Some of the very successful stocking operations in open waters are those carried out to build up fish populations in reservoirs formed by the construction of dams across rivers. Large land areas are inundated by the construction of dams and as a result very spectacular changes take place in the fauna above the dams. Due to increased water fertility caused by decaying vegetation and the flooded soils, explosive increases in fish fauna occur, but generally they are of the uneconomic species, considered as weed or trash fish. This is usually followed in a few years by a trophic depression, which results in the reduction of the fish populations. In the succeeding phase the productivity stabilizes, depending on the rate of growth of the biota and the amount of organic substances accumulated in the bottom soil. The fish populations in these reservoirs can be manipulated to provide a lucrative fishery by judicious stocking.
If a species that can breed in the reservoir is selected, initial stocking of an adequate number of spawners or adults may prove to be of con siderable value. If the conditions in the reser voir are favourable, a breeding population of the species can be expected to develop in a reasonable period of time, depending on the magnitude of the initial stock, spawning success and environmental conditions, including pro duction of food organisms and protection from predators.
Where there is a lack of a suitable spawning habitat, or when the species selected will not breed in the lentic environment of reservoirs, it will be necessary to establish hatcheries and nursery farms to produce fingerlings or year lings for stocking purposes. When non-indige- nous species are transplanted to fill ecological niches or to build up a dominant fish popula tion, there is usually the need for a steady source of fingerlings and yearlings for at least a number of years. This necessitates access to hatchery and nursery facilities.
A considerable amount of experience has accumulated in different parts of the world in the establishment and management of fish populations in reservoirs. It is now widely ac cepted that pre-impoundment studies for reservoir construction should include detailed investigations of the fish fauna, the possible effects of the dam and reservoir on the fishery resources and the possibilities of preserving them as well as developing new resources.
Some of the largest reservoirs utilized for fishery resource development are in the former USSR and very impressive efforts have been made there to stock and manage several species for commercial fishing. Reservoirs on the Volga and other rivers in parts of Eastern Europe are stocked with bream (Abramis brama), common
carp, white fish (Coregonus lavaretus and C. albula) and pike perch (Stizostedion luciop- erca). Southern reservoirs are also stocked with silver carp for effective utilization of phyto- plankton. Reservoirs created by hydro-electric
dams on the Black, Caspian and Aral sea basins have been stocked with young of migratory cyprinid species like roach (Rutilus rutilus), bream, vimba (Vimba vimba) and shemoia (Chalcalburnus chalcoides). Propagation and
continued stocking in the reservoirs of the Volga are reported to have been largely respon sible for the maintenance of the fisheries of the Caspian sturgeons, namely beluga (Huso huso), spiny sturgeon (Acipenser nudiventris), Russian sturgeon (A. guldenstadti) and sevryuga (A. stellatus).
Enhancement of fish resources forms an integral part of the hydro-electric reservoir management in the former USSR. It starts as soon as filling of the reservoir commences, and spawners are transplanted to build up a spawning stock. Hatchery and nursery facilities are established near the reservoir to propagate valuable species for stocking, to add to those which are produced by natural breeding. Fishing is prohibited in the reservoir until a satisfactory population level is achieved. Very often weed-fish are captured and destroyed, in order to reduce competition for food and space.
Food organisms of the stocked species are transplanted and provided with favourable con ditions for growth during the period of trophic depression. Special care is taken to maintain the optimum water levels required for breeding
and survival of the stocked species. Yields of fish from the reservoirs vary depending to a large extent on the climatic conditions in the area. Yields in the order of 25–45kg per ha have been reported from the southern areas, but pro duction is much lower further north.
Hydro-electric and irrigation reservoirs in a number of other countries also, especially in India and China, have been stocked regularly to develop commercial fishing. Most of the Indian reservoirs are regularly stocked with fry and fingerlings of Indian carps grown in nearby nursery farms. Although there is evidence that these carps can breed in the reservoirs or in streams draining into the reservoirs, the pro duction of fry and fingerlings appears to be low and so stocking is required to maintain the stock size. Some of the reservoirs in the north are stocked with varieties of common carp and some in southern India with tilapia, along with other local species. Breeding populations of tilapia have established in some of the lakes.
Experience has shown the need for using large fingerlings or yearlings for stocking to obtain satisfactory survival, and presently sizes above 15cm are used for this purpose (Jhingran, 1982). The rate of stocking varies very considerably, but where regular stocking is practised it ranges between about 3700 and 5000 finger-lings per ha. The yield per ha of reservoir also varies considerably and is reported to be between 6.2 and 39kg.
Some recent instances of stocking the giant river prawn (Macrobrachium rosenbergii) in South Indian reservoirs have been reported (New et al., 2000).
In China, reservoirs and lakes are managed very much on the lines of large fish ponds, to derive maximum production. Those under about 100ha in area are managed more intensively with supplementary feeding and heavy stocking. Management of larger ones of over 10000ha in area involves stocking, protection of natural spawning sites and creation of additional spawning grounds, as well as regulation of fishing through restrictions on fishing equipment and fishing season. The more common species stocked are the Chinese carps (big-head, silver carp and black carp). Wuchang fish (Megalobrama amblycephala), common carp and the crucian carp (Carassius auratus) are also used. Fingerlings about five months old, of 15–20cm length, are preferred for stocking and the stocking rate may be as much as 2250 per ha. The average production in large shallow lakes is reported to be about 60kg/ha, whereas a production of up to 750kg/ha has been reported from smaller reservoirs.
Many of the South and Central American countries, particularly Mexico, Brazil and Argentina have been stocking their inland waters, including reservoirs, with indigenous and introduced species of fish. The large-mouth bass (Micropterus salmoides), trout (Salmogairdnerii, Salvelinus fontinalis), tilapia (Tilapia spp.) and the common carp are the more important non-indigenous species used. Hatchery production of the commercially important local species and their regular stocking in reservoirs have been an accepted practice in hydroelectric projects in Brazil. Reservoirs in north-eastern Brazil have now established populations of species such as Prochiloduscearensis, P. argenteus, Pimelodus clarias, Salmi-nus maxillosus, S. brevidens, Cichla ocellaris and C. temensis. The pejerry (Bacilichthys bonariensis) is one of the common indigenous speciesused for stocking reservoirs in Argentina, and this species has been introduced successfully into reservoirs in Brazil and Chile. In North America, stocking of reservoirs has been largely confined to selected species of sport fish. Besides salmon and trout, the fry of white fish (Coregonus clupeaformis) have been regularly stocked for over six decades in the Canadian waters of Lake Ontario, but there does not appear to be any conclusive evidence of its contribution to the fishing of the lake. The striped bass (Morone saxatilis) has been stocked in reservoirs, lakes and rivers all over the USA to enhance sport fisheries.
Both accidental and intentional stocking of lakes and reservoirs have occurred in the African continent. For example, an organized stocking took place in the Kariba reservoir, where Tilapia macrochir has been stocked regularly for a number of years.
One of the major problems in the exploitation of fish stocks from reservoirs is the difficulty in operating nets due to the uneven nature of reservoir bottoms. Large rocks and tree stumps covered with water during the formation of the reservoir hinder the operation of seines and gill nets. Specially designed gill and entangling nets are only partially effective when there are so many obstructions. So it became an accepted policy in the former USSR to level the reservoir bottom for commercial fishing by trawling and clear-hauling areas for seine nets. Reservoir bottoms are also graded in China to enable the operation of pair trawls, seines, gill nets and encircling nets. Enforcement of strict regulations relating to the size and periods of fishing has also contributed substantially to the success of stocking programmes.
Through a coordinated programme of reservoir management by engineers and fishery biologists in both China and the former USSR, it has been possible to maintain the required water levels for fish production and spawning. The relatively shallow lakes and reservoirs in China are often partitioned by means of dikes and artificial islands to enable more intensive management, often combining fry and fingerling rearing in sections of the reservoir itself.