Estuary tidal mouth of a large river formed by an interaction of sea, land and fresh water forming a transition zone between fresh water and marine environment. Estuary is a productive place where many of the marine and fresh water organisms occur at particular phases of their life cycle. Hydrological features of an estuary are so dynamic that variations take place monthly, daily and even hourly.

These variations have a profound influence on the life cycle of all the migratory organisms and also on physiology of all other organisms which live there permanently. The coast of Bangladesh extends from Teknaf, in the southeast to Satkhira in the southwest (about 710 km) bordering the bay of bengal. The coast is ornamented by a network of complex estuarine system, some of the big rivers such as the padma, brahmaputra, meghna, and karnafuli open as estuaries into the Bay of Bengal. The estuaries of Bangladesh even a few decades ago were quite resourceful in fisheries and other aquatic organisms of commercial importance, but declined rapidly because of overexploitation and pollution. [Nuruddin Mahmood]