Temporal and Spatial Variations in Fish Assemblage Structures in Relation to the Physicochemical Parameters of the Merbok Estuary, Kedah

The effects of seven variables–rainfall, water depth, salinity, turbidity, temperature, conductivity and pH–on fish assemblages were evaluated in this study. Fish were sampled on a monthly basis using a barrier net deployed by artisanal fishermen at six physicochemical sampling stations. The Merbok estuary was influenced by variable river discharges and mainly affected by primary and secondary wet seasons in March–June and August–November, respectively. This impacted the salinity gradient which ranged from 3.50 ppt to almost 30.75 ppt, resulted in two different salinity regimes, i.e. mesohaline and polyhaline. The temperature varied with a pronounced peak in both the primary and secondary rainy seasons. Other parameters such as conductivity, turbidity and pH fluctuated temporally, but no significant differences were recorded among the sampling sites. Fish species accounted for 72.06% (897.9 g/b/t), while marine and freshwater shrimps accounted for 27.94% (350.7 g/b/t). Almost 80 species of fish, representatives of 45 genera from 36 were recorded in the present study. Temporally, the mean abundance of fish was lower during the primary wet season than during the secondary rainy periods while spatially, the mean abundance of fish species was higher in the middle zone of the estuarine systems. The correlations between species and variables, suggesting the importance of environmental parameters in determining fish distribution, abundance and assemblage. Some fish species such as Butis gymnopomus showed a strong correlation with turbidity and pH, whereas others such as Lates calcarifer were strongly correlated with salinity. Key words: physicochemical, estuarine fishery, resource management, Merbok estuar