Estuaries form the transitional zone between freshwater and saltwater where a river meets the sea. They can be classified based on their geological/morphological features into four categories namely coastal plain, tectonic and bar built estuaries and fjords. These features have a significant impact on the way that the fresh and salt water mix within the estuary which in turn affects the species which are found there.

Due to the changes in salinity within the estuary as the tide fluctuates estuaries present significant stresses to aquatic organisms. To survive these, organisms exhibit behavioural or physiological adaptations, including capacity to tolerate changing salinity and temperature condition.

The high productivity and abundance of both freshwater and marine animals makes estuaries an extremely important aquatic resource for man. This exploitation together with increased urbanisation and cumulative pollution from river side settlements upstream has caused many problems such as loss of habitat and pollution, significantly impacting estuarine ecosystems worldwide.