"Biodiversity" concept seems to provide a scientific foundation for wide spread desires to preserve natural ecosystems in aquaregimes. While, adequate protection of fish diversity often requires an extensive network of aquaregimes which exclude human intervention, its conservation elsewhere does not necessarily prohibit resource use. It is important for fishermen, fish farmers and planners to understand the basic concepts of fish diversity. "Ichthyological diversity" in aquatic eco system refers to variety of fish species. Depending on context and scale, ichthyo diversity can refer to alleles or genotypes within a piscian population, to species or life forms within a fish community, and to species or life forms across an aquaregime (Burton, et. al 1992). Further, biodiversity is essential (whether for agriculture, fishery, forestry systems or evolutionary processes) for stabilisation of ecosystems, protection of overall environmental quality, for understanding intrinsic worth of all species on the earth (Ehrlich, and Wilson, 1991).

Positive correlations between biomass production and species abundance have been recorded in various earlier studies (Elton, 1927; Nikolsky, 1978). The species diversity of an ecosystem is often related to the amount of living and non-living organic matter present in it. However, apparently, species diversity depends less on the characteristics of a single ecosystem than on the interactions between ecosystems, e.g. transport of living animals across the different gradient zones in the waterbody. The effect of such transport is an important "information" exchange enhancing the genetic diversity. The genetic imprinting of various populations of lentic fish species is essential since the freshwater ecosystems constitute crucial parts of their life-support systems by providing nursing grounds and feeding areas (Hammer, et.al., 1993).

Fish catches have always fluctuated widely, as reported in the very earliest historical records (Bertram, 1985) and is documented repeatedly in modern times. The aspect of variation in the fish catches began to receive scientific attention in the middle and late 1800's (Whymper, 1883). Attempts had been made to overcome difficulties in interpreting data collected in different ways and to establish standard approaches for estimating total catches (Ledig, 1986).

Fish is captured in natural Lakes, reservoirs, streams, rivers and oceans. From an estimated 500 million hectares of inland waters of the world, the estimated total catch of fish is about seven million metric tonnes per annum. This excludes the catch of subsistence and sport fisherman. In many Asian countries, inland catches make up 40-70 per cent of the total fish production (FAO, 1986).

However, few species, inspite of their great commercial interest, have been comprehensively studied to establish the importance of their distribution for their successful management (Ludwig, and Reynolds, 1988). It is in this context, this study assumes importance reflecting the fish species diversity and their conservation in lake ecosystem.

Species diversity is a property at the population level, while the functional diversity concept is more strongly related to ecosystem stability and stresses, physical and chemical factors for determining population dynamics in the lentic ecosystem. Inspite of its tremendous significance in determining productivity and calculating species diversity, few studies had been done on the fish population dynamics, ichthyo diversity and conservation of fishes in Lake ecosystems in India in general, and North-East India in particular (Jhingran, and Tripathy, 1969) as compared to elsewhere in the world (Janzen, 1981).

The management of ecosystems supporting fisheries has an extensive history of specialisation. In freshwater fisheries, the trend towards little mechanisation of gears and crafts and the cultivation of fish in large monocultures have directed fisheries management practice towards a small number of particular species of commercial interest.

Information on the interaction between hydrobiological conditions and fish yield in any water body is of prime importance before endeavoring to utilise it as a productive fishery. Limnological and watershed parameters such as water temperature (WT), turbidity (t), Water pH (WpH), dissolved oxygen (DO), free carbon dioxide (FCO2), total alkalinity (TA), water conductivity (WC), soil temperature (ST), soil pH (SpH), soil organic carbon (SOC), soil phosphorus (SP), soil potassium (SK) and aquatic macrophytic biomass (AMB) have significant role on fish yield in a lake ecosystem.