Inland Fisheries

Categories of Estuaries

1. Open estuary

It is always connected with the sea. It is located at the mouth of the river or lake. It may be perennial, filled with the water of the river throughout the year or seasonal in which the river dries up during summer, isolating the estuary from the sea. Chilka lake is an open estuary.

2. Embanked estuary

It is a confined area of brackish water, exposed to sea only during low tides or else it may remain surrounded by continuous chain of barrier islands. Examples: Adyar estuary and Cauvery estuary in Tamil Nadu. They are also situated in West Bengal, where it is called ‘Bhasabadha’ fisheries or ‘Bheris’.

Based on mixing and patterns of salinity distribution caused by various factors, there are two types of estuaries.

1. Positive estuary

It is the one in which the influx of fresh water is sufficient to undergo mixing and there occurs a pattern of increasing salinity usually towards the mouth of the estuary. This type of estuary has low oxygen concentration in the deeper waters and considerable organic material in the bottom sediments. Vertical distribution of salinity may range from top to bottom uniformly.

2. Negative estuary

In arid regions, where the rate of evaporation in estuaries exceeds the inflow of fresh water, salinity increases in the upper part of the basin, especially if the mouth of the estuary is restricted by shoreline features that check the tidal flow. The salinity distribution in this type of estuary is the reverse of the positive estuary. Negative estuaries and lagoons are hypersaline but possess a moderate oxygen concentration at depths. Bottom mud is generally poor in organic content.