community. Abiotic factors may set up the community framework, while biotic interactions refine species distribution patterns within this structure. However, much research on fish assemblages in estuaries has shown that salinity plays a major role in shaping assemblage structure (Whitfield, 1999). 
 

Fisheries have a vital importance in contributing beneficial nutrition for human beings, providing raw material for the industrial sector, creating employment possibilities and high potential for export (Can and Demirci, 2012). Various estuarine wetland systems spreading over three lakh hectare form an important component of the inland fisheries resources of India (Sugunan, 2010). India produces an average of 4.6 million tonnes of fish annually from inland water bodies. The average yield of estuarine fish production in India was estimated to vary from 45 to 75 kg/ha (Jhingran, 1982). Thirty major backwaters of Kerala forming the crux of the coastal wetlands form an abode for over 200 resident and migratory fish and shellfish species and fishing activities in these water bodies provide the livelihood to about 200 000 fishers and also provide full time employment to over 50 000 fishermen (Bijoy Nandan, 2008). These are indispensable habitat to a variety of biologically and economically important aquatic fauna; moreover, the inter dependence of the adjoining marine and estuarine zones in completion of the life cycle of the finfish and shell fish species (Jhingran, 1982; Chao et al., 1982; Muelbert and Weiss, 1991; Vieira and Castello, 1997). 
 

The Vembanad backwater has been extensively studied on the composition, distribution and gear wise catch of major fishery (Shetty, 1965; Kurup, 1982; Kurup and Samuel, 1985a; Kurup and Samuel, 1985b; Anon, 2001; Bijoy Nandan, 2008; Harikrishnan et al., 2011; Bijoy Nandan et al., 2012). Annual average fish production in the Vembanad lake and including other back waters of Kerala was estimated at 14 000 t ~ 17 000 t (Sugunan, 2010). Kodungallur-Azhikode estuary (KAE) is a northern extremity of Vembanad wetland ecosystem, is an ideal habitat for several species fin fish and shellfish species (Anon, 2001). 
 

A comprehensive study in the fish diversity and abundance in relation to environmental variability in the Kodungallur-Azhikode estuary (KAE) in particular was lacking. This study discussed the diversity and abundance of fishes in relation to environmental quality and use of fishes as Indicators of ecological change and estuarine health. 


2 Results 

2.1 Environmental variables 

The study area covered a both marine side (EMZ) and low saline side (EUZ) of estuarine transition zone. Wide range of variations in environmental variable was monitored during the study, which may potentially affect fish assemblage. Annual mean water column temperature in the KAE was 28.9℃ and it showed a clear vertical stratification especially during post monsoon season. Temporal variation was also noticed in the water column in both zones and it was lowest during south west monsoon (EMZ, 27.5℃; EUZ, 27.6℃) compared to pre monsoon and post monsoon seasons. The ANOVA of water temperature showed the variation between months were significant (p<0.01). The mean dissolved oxygen (DO) content of 5.1 mg/L was noticed in the KAE and monsoon period showed highest concentration (av. 5.8 mg/L) as compared to post monsoon period (5 mg/L) and pre monsoon period (5 mg/L). A noticeable trend was observed in the DO regime in the estuary, where surface water was higher than bottom waters. Surface water DO (5.6/mg) displayed comparatively higher values than that of bottom waters (4.7 mg/L). The ANOVA of DO showed that the variation between months were significant at 1% level (F=7.113). Carbon dioxide (CO2) values displayed highest mean in Station 5 (7.1 mg/L) and minimum in Station 7 (5.3 mg/L); temporarily the values were high in post monsoon (6.9 mg/L) as compared to monsoon period (6 mg/L) and pre monsoon period (6.3 mg/L) in the KAE. A remarkably high CO2 value of 14 mg/L was recorded in the bottom water in Station 2 (EMZ) during September and also comparatively high values were observed in the most of the stations particularly in stations 1 (7±3 mg/L) and 5 (7.1 mg/L). The ANOVA of CO2 between months showed variation and were significant at 1% level (F=18.324). The average biological oxygen demand (BOD) during the present study was 2.6 mg/L; it was high in the station 1 (3.1 mg/L) and temporarily it was high during monsoon (3.1 mg/L) as compared to the post monsoon (2.2 mg/L) and pre monsoon (2.3 mg/L) periods. Transparency values were generally low (0.6 m) in KAE particularly during monsoon season and it was negatively correlated with BOD values at 5% level (r =-0.688, p<0.05). In fact, high turbidity values were observed in the KAE with an average of 9.8 NTU with a peak concentration was recorded during south west monsoon season (20.2 NTU). Highest mean turbidity value was observed at mouth of the estuary (EMZ) represented by Station 1 (13.1 NTU). 


The discernible spatio-temporal variation was also observed in the pH values and it was generally on an alkaline side (7.4). However the peak monsoon was marked by heavy rain, pH values tended to fall in all the stations of KAE (6.9). Higher pH values were observed in stations EMZ (station 1; 7.5) when compared to the station in EUZ (Station 7; 7.2). Relatively high alkalinity was observed during pre-monsoon period (43.7 mg/L), when compared to the monsoon (24.4 mg/L) and post monsoon (36.9 mg/L) seasons. Highest mean alkalinity value was recorded at station 1 (40.3 mg/L). The ANOVA of alkalinity showed the variation between months were significant at 1% level (F=22.490). Average salinity of estuary showed mesohaline nature. The maximum average salinity was recorded at mouth region (Station1; 18.9 psu) and minimum at EUZ (Station 7; 10.2 ± 8.6 psu). Clear vertical stratification and seasonality were observed in salinity pattern. The salinity values showed a definite trend, where it decreased from estuarine mouth to head. During the monsoon period (June to September) salinity values were comparatively low (5.4 psu); however, salinity enormously increased (21.6 psu) during post monsoon period (October to January). But, salinity tends to decrease (16.1 psu) during pre-monsoon period (February-May), as a result of commencement of south west monsoon. The ANOVA of salinity showed that the variation between months were significant (F=33.433, p<0.01). Tides in the KAE are semidiurnal, with amplitude of 1m during spring tide and 60 cm during neap tides and average rain fall in the area was 310 cm (Revichandran and Abraham, 1998). During the present study average macronutrient concentration observed in the KAE were (15.0±12.1) µmol/L for dissolved inorganic nitrogen (DIN), 49.1 ± 28.7 µmol/L for dissolved inorganic silicate (DISi), and 1 ± 1.3 µmol/L for dissolved inorganic phosphate (DIP). Among the three major macronutrients, DIP concentrations were comparatively low in the KAE. The average nitrate- nitrogen (NO3-N) of KAE water was 10.2 µmol/L. The average NO3-N values ranged from 7.9 ± 9.9 µmol/L in Station 2 (EMZ) to 13.6 µmol/L in Station 7 (EUZ). Comparatively high NO3-N was observed during monsoon period (19.1 ± 19.4 µmol/L), whereas relatively low NO3-N content was observed in post monsoon (7.4 ± 3.6 µmol/L) and pre monsoon periods (3.8 ± 3.3 µmol/L). The ANOVA of macronutrient showed monthly variations significant at 1% level; (nitrate-nitrogen, F=50.537; silicate-silicon, F=38.965; phosphate-phosphorus, F=10.897). The average Chl-a for the seven stations of KAE was 6.42 mg/m3 and varied from 5.07 mg/m3 in Station 2 (EMZ) to 7.80 mg/m3 in Station V. Peak value of Chl-a was observed during pre-monsoon period (10.89 mg/m3) and decreased to an average of 5.16 mg/m3 during the monsoon season. The ANOVA of Chl-a showed the variation between months were significant at 1% level (F=14.295). The GPP showed an average of 1580 mg C m-3d-1 and NPP was 790 mg C m-3d-1 during the study period. Highest GPP was observed during pre-monsoon (1785 mg C m-3d-1) followed by post monsoon (1589 mg C m-3d-1) and monsoon (1 517 mg C m-3d-1). Generally increased GPP was noticed in the stations of EMZ (Station 1; 1625 mg C m-3d-1, Station 2; 1750 mg C m-3d-1 and Station 3, 1750 mg C m-3d-1). Highest NPP was observed during post monsoon (1035 mg C m-3d-1) followed by monsoon (828 mg C m-3d-1) and pre monsoon (585 mg C m-3d-1) respectively. Relatively high mean NPP values were observed in Station 3 (921 mg C m-3d-1) but Station 6 showed comparatively low average NPP values (588 mg C m-3d-1).


2.2 Fish abundance, species richness, and diversity

A total of 144 tows (EMZ, 72; EUZ, 72) data were collected representing 63 fin fishes belonging to 37 families (Table 1). Six species of penaeid shrimps, one species of palemonid prawns, two species of crabs, four species of clams and two species of edible oysters were observed. The average fish production in the KAE was estimated at 908.6 tons during the present study (2009-2010). Fin fish catches were dominated by Ambassis ambassis, Eubleekeria splendens, Mugil cephalus, Leiognathus berbis, Etroplus maculatus, Lisa parsia, Lisa macrolepis, Oreochromis mossambicus, Photopectoralis bindus, Plicofollis dussumieri, Etroplus suratensis, Valamugil speigleri, Gerres erythrouru. Fenneropenaeus indicus, Penaeus monodon, Penaeus semisulcatus, Metapenaeus monoceros, Metapenaeus dobsoni, Metapenaeus affinis, Macrobrachium rosenbergii, Scylla serrate, Scylla tranquebarica (Table 2) were the shell fishes observed in the catch. Villorita cyprinoides, Paphia malabarica, Meretrix casta, Meretrix meretrix, Crassostrea madrasensis and Saccostrea cucullata were the bivalves noticed during the study. Comparatively good Fish diversity (Figure 1) was observed in both zones of KAE (EMZ, 3.264; EUZ, 3.183). Fish assemblage in the estuary showed fish richness (d) value of 8.32 (EMZ) and 9.30 (EUZ); evennes of 0.79 (EMZ), and 0.75 (EUZ); dominance (D) of 0.915 (EMZ) and 0.912 (EUZ). 

 

 
Table 1 Species composition, Habitat, Environment, CPUE (number of fish per operation) and standard deviation (SD) of fin fishes collected from estuarine upper zone (EUZ) and estuarine marine zone (EMZ) of Kodungallur-Azhikode estuary (KAE) during July 2009-2010 period

 

 
Table 2 Species composition, mean CPUE (number of fish per operation) and standard deviation (SD) of shell fishes collected from estuarine upper zone (EUZ) and estuarine marine zone (EMZ) of Kodungallur-Azhikode estuary (KAE) during July 2009-2010 period

 

 
Figure 1 Seasonal variation of fish diversity in the Kodungallur-Azhikode estuary during 2009-2010

 

2.3 Fish assemblage – environmental relationships and landing pattern
The direct gradient analysis, using CCA approach, elucidated the role that the measured environmental variables played in shaping assemblage structure by comparing the species compositions among the collections from each zones. A test of significance (p=0.005) of all canonical axes using a Monte–Carlo permutation and a relatively high ratio of the sum of all canonical eigenvalues to the sum of all unconstrained eigenvalues indicates that the selected environmental variables are influential in driving the variation in the fish assemblages. The direct gradient analysis can be summarized on a two-dimensional plot, because 72% of the cumulative percent variance, of this species-environment relationship, was explained on the first two CCA axes. The first CCA axis, which explained most of the variation (45%) in the species data, was related to transparency, pH and salinity. The relatively large vectors representing salinity, pH and transparency the parallel orientation of these three vectors with axis 1, indicate their importance in shaping assemblage structure across the study site. Variation along the second CCA axis was driven by depth, turbidity and BOD (Table 3). The direction of the depth vector was towards the monsoon season (Figure 2), indicating the relative importance of each of these variables in shaping these assemblages (Table 4, 5).

 

 
Table 3 Environmental variables and Standard deviation (SD) of estuarine upper zone (EUZ) and estuarine marine zone (EMZ) of Kodungallur-Azhikode estuary (KAE) during July 2009-2010 period

 

 
Figure 2 CCA ordination using all samples during the period from 2009 to 2010

 

 
Table 4 Results of CCA on all samples

 

 
Table 5 Inter-set correlations of environmental variables with CCA axes

 

Fish landing pattern in the estuary was tremendously distorted during monsoon season due to various stress factors induced by heavy river discharge. Many of the marine fish species in the estuary declined during monsoon due to sudden changes in salinity, temperature and other physico-chemical and biological conditions in the estuary (Table 4, 5). Whereas, shrimp catch slightly increased during monsoon. Fin fishes contributed 69.62% to total fishery of KAE; catfish (8.46%), Etroplus maculates (5.69%), Etroplus suratensis (6.43%), mullets (5.65%), and Sillogo sihima (4.95%) were the major fin fish group. Among cat fishes Plicofollis dussumieri, A. subrostus were the major species observed in the present study. Crustaceans contributed an average of 23.47% to total landing; Methapenaeus dobsoni (8.01%), Fenneropenaeus indicus (6.22%), Metapenaeus monoceros (4.52%), Penaeus monoden (1.45%), Macrobrachium rosenbergii (1.43%) and crabs (1.83%). Bivalves (6.84%) and oysters (0.07%) also contributed to total fishery of KAE and the major bivalve species were Villorita cyprinoides, Paphia malabarica and Meretrix casta, whereas Crassostrea madrasensis and Saccostrea cucullata were the oysters noticed in the KAE. Overall similarity 71.26 % was observed in the month wise total fish landings of the KAE with highest similarity observed between December and February months; an apparent cluster formed between the months of monsoon and post monsoon season. Seasonal mean fish landing in the KAE was highest during pre-monsoon (397.9 t) followed by post monsoon (311.5 t) and south west monsoon period (199.2 t). Monsoon season dominated by cat fishes (19.9 t), Etroplus suratensis (13.2 t), mullets (11.7 t) and Chanos chanos (2.9 t). Fenneropenaeus indicus (21.4 t) and Metapeaneus dobsoni (12.5 t) were the major species of shrimps supported monsoon fishery of KAE. Fishery in the post monsoon season formed by cat fishes (24.6 t), Gerres sp. (18.5 t), Etroplus maculatus (17.9 t), Etroplus suratensis (17.5 t), mullets (15.9 t), Sillago sp. (16.1 t), Ambassis sp. (15.3 t) and Chanos chanos (14.2 t). Metapeaneus dobsoni (27.5 t) and Fenneropenaeus indicus (16.4 t) was dominant in post monsoon fishery. Clupeids and Carangoides were also appeared in the fish catch during pre-monsoon period; 16.2 t and 3 t respectively. Among complex array of fishing gears in the KAE, gill nets contributed 45% of the total fish catch followed by Chinese dip nets (18.31%), Stake nets (19.96), cast net (7.15%), ring nets (4.19%), scoop nets (3.39%), hook and lines (2%). 
 
3 Discussions 
3.1 Environmental gradients and species richness 
This study investigated the variation in fish abundance and its species composition in the Kodugallur- Azhikode estuary (KAE) in relation to prevailing environmental conditions. A total of 63 fin fish species were collected from the estuary during the study. Six species of penaeid shrimps, one species of palemonid prawns, two species of crabs, four species of clams and two species of edible oysters were also observed in the assemblages. In this estuarine system, environmental gradients are often steep due to the large watershed to surface area ratio and associated freshwater influence. This can cause dramatic shifts in salinity and temperature that estuarine organisms must either adapt to, or avoid (Vernberg, 1982). The estuarine fishes can be divided into two broad categories according to whether they spawn in estuarine systems or the sea; the former group is referred to here as estuarine and the latter group marine. The main feature of the life cycle of most marine species utilizing estuaries are divided into a juvenile period that is predominantly estuarine and an adult stage that is primarily marine (Wallace, 1975a). Although some species may attain sexual maturity within the estuarine environment, spawning always occurs in the sea (Wallace, 1975b) where the relatively stable marine environment is more suitable for the survival of the egg, embryonic and larval stages. Similarly, Fish diversity in the estuary was quite high in saline water dominated areas compared to fresh water dominated sites. Estuarine mouth zone (EMZ) dominated by marine and true estuarine fish species whereas estuarine upper zone (EUZ) supported mainly fresh water species.

3.2 Assemblage structure and assemblage- environment relationships
Estuarine environment contribute substantial amount to the total inland fish production and it also support diverse aquatic fauna as a location of breeding, spawning, larval development and feeding. Climatic conditions have direct relationship with growth, reproduction, abundance and distribution of fishes they were mainly temperature, salinity, dissolved oxygen, nutrients, pollutants, water current, tide and turbidity of water column (Vivekanandan and Sivakami, 2007). Fish assemblages in estuaries are largely structured by abiotic gradients (Kupschus and Tremain, 2001) that include salinity (Martino and Able, 2003), temperature (Maes et al., 2006) and dissolved oxygen (Weisberg et al., 1996; Eby and Crowder, 2004). The significant seasonal variation was observed in the fish assemblage in the KAE during present study. South west monsoon and associated river discharge; which finally resulted in the decline in distribution and abundance of fish community, fin fishes were suddenly responded and avoided stressful conditions by migration and most of the sessile organisms like molluscs died due sudden environmental changes and heavy fresh water discharge. 
 
High rate of primary production was observed during pre-monsoon period in the KAE and it has been support fish assemblage in the estuary by providing suitable energy sources. According to (Day, 1889) high rate of coastal primary productivity, which suggest high availability of pelagic and or benthic prey; this prey availability could be the one of reason for observed maximum fish production in the KAE during Pre monsoon period. Similar trend was also reported from the same estuary by (Harikrishnan et al., 2011) during 2005 to 2007 period. Whereas, fish production in the estuarine mouth zone (EMZ) decreased to 348.5 t during the present study as compared to earlier studies (Harikrishnan et al., 2011) where 369 t (2005-2006) and 424.8 t (2006-2007) were recorded. Total fish production in the estuary also declined to 908.6 t, when compared to the earlier studies (Anon, 2001) where 2 747 t was observed. Average fish catch yield in the KAE (5.4 kg ha day-1) was well below the average yield of estuarine fish production in India where it was 45 to 75 kg/ha (Jhingran, 1982). However, compared to total annual average fish production in the whole Vembanad wetland and including other back waters of Kerala (14 000~17000 t; Sugunan, 2010) fish production in the KAE was moderately good.
 
Both fisheries exploitation and increased nutrient loadings strongly affect fish and shellfish abundance and production in estuaries (Breitburg et al., 2009). Turbidity level enormously increased particularly during monsoon could be due to high influx of silt content, agricultural runoff, sewages and other allochthonous organic matters. Clam fishery in the KAE drastically reduced from earlier reports and it could be due to increased turbidity level and bottom disturbances by intense sand and clam mining and these stresses could be also affected larval settlement pattern; similar situation was also reported from the southern part of Vembanad Lake (Menon et al., 2000). However, high turbidity level in the estuarine environment generally offering increased partitioning of resources and shelter from predators (Day, 1889). Heavy fresh water discharges from the rivers during monsoon and later caused to depletion of oxygen content by the degradation of organic matter accumulated in the basin of the estuary. The combined effect of eutrophication and fisheries exploitation is to simultaneously increase algal production, degrade habitat and remove fish and shellfish biomass (Breitburg et al., 2009). As well as, clam population structure in the KAE altered on a temporal and seasonal scale. Omnivore type of fishes in the estuary increased during post monsoon season due to organic detritus enrichment, high level of habitat diversity in the estuary. In contrast, nutrient enrichment can reduce sustainable harvest by reducing the growth, survival and reproduction of target species where negative effect of eutrophication (Breitburg et al., 2009). 
 
Seasonal mean fish catch in the KAE was significantly reduced during monsoon period and highest observed during pre-monsoon period and it could be due various stressors in the ecosystem. The observed reduction in the fish landing during south west monsoon season was not only due to environmental stress but also due to decreased fishing days; marked by heavy rain. In this ever changing environment salinity has an important role, not only in determining the distribution of fishes within an estuary, but also the abundance and diversity of ichthyofauna. Salinity was comparatively high during pre-monsoon months and it substantially reduced
 
during south west monsoon season; salinity variation in the water column could have resulted in the reduction of total fish landing pattern. Conversely, some fishes attracted to estuary during monsoon period due to physiological/behavioural attraction to river discharge and precipitation; lower salinity preference for part or whole of life cycle (Day, 1989). Correlation coefficient of fish abundance and its environment variables in the estuary showed positive correlation Salinity (r=0.959, p<0.01), pH (r=0.988, p<0.01) and transparency (r=0.962, p<0.01). Turbidity showed a negative correlation with fish assemblage in the KAE (r=-0.992, p<0.01) during the study period. Linear regression analysis shows that the multiple correlation coefficient (R), using all the predictors simultaneously, is 0.79 (R2=0.63) and the adjusted R2 is 0.50, indicating that 50% of the variance in fish landing can be predicted from the Chl-a, water temperature, salinity variations combined. The model of Chl-a, water temperature and salinity significantly predicts the fish landing, F (3, 8) = 4.69, P<0.05. Mass fish surfacing and few mortality was also observed in the EMZ of KAE during fall of south west monsoon by the influence of depleted oxygen content due to degradation of organic matter accumulated in the basins of estuary and changes in salinity. 
 
The major fishing gears employed in the estuary were gillnets, cast nets, stake nets, scoop nets, ring nets, traps and Chinese dip nets. Gill net was most widely used fishing gear in the estuary and it has contributed 53% to the total fishery; it also exhibited highest CPUE value among the several type of fishing gears employed in the estuary. Present study noticed a significant reduction in the fish production from the earlier studies by (Anon, 2001) and (Harikrishnan et al., 2011) in the same estuary, a significant intra annual variation in fish productivity were also observed during the study. This seasonality in fish catch at the estuary indicate that monsoon induced sudden changes in the water quality support less biological populations and communities while less disturbed seasons characterized maximum fish stock and variety of species. The drastic reduction in annual fish landing in the estuary could be due to habitat degradation caused by intense sand mining, aquaculture, pollution, artificial breaching, urban encroachment and harbour development. Unfriendly fishing practices, size and recruitment over fishing and associated factors were the major reason for fish diversity loss in the KAE.
 
Anthropogenic activities, over fishing and various types of pollutants from different point and non-point sources have a crucial role in the habitat degradation. The proper assessment of variation in estuarine fish production as well as fish diversity is important for management of fisheries resources of an ecosystem. Juvenile estuarine fish populations are strongly affected by climatic variability, which may affect fish production potential through changes in either growth or abundance (Martin and Michael, 2002). KAE was important ecological buffers for many of the marine fish species as a location for breeding and larval development. However, KAE was potentially critical for dampening climate variability induced stock fluctuations and this recall the important for sustainable management of fisheries; particularly during recruitment of fish stock and this highlights the importance of estuaries. The declined fish stocks in the estuary can be attributed to inadequate fishery surveillance by the authorities and it lead to overfishing, mortality juveniles and removal of brood stock during reproductive seasons. The large scale seasonal variability on environmental variables implies that seasonal variation can have an important role on fish abundance in the estuary. However, climatic variations can affect the natural fish stocks and these variations would be affect the fishery environment and ecosystem as a whole. 

4 Materials and methods 
4.1 Study site
The study site encompassed the transition from an oligohaline estuarine system to the adjacent marine environment with sampling stations spaced at each of two regions, including the estuarine upper zone (EUZ) and estuarine marine zone (EMZ) in the Kodungallur-Azhikode estuary (KAE), is a northern extremity of Vembanad wetland ecosystem (Figure 3). KAE (10°11'~10°12' N and 76°10'~76°13' E) having an area of 700 ha. Tides in the estuary are semidiurnal, with microtidal tidal range; tidal effects extend to approximately 25 km landward of Azhikode (Revichandran and Abraham, 1998; Martin et al., 2011). The KAE is fed by the tributaries of Periyar River and it has remained open to tidal flushing from the sea at Munambam. 

 

 
Figure 3 Location of the sampling sites in the Kodungallure-Azhikode Estuary.

 

4.2 Sampling methods

Estuary was classified into two zones based on general morphology and environmental characteristics of estuary, estuarine mouth zone (EMZ) and estuarine upper zone (EUZ). Landing centre based direct data collection method was adopted for the fish landing estimation (FAO, 2002). Azhikode, Anapuzha and Krishnankotta were the major fish landing centers of the estuary. 

The fish catch composition, gear wise catch (%) of the fish diversity were studied in the KAE during July 2009 to June 2010 period. Catch per unit effort (CPUE) is defined as one tow of the net which occurred once per site or the number of fish collected per tow. CPUE for fish assemblages were estimated using standard an otter trawl (3.5 head rope, 30 mm mesh wings, 18 mm mesh cod end) towed by small boat. CPUE was expressed as No. 100 m-net hr-1 and used as index of relative abundance (FAO, 2002). The total catch was sort out into finfish, shrimp, prawn, crab, molluscs and other species by visual assessment. After sorting and counting, representative samples were preserved in 10% formalin for taxonomic studies in the Laboratory. The species wise identification of fishery was done based on standard works (Day, 1889; Talwar and Jhingran, 1991; Jayaram, 1999; Munro, 2000) and also Fish Base (Fishbase, 2012). 


Water quality parameters from different zones were also collected on an array of environmental variables that can be potentially influence the fish communities. Water transparency (Secchi disk transparency; SD) was measured by Secchi disk in the field. Dissolved oxygen (DO) was estimated according to Winkler’s method (Grasshoff et al., 1983). pH by Systronics pH meter (No. 335; accuracy ± 0.01). For the estimation of Chl-a, acetone extraction method was employed (Parsons et al., 1984). Primary productivity was estimated by in situ incubation method using the light and dark bottle oxygen method (Strickland and Parsons, 1972). Temperature of water samples were measured with a centigrade thermometer, conductivity by Systronics digital potentiometer (No. 318), turbidity by Systronics water analyser (No. 317) and salinity by Systronics water analyser (Model No. 317; accuracy ± 0.01) calibrated with standard seawater (APHA, 2005). Carbon dioxide, alkalinity, hardness and biological oxygen demand (BOD) was determined by standard procedures (APHA, 2005).  


4.3 Data analysis 

The structure of the fish assemblage was analyzed via direct gradient analysis methods. Direct gradient techniques elucidate the role of environmental variables in shaping the assemblage structure. Direct gradient analysis methods are useful in determining the relative importance of measured environmental variables, but may lead to a biased ordination of the species data depending on the available environmental data. Canonical Correspondence Analysis (CCA) is becoming the most widely used method of direct gradient analysis in ecology (Rakocinski et al., 1996). The CCA algorithm performs better than other direct gradient techniques when faced with certain problematic characteristics of ecological datasets, such as skewed species’ distributions, quantitative noise in species’ abundance data, unusual sampling designs, highly inter-correlated environmental variables, and unknown species-determining factors. The software package CANOCO version 4 (ter Braak and Smilauer, 1998) was used to perform all ordinations. Two-way analysis of variance (ANOVA) was applied to calculate the variation in hydrographic and biological parameters (SPSS 18v.). Primer 6.0v was used to study fish diversity in the KAE (Clarke and Gorley, 2006).


Author Contributions 

SBN is the supervisor of the project. SBN, PRJ, OKS and VFS made the experimental design, data analysis, manuscript preparation. 


Acknowledgements 

This work forms part of the research project entitled “Ecology and fish production potential of the Kodungallur-Azhikode back water ecosystem” funded by the Kerala State Council for Science, Technology and Environment (KSCSTE) and the authors are thankful for financial assistance. Authors are thankful to The Head and Ecology Division, Department of Marine Biology, Microbiology and Biochemistry, Cochin University of Science and Technology for providing necessary facilities. 


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