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Synopsis Fish fauna of Sunderban estuarine system exhibit far greater diversity as compared to adjacent fresh or marine waters. While many of the species occurring in estuarine zone can tolerate wide range of salinity fluctuations and move freely in the entire salinity gradient during major part of the year, some others, mainly marine species, are restricted only to lower zones. Most species enter the estuarine zones and pass varying period of their life span for the purpose of either feeding or spawning. However, there are certain species which have made the estuaries their almost permanent abode and pass major part of their lives. Sunderban estuarine complex is formed by river Ganga and its distributories in Indian region. It is a cris-cross network of a large number of smaller distributories in lower Gangetic delta. The lower zone of the estuarine system falls under mangrove forests, which because of their abundant food supply and shelter, provide very suitable conditions, specially to youngones of many species. With the result, both diversity and density of fish species are always higher in lower zone as compared to middle and upper zones.