Inland Fisheries

4.1.2 Ecology of reservoirs of India

The reservoirs are man-made ecosystems. Besides adding substantially to fish production, they offer employment opportunities. While aquaculture in small water bodies such as ponds is capital intensive, development of fisheries in reservoirs a culture based capture activity is labour intensive. In other words, the reservoir is an ecosystem where fluviatile and lentic conditions co-exist. In reservoirs, the quality of impounded water varies from watershed to watershed and even with the same watershed depending on the soil climate and human activities. It also varies with shape of the reservoir basin, photoperiod, wind action and quality of water change. Owing to these variables evaluation of water quality and productivity of the reservoir have to be made separately for different sets of ecological families of reservoirs sharing similar eco-climate.

India has 19,370 small reservoirs with a total water surface area of 14,85,557 ha. At least 100 of them have been subjected to scientific studies. Habitat variables responsible for a reservoir's productivity can be summed up into climatic, morphometric and hydro-edaphic factors. The peninsular reservoirs are characterized by a narrow range of fluctuations in water and air temperature across seasons, a phenomenon which prevents the formation of thermal stratification. Many reservoirs in the Upper Peninsula show thermal stratification during summer. Wind-induced turbulence facilitates the return of nutrients to the trophogenic zone. Most reservoirs on the mountain slopes of Western Ghats, Himalayas and the other highlands are deeper, with steeper basin walls, compared to irrigation impoundments. Mean depth does not show any direct correlation with productivity, either at primary or fish level. A high shoreline development index gives a better indication of productivity. Plankton, benthos and periphyton pulses of Indian reservoirs coincide with the months of least level fluctuations. Oligotrophic tendencies shown by some reservoirs are mainly due to poor nutrient status and other chemical deficiencies. In most cases, poor water quality is accountable to poor catchment soil. Low levels of phosphate and nitrate are not indicative of low productivity due to quick recycling of these nutrients. Specific conductivity reflects the production propensities of reservoirs satisfactorily. Almost all productive reservoirs have a clinograde oxygen curve and a vertical stratification of chemical variables such as pH, carbon dioxide, total alkalinity and specific conductivity. High seasonal rainfall and discharge of water during monsoon result in high flushing rates, which do not favour colonization by macrophytic communities. Similarly, inadequate availability of suitable substrata retards the growth of periphyton. Plankton constitutes the major link in the trophic structure. A rich plankton community with well-marked succession is the hallmark of Indian reservoirs with blue-green algae as the major component. The main factors that retard the growth of benthos are a rocky bottom, frequent water level fluctuation and rapid deposition of silt and other suspended particles. Large reservoirs, on average, harbour 60 species of fishes, of which at least 40 contribute to the commercial fisheries. Fast-growing Indian major carps are the prominent commercial fishes. Dam construction has adversely affected populations of many other species such as Tenualosa ilisha, Tor spp. and Cirrhinus spp. While culture-based fisheries have been successfully practiced in many small reservoirs, the management norm followed in medium and large reservoirs is primarily on capture fishery. In large and medium reservoirs, stocking was successful only when stocked fishes bred. Two exotic fishes viz., Oreochromis mossambicus and Cyprinus carpio have been introduced into Indian reservoir with discouraging results.