Practical management of fish populations in reservoirs depends on the degree of environmental control that can be applied, on those factors that limit the size of the population, and on the goals of the fishery. Several of the following management measures have been devised to substantially increase production in the reservoirs.
A selective kill has been accepted as an effective reservoir management technique, particularly in the western hemisphere, but it is rarely found in Asia and the Far East. The method consists of application of toxicants to the impoundments for either partial or total elimination of unwanted species. Many toxicants have been used, but rotenone and its derivatives have been favoured (Ryder, 1970). Conservational methods of commercial fishing have occasionally been adopted as the most feasible way for controlling fish populations in reservoirs (Thompson, 1955).
In Southeast Asia and the Indian subcontinent where fish is an important source of cheap animal protein in the regular diet of the people, all freshwater fishes, regardless of species and size, are accepted as food fish; however, at different levels of preference. Any management practices must aim mainly to increase total fish production. Attention is therefore given to an intensive selective fishing for predatory fishes with selective fishing gears in order to suppress predators, such as snakehead (Ophicephalus), catfish (Wallago, Macrones, Mystus, etc.), carp (Hampala macrolepidota), and strengthening forage fish population (mostly carps) in the reservoirs. The elimination of predatory species in Gandhi Sagar reservoir in India is being achieved by keeping the royalty rate low and encouraging longline fishing with live baits (Dubey and Chatterjee, 1977). Intensive fishing for predators in Thai reservoirs using longlines, baited bamboo traps, cast nets, harpoons, etc., has shown good promise within a few years of operation. Predators have continuously decreased both in number and size of individuals, and carps are now becoming more abundant, contributing nearly 65% of total catches (Bhukaswan, 1979).
Fish stocking has proven to be one of the most successful tangible tools in reservoir fishery management (Jenkins, 1961). Two basic approaches have been used in stocking programmes. One system stocks with indigenous or native species, another stocks with exotic or foreign species. Introduced fishes could be either herbivores, omnivores or carnivores, depending upon certain circumstances and purposes. In implementation of this technique, it is generally recommended that the introduced species should not act adversely on economically or aesthetically valuable native stocks and environmental conditions. Introduced fish should be fast growing, be able to breed in confinement, have its feeding habit related closely to the base of the food chain or to food available in reservoirs, and be finally accepted by people (Adesanya, 1969). The purposes of stocking or introducing fish (indigenous and/or exotic species) to reservoirs are as follows:
To utilize ecological niches to which none of the existing species are adapted;
To increase fishing success by introducing species that are considered to be more desirable in the fisheries;
To restore “balanced population” by introducing substantial numbers of large predators, or to replace year-class failures of important species;
To provide a source of food for sport and commercial species;
To control aquatic weeds;
To provide more food fish;
To curb unemployment through fishery development.
The reasons for introducing fish into reservoirs are varied. In North America and Western Europe, freshwater fish are primarily a source of sport fishing. Introduced fishes are mostly predators, and only a few forage species are introduced as prey-fish. In the tropics, particularly in Southeast Asia, even relatively small fish are generally acceptable for consumption. Thus the introduction of predators to crop small forage fish is not desirable in this region (Fernando, 1979). Fishery management in reservoirs is mainly devoted to producing maximum yields of fish for food. Herbivorous and omnivorous species are stocked. Several strategies of stocking programme for optimization of reservoir fisheries have been recommended and discussed by Fernando (in press). These include:
Management of indigenous fish stocks derived from the riverine and marsh environments;
Stocking with domesticated and semi-domesticated fish annually or seasonally. This strategy is mainly to stock reservoirs annually and/or seasonally with common carp, Chinese carps and Indian carps. The purpose is to harvest the fast-growing introduced species together with resident fishes. This method gives poor returns in Southeast Asia; it was a failure in Sri Lanka, and has a moderate success in India. The reason is that most, if not all, reservoirs have indigenous predators which, combined with small numbers of stocked fishes, result in a high percentage predation on stocked fishes;
Introduction of carnivorous and herbivorous exotic species. This strategy has not been practised in Southeast Asia deliberately. Only herbivores and omnivores have been stocked for the very good reason that indigenous carnivores are almost invariably present and a maximum protein yield is the objective of management. Experience from several parts of the world indicate that the introduction of predators into reservoirs has been generally unfavourable to harvesting high fish yields. Therefore, the introduction of any predatory fishes into Southeast Asian reservoirs must be viewed with alarm. Fernando (1980) recommended that the only situation where both herbivores and predators should be introduced is where there are no indigenous predators, and only after the herbivorous component has been well established and the probable effects of the predator are carefully evaluated;
Introduction of herbivorous self-perpetuating exotics. Fernando believes from his experience that high fish yields in Southeast Asian reservoirs could be obtained only if they are stocked with exotic African Cichlidae (Tilapia spp.). The reasons for the success of tilapias in producing high fish yields in Southeast Asian reservoirs are their ability to maintain high densities in the face of substantial predatory pressure and their rapid growth and ability to utilize directly blue-green algae, digest bacteria and use non-protein amino-acids (Moriarty, 1973; Bowen, 1980).
Cage culture in reservoirs. In reservoirs which are dominated by carnivorous fishes, cage culture of fish might be the only alternative practice in raising the fish yields. Successful cage culture of bighead carp, Aristichthys nobilis, has been practised in Seletar reservoir, a drinking water eutrophic reservoir in Singapore (Anon., 1979). This strategy can be applied together with capture fishery, since they do not interfere with one another;
Introduction of deep water and plankton feeding fishes. This strategy is aimed at introducing deep water and plankton feeding fishes into large and deep reservoirs. Such fish species, e.g., Tilapia macrochir (Eccles, 1975) and the clupeid, Limnothrissa miodon, are available in the great lakes of Africa;
Annual or seasonal stocking of fish in small reservoirs. The most practical strategy seems to be stocking of fish in small reservoirs, varying in size from 1–300 ha, with fingerlings of fast-growing herbivorous fishes. The use of bigger fingerlings coupled with selective prestocking fishing to remove predators were recommended for obtaining higher fish yields.
In many cases, the natural reproduction of native fish is disrupted after impoundment, and the various improvements do not bring about a large enough replacement of the stock. Some introduced species fail to establish populations in the reservoirs. Attempts to solve these problems with stocking eggs and fry are not always successful. The failures have led to the idea of a nursery pond built adjacent to the receiving reservoir and connected by a canal. For example, the Kakhovka reservoir on the Dnieper River, USSR, has associated with it a large hatchery complex of 1 000 ha for producing 35 million yearling carp each year for stocking in this reservoir (Frey, 1967). Similar management has been practised in Indian reservoirs. Sreenivasan (1967) reported that fish farms and hatcheries have been constructed adjacent to several reservoirs. The present production of stocking fingerlings was 2.25 million at Stanley reservoir, 1.50 million fingerlings each at Bhavanisagar reservoir and at Anaravathy reservoir, and 4.50 million fingerlings at Sathanur reservoir. A hatchery was constructed on the bank at the Ubolratana reservoir in Thailand in order to produce annually 2.0 million fish fry and fingerling, mostly carps, for stocking in this impoundment. The use of a nursery pond to raise fish fry to a more resistant stage, particularly with respect to predation, before being released into the reservoirs, is highly successful in some cases of reservoir fishery management in temperate latitudes.
Stocking desirable fishes into reservoirs has been practised in tropical regions for the production of food fish rather than for recreational purposes as in Europe and North America. Fish stocked in reservoirs are usually plant eaters and plankton feeders, quite a few are predators.
Several species of Tilapia are making great progress in Southeast Asian and in African impoundments (Fernando, 1977). Spectacular results of higher fish yields in reservoirs following the introduction of exotic species have been noted in Sri Lanka (Fernando and Indrasena, 1969; Mendis, 1977). They reported fish production in Parakrama Samudra reservoir (2 246 ha) was very low (less than 10 kg/ha/yr) before the introduction of Tilapia mossambica into this reservoir in 1952. Thereafter the fish yield increased tremendously to about 180 kg/ha/yr in 1967 and 80% of the catch were Tilapia. A similar sharp increase was obtained within 3–4 years after the introduction of Tilapia melanopleura1 into this lake in 1969. The production of fish has risen from 400 t in 1973 to over 1 300 t in 1975. At this stage Tilapia melanopleura has displaced T. mossambica as the dominant species in the catches (Mendis, 1977), and fish yield taken from this reservoir was estimated around 450–500 kg/ha/yr in 1977 (Fernando, in press). Success in stocking Tilapia in reservoirs and lakes is also mentioned for Tanzania, Zimbabwe, Kenya, Uganda, Ghana (Hickling, 1961; Payne, 1974); the Philippines (Frey, 1969); Indonesia (Sarnita, 1977); and India (Sreenivasan, 1977).
In the Southeast Asian region, there are a number of fish species valuable for introduction into reservoirs. Fernando and Furtado (1975) reported that the introduction of an estuarine cichlid, Etroplus suratensis, into reservoirs in Sri Lanka has increased fish yield. Carps belonging to the genera Labeo, Puntius, Cirrhina and Catla have provided exploitable fish stocks when introduced into reservoirs in India and Indonesia. The sepat-Siam, Trichogaster pectoralis, has contributed significantly to the fish yield in reservoirs in Malaysia and Indonesia. Nilem, Osteochilus hasselti has contributed significantly to fish yields at Bukit Merah and Subang reservoirs in Malaysia. Clariid catfishes (Clarias spp.) and snakehead (Ophicephalus spp.) are important predators in fish catches. They also stated that all these species are riverine and marsh fishes, mostly detritivores or browsers and some predators, and their yields have been much lower than those of the African cichlids introduced, which are pelagic and planktivores.
The favourite fishes to be stocked in reservoirs in India are mostly the indigenous species, the Indian major carps. They are the catla (Catla catla), rohu (Labeo rohita), mrigal (Cirrhinus mrigala), and calbasu (Labeo calbasu). The fingerlings of major carps are introduced into reservoirs annually to replenish the stock and also to forestall possible breeding failures. Successful stocking rate for major carp fingerlings was recommended at a rate of 82 fish or more per ha. The annual stocking rates in Stanley reservoir were 105–500 fingerlings per ha, and 135–475 fingerlings per ha for Bhavanisagar reservoir (Sreenivasan, 1977). The results show different degrees of success among the reservoirs. The catla attained a high rank in Bhavanisagar reservoir where it participated by over 10% in catches in 1964–65, with 27% in 1970–71. This species plays an even more important role in the fisheries of Rihand reservoir in northern India. There, after it was introduced in 1957, it formed 25% in 1964–65, and 86.4% in 1973–74 of the total catch. Similar results were also found in the Gandhi Sagar reservoir (Dubey and Chatterjee, 1977). Catla was first introduced into this reservoir in 1959–60. By 1974–75, the percentage of this species has risen to over 70% (by weight) in the total catch.
The mrigal meanwhile has contributed 10% to the total catches in Bhavanisagar since 1968–69 (Sreenivasan, 1977). He also reported that the calbasu has established itself firmly in Stanley reservoir, and made a spectacular contribution up to 40% of total landings in Bhavanisagar reservoir. The rohu appeared in commercial catches in 1968–69 and became the first rank in 1973–74 by contributing 26.51% to total landings. The success in stocking of other species included Puntius curmuca, and Thynnichthys sandkhol (Jhingran, 1975). He reported that the former species forms an important fishery in Malampuzha and Mangalam reservoirs. The latter dominated the catches in Nizamsagar reservoir. Other stocking has been conducted with Etroplus suratensis, Osphronemus gouramy, Ophicephalus spp. and certain euryhaline fishes (Mugil spp., Chanos chanos and Megalops cyprinoides), but has not proved of any consequence (Jhingran and Tripathi, 1976).
Among the exotic fishes which have been introduced into Indian reservoirs, Sarotherodon mossambicus is the most promising species. Jhingran (1975) noted that this species formed sizeable catches in many reservoirs. Sreenivasan (1977) reported that this fish took a dominant status in the population of Amaravathy reservoir and accounted for 13.50% in 1965–66, and thereafter has risen to over 80% of the total catches since 1968–69. He estimated the average yield of this reservoir was 187.70 kg/ha/yr, which he claimed as the highest in Tamilnadu region, and perhaps in India.
In Indonesia, the common carp (Cyprinus carpio), Java carp (Puntius javanicus), nilem (Osteochilus hasselti), S. mossambicus and kissing gouramy (Helostoma temminckii) have been successfully stocked in several reservoirs (Hickling, 1961). Sarnita (1977) recently reported that, out of several species stocked in Selorejo reservoir, S. mossambicus has successfully occupied the lake habitat and increased fish production immensely.
In Thailand, stocking programmes are operated in several ways. Eggs and fry of Pangasius sutchi are stocked in some reservoirs such as in the Bhumipol reservoir but most reservoirs are stocked with fingerlings. Several freshwater fishes have been introduced into Thai reservoirs. They are both indigenous and exotic species. The introduction was carried out in some cases as early as the period of reservoir formation (the filling period) such as in the Ubolratana reservoir (Bhukaswan and Pholprasith, 1977). The purposes of stocking are mostly to produce more food fish of faster growth rate which are well accepted by the Thais, and for controlling aquatic vegetation in the impoundments. Introduced indigenous species are mainly the Thai silver carp (Puntius gonionotus), pla yee-sok or striped carp (Probarbus jullieni), snake-skinned gouramy or sepat-Siam (Trichogaster pectoralis), striped silver catfish (Pangasius sutchi) and several others. The exotic species include the grass carp (Ctenopharyngodon idella), bighead carp (Aristichthys nobilis), silver carp (Hypophthalmichthys molitrix), common carp (Cyprinus carpio), Sarotherodon niloticus and rohu (Labeo rohita). The number of fishes stocked annually in each reservoir has been very small compared to the water surface area. It was apparent that stocking rates in Thai reservoirs ranged from less than 1 fish to 8 fish/ha/yr. Therefore, the stocked fishes have not contributed significantly to the total catches. Only a few have been recovered and reported, but most of them showed a very good growth rate. For example, the grass carp reached a weight of up to 12.40 kg in 2 years; the rohu 5.40 kg in 3 years; the striped carp 10.70 kg in 4 years and the silver carp 9.40 kg in 6 years (Bhukaswan and Pholprasith, 1977). All of these fishes were caught from the Ubolratana reservoir. Amazing growth rates were also obtained from the Chulaporn reservoir where the striped silver catfish and Chinese bighead carp reached 100 cm and 150 cm in total length and weighed 12 kg and 40 kg respectively in 4 years after introduction (Chantarapakdi, 1976).
As it has been discussed previously, stocking may be the most successful management programme in Southeast Asian reservoirs, but with various degrees of success. Before stocking, however, one should carefully consider whether the introduced fish will be accepted by people as food fish. Introduced fishes must be free of parasites and diseases. They should not interrupt life cycle of native species and/or compete for food. They should not interfere and endanger the environmental conditions of the reservoirs, and should be self-reliable species in order to reduce possible cost of restocking.
Higher production of fish in large lakes and reservoirs can be achieved by raising the supply of food for fish. Fertilizing large reservoirs does not seem to be a practical approach because added nutrients may get locked out in the hypolimnion or in sediments, or lost with overflowing water, or for the simple reason of fertilizers being too expensive. A cheaper and better approach, by improving the fertility of the littoral area, has been recommended by Kimsey (1958). Wood and Sheddan (1971) found that fertilization increased the numbers of bottom organisms and zooplankton, but there was no significant change in numbers, size, species composition, or survival of fish. In the Kariba man-made lake, wild animals grazing on exposed and overgrowing draw-down areas fertilize them with their excreta; flooded again during the next water rise, these drawn-down areas contribute to the productivity of the littoral (McLachlan, 1974). Cattle grazing in such areas would have a similar effect.
The introduction of various invertebrates into reservoirs in order to increase the food available to fish appears to be profitable in some cases. In the Soviet Union, introduced invertebrates include cladocerans, an amphipod, a mysid, cumacean crustaceans, polychaetes and molluscs. Winberg and Bauer (1971) stated that “during the last 15 years, intensive work has been done in the Soviet Union to enrich the fauna of large lakes and reservoirs through herbivorous invertebrate introduction.” A successful introduction of 12 species of invertebrates into the Veselovsk reservoir on the River Manich (tributary of the Don River) has been reported by Kruglova (1962). Among successful species was the mysid (Paramysis kowalewskyi). It is now found in all parts of the reservoir and its yearly biomass is about 5.7 g/m2. In the Cimlyansk reservoir (2 700 km2) on the Don River, the mysids (P. kowalewskyi, P. intermedia) and the polychaetes (Hipania invalida and Hipaniola kowalewskyi) were introduced in 1954 with good results. In 1961 the mysids comprised one third of the total benthos biomass (2 g/m2).
In North America, the introduction of Mysis relicta and Pontoporeia affinis into large lakes as a source of fish food has been recommended since 1939 (Clemens, Rawson and McHugh, 1939). A decade later, approximately 25 000 Mysis and Pontoporeia were introduced into Kootenay Lake (399 km2) in British Columbia for the purpose of supplying large invertebrates as food for intermediate-sized rainbow trout (Sparrow, Larkin and Rutherglen, 1964). Mysis took approximately 10 years to become established in this lake. Studies show that they are utilized by rainbow trout (25–60 cm in length), Dolly Varden (Salvelinus malma), kokanee (Oncorhynchus nerka kennerlyi) and mountain whitefish (Prosopium williamsoni). Preliminary observations suggest that Mysis has increased growth rate of rainbow trout and kokanee.
Introduced Mysis is now established in Lake Tahoe, as well as in some other Californian lakes (Linn and Frantz, 1965), but its rate of utilization by fish has yet to be measured (Frantz and Cordone, 1970).
Three species of Crustacea (Mysis relicta, Pallasea quadrispinosa, and Gammaracanthus lacustris (Fuerst, 1970) have also been successfully introduced into Swedish reservoirs as a source of fish food.
The introduction of invertebrate species, planktonic and benthic, to enhance the food supply available for fish has been suggested for increasing fish production in Southeast Asian reservoirs (Bhukaswan and Pholprasith, 1977; Jhingran and Tripathi, 1977; Fernando, 1977). A Japanese cladoceran, Daphnia carinata, has been introduced into Jatiluhur reservoir in Indonesia in 1970. This invertebrate has established itself successfully and became a major source of food for the plankton feeders (Sarnita, 1977).
The success in Indonesia has shown that this measure is an applicable management method for the region. Small crustacea seem to be the most suitable invertebrates for introduction into tropical reservoirs rich in phytoplankton.
The manipulation of the water level in a reservoir has a profound effect on the production of fish and the composition of the biotic community. Among several design and operational factors which are sometimes adjustable are the extent and timing of the increase or decrease in water level and the size of minimum pool. Water level fluctuations may produce many effects on aquatic resources. Among these are:
standing or trapping aquatic organisms;
destruction and lower production of rooted aquatic plants, benthic algae, and benthos;
affecting the success of spawning by uncovering nests or former spawning areas, or covering them to excessive depth, and
affecting migration of fish (Dill and Kesteven, 1960; Hunt and Jones, 1972).
On the other hand, benefits to aquatic stocks may increase through fluctuation in water levels. For example, a reduction of levels immediately following egg depositions may permit the control of unwanted fish. Conversely, flooding of reservoir margins during the spawning season may aid the reproduction of desired species (Fraser, 1972).
However, these effects are not only dependent upon degree of variation in depth, but also on time, area, and duration of flooding or lowering (Wood and Pfitzer, 1960). Decreasing of water level of large shallow reservoirs in summer will destroy the conditions for the summer spawner and will reduce the area of feeding ground, whereas lowering the level in winter will result in mass mortality of young fish (Aronin and Mikheev, 1963). Quennerstedt (1958) reported that divergences in water level conditions affect the zonal distribution of aquatic vegetation. He found the 3 m fluctuation of water level in Lake Hotagen, northern Sweden, during the winter had caused a disappearance of the rooted vegetation and all vascular plants. The fall of water level not only inhibits the permanent growth of aquatic plants in the marginal areas, but it may also expose areas being used for spawning, killing eggs and fry (Jackson, 1966). Davis, Hughes and Schafer (1964) stated that the effectiveness of water level fluctuation on aquatic weed control was directly correlated to the species present and duration of fluctuation. They found an excellent control of Najas guadalupensis and Potamogeton spp. was achieved through the winter drawdown in Bussey Brake reservoir, Louisiana, in 1962. The pool elevation fluctuations of Lake Francis Case precluded the establishment of a vigorous periphyton growth in this reservoir (Claflin, 1968).
The effects of water-level fluctuations on the reproduction of fishes and the effectiveness of spawning in reservoirs are reported by several investigators. Runnstrom (1960) reported that up to 50% of the roe of the char in Lake Torron in Sweden was destroyed when the fluctuation reached 14 m in winter, because the major part of the roe came above the water and dried up. Yakovleva (1965) noted that the reproduction of the phytophilic fishes in the Volgograd reservoir, USSR, was inhibited as a result of the removal of water to irrigate the Volga delta during the spawning season. Il'ina and Gordeyev (1970) found that a gradual reduction in the area of the spawning grounds and a deterioration in the breeding condition of the phytophilous fishes such as bream and pike in Rybinsk reservoir occurred in years when water level was low and the marginal vegetation was not submerged. A similar phenomenon was also reported in the Kuybyshev reservoir (Kuznetsov, 1971). He stated that the effectiveness of reproduction of the zope (Abramis ballerus (L)) is closely connected with the fluctuations of water level in the spring. A relatively strong year class was found only in years in which water level was high during the spawning period in May.
The fluctuations of water level in reservoirs are not always injurious, a deliberate drawdown is sometimes used as a management tool in reducing stocks of undesirable fishes. Otherwise, it may be used to establish favourable spawning and feeding areas for desired species in the impoundments by a practical application of managed fluctuations. Hulsey (1957) reported that following autumn and winter drawdown of the Nimrod reservoir, the subsequent filling caused a large increase of young large-mouth bass (Micropterus salmoides) and whitebass (Morone chrysops), with a resultant decrease in a number of young channel catfish (Ictalurus punctatus), carp (Cyprinus carpio), drum (Aplodinotus grunniens) and buffalo (Ictiobus sp.). Shields (1958) found the water drawdowns of 1 1/2 ft to 2 ft in Fort Randall reservoir, South Dakota, following the peak of carp spawning in late spring showed an effectiveness in limiting its production. Yakovleva (1969) stated that the fluctuations of water level at specific periods possibly improve conditions and increase the breeding efficiency of the fishes. A reduction of water level in Volgograd reservoir, USSR, by 1.5 m to 2.0 m from normal preserve level in summer led to improving the breeding conditions of fishes throughout the reservoir in the spring. Allen (1970) has suggested that the water level in reservoirs must be maintained or slightly increased during the spawning activity of desirable fishes. A level between reasonable limits is maintained for about two to three weeks or until the spawning is completed. This will avoid exposing the eggs to an extreme low level with a risk of stranding, or submerging the eggs in too deep water.
Although the manipulation of water level fluctuations in reservoirs has shown very good promise for the management of fish populations and aquatic weeds, it may be considered as an impractical measure to apply for reservoirs in Southeast Asia where the priority objectives of reservoir construction are to provide water for power generation and irrigation. Water is therefore preferentially used to fulfill these purposes. However, a multiple use of reservoirs including a possibility of future fisheries development should be considered prior to the final decision on building a new reservoir.
The annual water level fluctuations in reservoirs in Southeast Asia already create benefits for the fisheries. Water level starts rising with the onset of the rainy season and a high water is maintained till the dry season. As water starts flooding land, which is usually covered with grasses and shrubs, the fish start spawning. Spawning occurs throughout the rainy and flood seasons. Fry and fingerlings feed in shallows with abundant food resulting from the newly flooded land and decomposition of flooded vegetation which releases nutrients to water and encourages the bloom of phytoplankton, followed by zooplankton and benthos. These conditions lead to a good survival rate of young fish as well as a good growth rate. With the onset of the dry season, water level starts decreasing as a result of the diminished inflow, power generation and irrigation, resulting in the discharge rate exceeding the rate of inflow. The lowest water level is reached prior to the next floods. As the water level decreases, fish migrate to deeper zones. Aquatic vegetation, particularly submerged species, become stranded and dry out. The drawdown area may start overgrowing with terrestrial vegetation.
Parasites and diseases of fish may be one factor contributing to the fishery decline in lakes and reservoirs. They result directly in fish kills, and affect the productivity of the stock through changes in growth rates and reproductive capacity of the individuals. Fernando (1965) noted that the mortality of Glossogobius giurus and several carnivorous fishes in reservoirs in Sri Lanka was a result of heavy infestation with larvae of nematodes (Hedruris sp. and Eustrongylides sp.). Bailey et al. (1978) found that the pericardial cavities of cichlids in Nyumba Ya Mungu reservoir in Tanzania commonly contained larvae of the nematode (Contracaecum sp.). The incidence of infection in a sample of 184 tilapias (Sarotherodon jipe, S. pangani and S. esculentus, Tilapia rendalli) was 70.6%. The mean number of worms per fish was 4 and the highest individual count was 14. Haplochromis sp. was less frequently parasitized, nematodes occurring in only 13.8% of 116 pericardial cavities examined. Tilapias are presumably infected as juveniles when planktonic crustaceans, which act as the first host, may be included in the diet. The parasite's life cycle is completed in fish-eating birds. It is not known what effect this infestation has on the fish.
Hoffman and Bauer (1971) quoted a report of Reshetnikova, who described the effect of a parasite, Digramma interrupta, on the productivity of bream in the Cimlyansk reservoir, USSR. This parasite caused mortality of young fish, growth retardation and sexual sterilization of adult fish. The losses were about 500–700 t/yr (12–14% of the total catch).
The parasitofauna of fish in reservoirs forms along with the establishment of fish and populations of other aquatic organisms. Izyumova (1964) commented that there was a certain decrease of the parasitofauna of fish during the first years of reservoir existence, particularly parasites that were bound to intermediate hosts. She reasoned that this might be a result of host shortage in newly established reservoirs, because zooplankton, benthos and other macroinvertebrates are still in the developmental phases. Becker and Evans (1967) reported for the temperate waters that snails, amphipods, copepods and ostracods, which serve as intermediate hosts for many species of helminths, evidently take a long period of time to adjust to the new environment. Thus, the incidence of infection of the helminths in their definitive hosts occurs at a lower rate, and development takes a longer period of time in newly created impoundments. Whether this is so for tropical impoundments is not known. Hoffman and Bauer (1971) stated that the period is relatively short (about one year) for Protozoa, Monogenea and helminths using copepods as intermediate hosts, and is longer (three to five years) for trematodes using snails and vertebrates as intermediate hosts. Whenever the intermediate hosts became established, the number of parasites bound to these intermediate hosts increased. This indicates that the process of formation of the parasite fauna of fish usually lags behind their intermediate host formations. For instance, the formation of zooplankton and benthos populations in the Rybinsk reservoir in the Soviet Union was completed during the first 6–8 years of its existence, whereas the parasitofauna still continued their development in the following years (Izyumova, 1964). She also indicated that changes of environmental conditions in a reservoir following the impoundment, such as slow current and higher temperature of impounded water, have a tendency to favour the multiplication of parasites and also their invasion of fishes. In another example, the parasitofauna in large southern plain reservoirs in the USSR became stabilized in about 10–12 years after impoundment (Hoffman and Bauer, 1971).
The fish parasites that succeed in a reservoir usually derive from various sources. They may stem from:
fish parasites present in the rivers, streams, ponds and other water bodies which are inundated after impoundment;
fish parasites which were added with introducing new fish species to the reservoir;
fish parasites which were transferred by other animals such as helminth eggs being distributed by fish-eating birds, and
fish parasites present in the watershed and being distributed into the impoundment by means such as surface runoff.
The number of parasites in a reservoir tends to increase directly with age of the impoundment.
Hoffman and Bauer (1971) recommended that for obtaining precise data about the species composition and density of parasites in a given reservoir, extensive investigations should be made in both pre- and post-impoundment periods. A continuous study is required for the first three or four years; thereafter, a biennial and triennial survey is necessary. It is also important to maintain the same sampling stations, collect during the same seasons, and take samples of standard size.
In general, an abundance or outbreak of a given parasite is influenced by environmental factors, either abiotic or biotic (Bauer, 1962). The abiotic factors include temperature, light, pressure of water column, oxygen content of water, hydrogen-ion concentration and salinity. The biotic factors are based on the mutual relationship between the parasite and other living organisms. Among abiotic factors, temperature is considered most important. It determines not only the distribution of certain parasites and the character of its life cycle, but also affects their abundance. For example, the extent of trematodes, nematodes, cestodes, acanthocephalans and parasitic copepods on fishes in Wikes reservoir, Texas, was a result of thermal effluent drained into the reservoir which kept temperature at or near optimum conditions for their activity all year round (Smith, 1972). Other abiotic factors have more limited effects in freshwater reservoirs. Although the biotic factors undoubtedly play an important role in the ecology of fish parasites, unfortunately their roles have been so poorly studied, it is difficult to evaluate their effects. Further experiments and observations are needed.
The presence of fish parasites in reservoirs has been reported by many investigators. For instance, Iskov and Koval (1965) studied the parasitofauna of fish in both the upper and lower reaches of the Kakhovskoe reservoir, USSR. They found the fish parasites in this reservoir comprised 79 species, of which 41 species were common in both regions. The most dangerous parasites are: Glugea luciopercae, Cotylurus spp., Diplostomulum spp., Bothriocephalus gowkongensis, Erga silus boldi, Tracheliastes maculatus, Ligula intestinalis, Digramma interrupta and Henneguya cutanea longicauda. Devaraj and Ranganathan (1967) observed the incidence of Isoparorchis hypselobagri (Trematoda) among the catfishes (Wallago attu, Callichrous bimaculatus, Mystus aor) of the Bhavanisagar reservoir, India. An early post-impoundment investigation of fish parasites of Volta Lake, Ghana, revealed that fishes are heavily infected with parasites. Those found were: Trichodina, Myxobolus, Thelohanellus, Henneguya, Dactylogyridae, Gyrodactylidae, Ergasilidae, Lamproglena and Argulus (Paperna, 1968). About one fifth (21.9%) of fish taken from Nungua pond were parasitized with Clinostomum sp., Lernea sp., Ergasilus sp. and pentastomid larvae (Prah, 1969).
In North America serious problems are caused by Ichthyophthirius, Lernea, monogenetic trematodes and Argulus (Hoffman and Bauer, 1971). Becker and Evans (1967) reported that the parasitic copepod, Achtheres micropteri, in the host, Micropterus salmoides, seemed to adapt itself from the river to the lake environment sooner than parasites with intermediate hosts in their life cycle. Spall and Summerfelt (1969) studied host-parasite relations of the endoparasitic helminths of the channel catfish and white crappie in an Oklahoma reservoir. The ontogenetic change in the food habits of channel catfish, from a diet of invertebrates to fish, was apparently the reason for changes in the occurrence of many enteric helminths.
The feasibility of the control of parasites of fishes presents complicated problems in reservoir fishery management. Almost nothing is known about the control of dangerous fish parasites in large bodies of waters. However, prevention and some means of control can be achieved in various circumstances. Hoffman and Bauer (1971) recommended that the fish parasites present in the watershed be determined before the dam is constructed. If dangerous parasites are found and the water bodies are small, possible control may be done by fish eradication and chemical disinfection. In some cases, where the control or eradication of parasites in an existing watershed would be economically impossible, the best solution seems to be the control of the parasites of fish introduced into the reservoir from hatchery ponds. This can be accomplished by checking hatchery fish for parasites before they are introduced into the reservoir and by controlling intermediate hosts in the hatchery ponds where it would be economically feasible (Becker, Heard and Holmes, 1966).
It appears that biological control methods have promise in controlling parasitic diseases of fish in reservoirs. The methods are based on an accurate knowledge of the biology and ecology of both parasite and host.
The abundance of intermediate hosts in reservoirs may, apparently, be regulated by changing the types of aquatic animals in a given reservoir. It appears that an abundance of the intermediate invertebrate hosts in a reservoir can be reduced by introducing aquatic animals feeding largely on these invertebrates. In some cases it might be possible to stock fish species which are not hosts of the parasites already present in the reservoirs. Bauer (1962) gave the example that wild carp should be introduced into reservoirs to replace the bream where their stocks suffer from ligulosis. This measure is believed to decrease the disease in bream. Some parasites that develop in second intermediate or additional hosts can be decreased through fishery management. This method calls for the reduction in numbers of fish heavily infected by a given parasite. Intensive catch of pike (Esox lucius) and burbot (Lota lota) was recommended as an active means for controlling diphyllobothriasis in Soviet reservoirs. Since these two fishes are the main carriers of broad tapeworm plerocercoids, a decrease in their abundance in the reservoirs would cause a decrease in the diphyllobothriasis.
It is very interesting to know that some aquatic invertebrates and fish feed on fish parasites, thus, the introduction of these animals into reservoirs could limit parasite abundance. Minnows (Phoxinus spp.) were used effectively to control mature Argulus and their larvae (Bauer, 1962). Hoffman and Bauer (1971) had reviewed several Soviet publications on this measure; for example, the ligulosis and digrammosis of carp in Cimlyansk reservoir could be controlled by extensively harvesting the young infected fish. The idea was practicable because the infected young fish remained isolated from healthy ones and they gathered in coves and creeks of the reservoir. Furthermore, ligulosis can be controlled by increasing the population of pikeperch. This fish was found to intensively feed on infected fish, and such a method was successful in the Simferopol reservoir and the Karachunskoe reservoir.
Freshwater molluscs and oligochaetes also served as intermediate hosts for several fish parasites. It is possible to control these intermediate hosts by regulating the water level. Bauer (1962) stated that if the water level in reservoirs were rapidly decreased when large numbers of snails are developing, most pulmonate molluscs would be stranded and could then be raked away. If the level could be kept low for several days, most of the snails would die. As fisheries is usually secondary to other uses of water reservoirs, this method may be difficult to apply. Devaraj and Ranganathan (1967) reported that the snails (Melanoides scabra and M. tuberculata) serve as the intermediate hosts for Isoparorchis hypselobagri in the Bhavanisagar reservoir in India. They recommended that the most feasible method to reduce the snails in this reservoir was by introducing a snail-eating catfish (Pangasius pangasius) into the reservoir to feed on them.
Furthermore, fish-eating birds also serve as definitive hosts for several fish parasites such as Ligula, Postodiplostomum, Hysteromorpha, and similar helminths (Bauer, 1962). These parasites become sexually mature and begin to produce eggs in the birds' intestines only a few days after the birds become infected. The eggs thus begin to be dispersed very soon after the birds arrive at a reservoir. Therefore, it is desirable to control them by any means before they can infect the invertebrates with a new generation of parasites.
Elimination of fish parasites and diseases in reservoirs is a worldwide problem. For reservoirs in Southeast Asia and the Indian subcontinent the most suitable method of controlling may involve the reduction of intermediate hosts of parasites and diseases by using biological control. This can be done with the introduction of fishes that feed directly on intermediate hosts. For instance, freshwater snails have been found as the intermediate hosts of several parasites and diseases. They can be controlled by introducing certain species of fishes such as the catfish (Pangasius pangasius, P. sutchi), carps (Probarbus jullieni, Cyelocheilichthys enoplos), etc. Another effective measure is controlling parasites and diseases of fishes introduced into the reservoirs. Before stocking, fish must be checked to ensure that all introduced fishes are free of parasites and diseases.
