Fisheries :: Reservoir Fisheries
India has a large spread of fresh water resources in the form of rivers, reservoirs, lakes, ponds. Indian reservoirs, being in the tropics, have high primary productivity and have the capacity to produce more fish than their present low Indian average of 29.7 kg/ha/yr in reservoirs. Reservoir fisheries are essentially a stocking cum capture system. There are 975 reservoirs in the country with a total area of more than 3.15 million hectares.
River water is usually running or flowing water. Construction of dam results in the creation of a reservoir or dam- lake, in which the lotic water of the upper reaches becomes lentic as water approaches the dam. Rise in reservoir depends upon river flow and rain water. A new
reservoir passes through three trophic phases- initial fertility, trophic depression and final fertility. Filling of a reservoir inundates vast area bearing a cover of vegetation. It starts decaying and putrefaction results initial fertilization of the water leading to an intense development of
fish food in the form of benthic micro and macro flora and fauna. The initial increase of biota is often spectacular. After the initial high fertility, trophic depression phase sets in. This is cause by gradual diminishing of the rate of nutrient release. This is due to increase in the volume of the impounded water and available nutrients used by the vegetation. After this phase is passed the final fertility level is reached in the reservoir, which is a much lower level than that of initial fertility.
Reservoir ecology is changed from the reverine ecology because, in reservoirs, the lotic water of the upper reaches becomes lentic as water approaches the dam. This facilitates entirely different types of fishery called reservoir fishery, to suit the ecology of the reservoir. A reservoir has its own peculiarities in which it differs from natural lakes. The revirine ecology of the water of the upper reaches becomes increasingly changed into lacustrine ecology in the reservoir. The benthic riverine fauna disappears and it is replaced by typical lacustrine benthic fauna. With the change in from the lotic to lentic conditions of the water current, riverine plankton are replaced by lacustrine plankton. The turbidity level also reduced as reservoir act as settling basins. Fish fauna is greatly affected. The running water fish species become fewer or completely eliminated. Slow water fish species are predominant. Floating plants may come up, particularly in tropics where they create deoxygenating conditions or cause other serious ecological problems.
The dam in some way interferes with the ecology of the upper reaches of the river. Migratory fishes are completely wiped out from the upper reaches. This often leads to disturbances in the ecosystem especially with advantage to the prey. The reservoir it self may affect the ecology of the lower reaches of the river. Periodical discharge of sediments from the reservoirs may cause mud and silting in the lowe reaches with serious consequences on the fauna. Reservoir acts as fertility traps, reducing the amount of dissolved plant nutrients which would other wise be freely arriving at the lower reaches.
Reservoirs in India:
The following reservoirs are presently suitable for the fish culture in
Nagarjuna Sagar - Guntur and Nalgonda Districts.
Classification of reservoirs:
The reservoirs are classified by many authors in different ways mainly
based on the area of reservoirs.
Mohanty (1984) reported three types of reservoirs.
Pathak (1990) classified the reservoirs into three categories.
Agarwal (1990) classified the reservoirs, keeping in view the availabilities and other factors of management into four categories.
Jhingran and Sugunan (1990) classified the reservoirs into these groups.