Inland Fisheries

5.1.5 Classification of estuarine animals

Estuarine animals are classified into five main groups as below:

1. Oligohaline organisms

This group includes most of the freshwater forms inhabiting rivers which cannot tolerate variations in salinity of more than 1 ‰ and which are not found at the head of the estuary.

2. True estuarine organisms

True estuarine organisms are such that they are geographically restricted to estuaries only and are best represented in their upper and middle reaches in low salinities. Although these organisms have marine affinities, they have adapted themselves to tolerate a wide range of salinity.

3. Euryhaline marine organisms

These organisms extend in their distribution from the sea to the upper reaches of the estuary and can tolerate salinity as low as 15 ‰. A few, however, can tolerate salinities of even 5%o. These organisms form the majority of total estuarine biota.

4. Stenohaline marine organisms

These organisms live on open seashore and at mouths of estuaries. They do not enter estuaries below salinities of 25 ‰ in the lower reaches.

5. Migrants

Certain euryhaline marine migrants spend only a part of their lives in estuaries. Many are predators subsisting on resident estuarine benthic organisms.