Limnology (2+1)

Unit 8- Nekton

8.1 Nekton- Composition, Distribution, Movements
Nekton
The term nekton has been coined by Ernst Heckal (1890). Nekton is derived from Greek word means swimming. The term nekton is used to designate those organisms which swim freely in water and possess locomotion enabling them to have independent drifting movement along the water flowage system. In limnetic regions of inland waters, the nekton is composed almost entirely of fishes.
The limnetic nekton may inhabit the whole of open water of a lake and down to its greatest depth. In contrast to the limnetic region, the littoral area is the zone of greater nekton population. In addition to the fishes (young and mature), numerous free swimming invertebrates occur mainly insects. Vegetative zone particularly the pondweed zone, contain largest nekton population. Major groups of vertebrates including fishes, Amphibia, Retillia, Aves and Mammalian are more or less representative of water and some time free swimming in nature.
Composition: Nekton comprises three phyla, viz, chordates, molluscs and arthropods, but molluscs composition in fresh water is very less in the case of nekton.
Among Arthropods the dominant groups comprising Insects and crustaceans are involved. However, the phylum, Chordata, is the largest group in Nektonic community, mainly fishes, Amphibians, Reptiles, Birds and mammals.
Distribution
Distribution depends on the following factors
- seasonal changes
- Physiological changes (competition for space).
- Food availability and distribution
- Life cycle - young ones of white fishes goes to the surface for feeding whereas adults inhabit the column water.
- Formation of local aggregations.
- Seasonal migration
- Diurnal movement
- Reproductive cycle
- Presence or absence of shelter
- Oxygen distribution
- Temperature conditions.
It has been observed that certain fishes select deeper, cooler waters, intermediate regions and some select upper strata. Such a distribution depends upon the season, physiological state of the fish and the stage in life history. According to Shelford (1913) in Great lakes, the white fish exhibit horizontal depth stratification and are arranged one above the other in their distribution. Koelz (1929) claimed that this fish in Great lakes have shown vertical distribution pattern. In some lakes the species that regularly inhabit shallow water may be driven by competition on the shoals or by absence of shoals found in the deeper waters eg, lake Ontario. In lake Nipigon the species inhabiting deeper waters are known to occur in shallow waters are Clupeaformis, alpenae, zenithicus etc; but Johannae, Nigripinnis and Kiyi found in deeper waters.
The food of great lakes white fishes seams to be secured from the plankton and the immature fish come to uppermost waters giving them a vertical distribution from that of mature stages. Fishes like lake trout (Cristivomer namaycush) and turbot (Lota lota) occur in deeper waters.
Like plankton, the nekton also have uniform distribution pattern. They have a complex background of distribution with regard to seasonal migrations, diurnal movements reproductive cycle, movement of water, temperature conditions etc.