Brackish-water Environment

Brackish-water Environment conditions of the mouths or lower courses of rivers that meet the sea tide, having slightly salty water. In the open sea, salinity is consistent between 33 and 37%, and for fresh water it is always less than 0.5%. In an estuary, ie in brackish water, salinity ranges between 0.5 and 35%. Three big rivers, the ganges, the brahmaputra, and the meghna converge in Bangladesh, and drain vast areas of India, Bangladesh and the himalayas, emptying into the bay of bengal with huge quantities of fresh water (some 1100 cubic km annually). The inundation by such an enormous amount of fresh water turns the surface water of the northern part of the Bay of Bengal almost into a riverine condition during post-monsoon months (September-November) and estuarine condition during pre-monsoon months (December-May). The oceanography of the Bay of Bengal, and particularly the offshore waters of Bangladesh, is dominated by three main factors: i) monsoon wind; ii) precipitation as a consequence of the monsoon climate and the presence of the Himalayan mountains; and iii) river discharge, also related with monsoon climate and precipitation, but intensified by the fact that the major river systems in India, Bangladesh and Myanmar empty into the Bay of Bengal. These factors have a strong influence in creating a 'brackish water environment'; in the northernmost part of the Bay of Bengal as they affect to a varying extent and in varying time sequences, the water circulation, salinity, turbidity, productivity, and nature of the bottom. These in turn influence the distribution of fish and their migration in the offshore waters. [Nuruddin Mahmood] [Mahmood, Nuruddin Professor, Institute of Marine Sciences and Fisheries, Chittagong University]