Inland Fisheries

5.1.2 Ecology of estuaries

Several unique features are seen in most estuaries with regards to the physics, chemistry, geology and biological features.

• The fast flowing rivers of upland regions lose their velocity and turbulence and become nearly still except for the changes brought about by tides.
• The depth and width generally increases.
• Waters tend to become clear due to settling of suspend matter caused by great reduction in velocity.
• The bottom of the estuary gets built up due to deposition of materials brought by the flood waters.
• Physical environmental characteristics like penetration of light and temperature change compared to shallow upland river stretches.
• Salinity, dissolved gasses and chemical nutrients again show variations from that in the upper stretches.
• The estuary shows distinct seasonal variations in several of the physic-chemical and biological characters, particularly those waters, which come under monsoonal regime. The estuary can show marine, freshwater and intermediary characters nearly freshwater during monsoon season and flooding periods, marine characteristics in pre-monsoon or summer and between monsoons an admixture feature of seawater and freshwaters.
• Tidal regimes also influence the estuarine characters with high tide bringing more seawater into the region and on withdrawal of same during low tide with more freshwater entering the area.
• Biologically an estuary is more unique in allowing the development of a set plants and animals that only can thrive in a buffer media of estuary. Euryhaline organisms capable of tolerating some degree of variation in salinity of the waters develop and thrive here.
• Estuaries thus support freshwater life forms, marine forms and finally the brackish water forms, capable of inhabiting waters with changing salinity features. Also, this environment will support pure freshwater forms in the upper reaches, euryhaline forms in the middle regions and stenohaline forms in areas near to the mouth. During flooding and monsoon conditions, only freshwater forms are present and during dry period or summer with no or less freshwater inflow more marine forms inhabit the region and the euryhaline brackish water forms exist during rest of the period.
• The estuaries are considered as the highly productive aquatic ecosystems for all forms of aquatic life including benthic forms.
• They are ideal and favourable nursery areas for a wide variety of commercially important marine finfishes and shellfishes, as the waters are more sheltered with favourable conditions for breeding and feeding. An assortment of food components, i.e. phytoplankton and zooplankton, thrive here to help for the growth and development of different fishes and shellfishes.
• Many estuaries support a thriving commercial fishery of brackish water finfishes and shellfishes.
• Owing to their sheltered nature, estuaries offer safe navigation and anchoring of boats and ships