Fish Ecosystems in Estuaries

The following factors define an estuary. First, it is a partially enclosed area from which freshwater discharges into a large body of saltwater, such as an ocean or sea. Secondly, it receives some saltwater from the ocean or sea by tidal action. These factors make an estuary the site of an interesting fish ecosystem.

Differential Salinity

• Since high tide brings the saltwater of the ocean into contact with the freshwater coming down from rivers, an estuary is a congenial home for such fish as the killifish that can adapt themselves to varying degrees of salinity. But freshwater fish will tolerate the less saline estuarine regions farthest from the mouth, while a saltwater fish like the flounder will roam about the saline area closer to the ocean.

Mangrove Ecosystems

• Red mangroves often grow in shallow water along the shoreline of estuaries in tropical and subtropical areas. The calm waters under and around the stilt-like roots of the mangroves provide a quiet haven for fish, especially young fish that are growing up. The mangroves furnish an excellent hiding place from the many birds that come to an estuary to feed on fish.

Rich in Nutrients

• Rivers bring nutrient-rich sediments into estuary ecosystems. The incoming detritus and such plankton as diatoms lie at the bottom of the estuarine food chain. Fish benefit from this bounty, either directly or indirectly, when a fish feeds on the organisms that eat the detritus and the diatoms.