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3. Habitat and ecological niche

The habitat of an organism is “the place where it lives”, whereas the ecological niche, a term coined by Elton in 1927, can br termed as “the position or status of an organism within its community and ecosystem resulting from the organism's structural and functional adaptations”. By analogy, it is often said that habitat is the organism's “address” and the niche is its “profession”.

The concept of ecological niche is of much interest to the aquaculturist, for it is around it that the practice of “polyculture” of fishes is centred. The wellknown examples are the polyculture of Chinese and Indian carps including in some cases a predator species also. Here it is basically the complete exploitation of the feeding niches in the water body as examplified by the mixing of surface feeders, some depending predominantely on phytoplankton (silver carp), some on zooplankton (Catla), columinn feeders (rohu), bottom feeders (common carp and mrigal), feeders of aquatic (grass carp) and predators (snakeheads). While the practice of polyculture of fish in Asia is old and based mainly on empirical information, it is necessary to critically examine this practice and adapt or improve the presently practised systems. In Africa polyculture of compatible species of tilapias in combination with predator fishes (Clarias gariepinus/ Ophiocephalus obscurus, for e.g.) have been practised in some countries. We shall be looking into these aspects more closely, in the section on “Pond Culture”, in the present series.

Fish culturists and ecologists often refer to ‘vacant’ niches, when all the niches, spatial as well as habit-wise, are not fully exploited by fish stocking. For example it is recognized that some of the man-made reservoirs have a ‘vacant’ niche of phytoplankton feeders, often in tropical waters addition of silver carp to such a water body can cause increase in production.

It is also known that in the case of some natural lakes such introductions have improved the production of fish. In Africa, the introduction of the clupeid, Stolothrissa tanganicae a plankton feeder is expected to have filled the vacant niche of a phytoplankton feeder, and resulted in higher fish production. The concept of ‘niche’ in space should also be considered along with a ‘time niche’ for there are examples where the f feeding regimes of closely related species feeding in the same spece, feed at different times to avoid competition. It would be useful for the fish culturist/fish ecologist to look in more closely to the actual partitioning and mechanics of the various niches within the fish pond to facilitate more effective use of productivity of the water body in increasing fish production. Herein comes the selection of species for polyculture, including induction of new species and improved strains and also manipulation of stocking density and stocking ratios, fertilization, supplementary feeding etc.

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