Ministry of Fisheries, Sri Lanka
Purpose of stocking reservoirs
Stocking is a powerful tool in the management of reservoirs. The extent and periodicity of stocking and the species to be stocked should be determined by ecological considerations. Some of the objectives of stocking are: (i) to supplement fish populations where natural populations of species of economic importance do not exist or where repopulation is inadequate; (ii) to maintain fish populations in ecosystems lacking spawning sites, nursery grounds, etc.; (iii) to add new species to an existing fish population complex, and (iv) to restock as replacement in areas where species have declined due to changing conditions. Introduction of new species help to fill unoccupied ecological niches, to add superior species of higher market value, or to control forage or trash fish which compete for food with more valuable species.
The important qualifications for candidate species for introduction and stocking are: rapid growth, non-competitiveness, non-cannibalistic habits, low susceptibility to diseases, hardiness, high fecundity and breeding success, tolerance to hydrostatic pressure in deep reservoirs, keeping quality, culinary properties, etc. A supply of seed must also be available in close proximity to the reservoir.
In most of the countries, especially in India and Sri Lanka, an adequate supply of fish seed is still a limiting factor in fisheries development. Concerted action is being taken to boost fingerling production, mainly through artificial breeding techniques. Because of the acute shortage of seed, it is imperative that the available fingerlings are used judiciously and not wasted in unplanned stocking. Repeated annual stocking is an expensive exercise which should be justified only by adequate recaptures.
With this objective in view, the results of stocking and fish production in a representative sample of Indian reservoirs is presented here.
Fish population changes in reservoirs
Stocking programmes have to be based on anticipated alterations in the complex fish communities following the impounding of rivers. When a lotic system is dammed, a lacustrine system (but not a strictly lentic system) develops, producing changes in the fish populations. This warrants a pre-impoundment assessment of fish population and planning of fish transplantations. Such a study was made by Chacko and Dinamani (1949) in India on the River Bhavani. Earlier Sunderaraj (1942) made observations on the Stanley Reservoir (Mettur Dam), Tamilnadu, and transplanted Catla catla from the Gangetic system to the Cauvery system. This pioneering action produced spectacular results in that in large measure it made up for the loss of the anadromous migratory Hilsa fishery of the Cauvery system. Strictly fluviatile species like the mahseer (Tor khudree) and Acrossocheilus (Barbus) hexagonolepis were doomed to disappear but the extinction of the native and predominant Puntius dubius was unexpected. From forming 28 percent of the catch during 1943–44, this species was cut down to 0.33 percent of catch in a decade and subsequently disappeared. Another native, Cirrhinus cirrhosa was reduced to less than 2 percent of the catch from 1970 to 1971 onward, whereas it formed 20–47 percent of the catch between 1943 and 1966. Two other native carps of the Cauvery, Labeo kontius and L. fimbriatus, were also reduced to insignificant status in this reservoir. The eels, Anguilla sp. continue to exist in this reservoir. Two major catfishes, Pangasius pangasius and Silonia silondia dwindled signficantly while two others, Mystus aor and Wallago attu increased signficantly, contributing to not less than 35 percent of yield. P. carnatius and C. reba also disappeared.
Bhavanisagar Reservoir (upstream of Stanley Reservoir) was formed by damming the Bhawani and Moyan Rivers below their confluence. As in Stanley Reservoir, both the mahseer and A. hexagonolepis more or less disappeared. C. cirrhosa was reduced to an insignificant level. P. dubius decreased but did not disappear as its breeding grounds were not submerged. As in Stanley, W. attu and Mystus increased to a significnat level of over 25 percent of catches (Sreenivasan, 1976).
In Sathanur Reservoir, Tamilnadu, there were practically no native valuable species except L. fimbriatus, associated with Wallago attu and Notopterus notopterus. During the initial years after damming, L. fimbriatus dominated the catches, yielding up to 36 percent in 1964–65, but subsequently decreased to less than 1 percent (Sreenivasan and Chandrasekaran, 1980).
Amaravathy Reservoir had a rich indigenous fishery of P. dubius, T. khudree, P. carnaticus, etc., and of less important species like Barilius sp., minor catfishes, etc., Puntius dubius formed 55 percent of the catch until 1964–65 but dwindled and later disappeared. The Amaravathy River is also a tributary of the Cauvery.
In the northern Indian reservoirs such changes have also been noted. Among the Damodar Valley Corporation (DVC) reservoirs, Wallago attu increased in abundance but data on the other DVC reservoirs is meagre. In the large Gobindsagar, the mahseer decreased drastically from 10.41 percent in 1977/78 to 0.42 percent in 1980 (the same pattern as in southern Indian reservoirs). The native fish L. dero (not an economically valuable major carp) continues to maintain its population. In Nagarjunasagar, Andhra Pradesh, the catfish population multiplied tremendously, exceeding 60 percent of catch and the indigenous L. fimbriatus still contributes a sizeable percentage to the yield.
Impact of stocking on fish yield in reservoirs
There are valuable indigenous species in many of the major Indian river systems, of which the principal ones are: catla, rohu, mrigal, L. calbasu in the north; L. fimbriatus, C. cirrhosa, P. dubius, L. kontius in the south. Catfishes such as Wallago attu, Mystus aor, M. seenghala, Pangasius pangasius, Silonia silondia, S. childreni and other minor species also are abundant, in most of the river systems. In the upper reaches, the mahseers (Tor tor, T. khudree, T. putitora), Acrossocheilus (Barbus) hexagonolepis, P. carnaticus etc. are important.
Post impoundment changes have to be anticipated by an early transplantion of useful species. This was exactly what was done in Stanley Reservoir (even before impoundment) and in Bhavanisagar (during the impoundment stage). This is an essential step in planning reservoir fisheries development as fish ladders or passes are totally unnecessary and expensive under our conditions.
IMPACT OF NEW INTRODUCTIONS
Stanley Reservoir (Mettur Dam), Tamilnadu
This is the largest and earliest manmade lake in Tamilnadu. Development of the fisheries was scientifically planned (Sunderaraj, 1942). Even before the sealing of the dam in 1934, a few catla obtained from the Gangetic system were planted in the upper reaches (between 1922 and 1935, only 10 000 catla were introduced). These formed the nucleus of subsequent catches of millions of catla in the reservoir. No intensive stocking was done with the species which established a population of its own by successful breeding. Of course, in certain years failure of breeding caused by meteorological and hydrographic factors lead to a downtrend in catches but by and large, a self-sustaining population was established. This species became important in reservoirs generally both in terms of weight and earning of the fishermen. Catla contributed 15.98 percent of the yield in 1943–44 and in succeeding years rose to reach a peak of 39.60 percent in 1961/62. Thus the initial stocking of catla was a great success in Stanley Reservoir. Rohu, mrigal and L. calbasu were also transplanted in the early fifties and more than made up for the loss of the native species. From 1966/67 onward they have contributed substantially to the catches. The introduction of giant guramy and chanos was unsuccessful but the estuarine cichlid Etroplus suratensis established itself to contribute to a small fishery, finding a good market in neighbouring Kerala State. This species has not been stocked for nearly three decades and yet the species is still landed. Eels (Anguilla sp.) are also caught even though dams and other barriers impede their migration. Catfishes contribute to 35–50 percent of reservoir production in Mettur although they are not stocked. Stocking of the following species had no impact at all in their yields in Stanley Reservoir: L. fimbriatus (over 2 million fingerlings planted between 1972/73 and 1975/76 alone), L. kontius (432 663), P. carnaticus (355 505) and tens of thousands of C. reba and P. dubius. During the same period over a million common carp were stocked which also proved futile, possibly because in deep reservoirs with a dense predator population, this sluggish fish may not survive. By contrast, in large static, seasonal tanks (e.g., in Madurai district) common carp has been a success and in great demand for stocking. From 1978/79 to 1982/83 the wasteful stocking of Stanley Reservoir was repeated - L. fimbriatus (889 512), L. kontius (298 566), P. carnaticus (1 076 855), C. reba (1 563 563), A. hexagonolepis and P. dubius - some of which were recaptured. L. fimbriatus however, still forms the basis for a fishery in the river system downstream.
Fish yields ranged from 9.75 to 42.8 kg/ha (average 29.49 kg/ha) but conversion of primary production to fish biomass was low, mainly because of the high percentage of predatory species occupying the higher trophic levels. Control of predatory species by intensive selective fishing is necessary to improve catches of other species.
Bhavanisagar Reservoir, Tamilnadu
Stocking was initiated in 1953 when the dam was being sealed. Catla, rohu and mrigal were introduced from northern India and have since contributed to the catch without making a major impact. However, L. calbasu, which was transplanted in 1962/63 has been making a spectacular contribution to production. It has supported a fishery for over two decades, with only limited stocking and has been yielding over 40 percent of the total catch. This has more than offset the disappearance of C. cirrhosa and decrease in abundance of P. dubius. Only 4 618 fingerlings of mrigal were stocked in the nine years prior to 1962–63. Rohu and mrigal may breed to a limited extent so stocked fish seem to be the main source of the catch. The two unstocked catfishes, W. attu and M. aor contribute a significant portion of production (about 30 percent) and selective fishing is suggested for reducing their population. Over 1.6 million common carp were planted over a period of seven years from 1970/71 to 1978/79 and more than 63 000 mirror carp (German carp) were planted earlier (1955/56). However, only a few hundred well grown specimens have been recaptured. Heavy stocking of L. fimbriatus also failed to make any impact on the yield. Over 215 730 were planted during 1953 to 1966/67 and a further 833 520 during 1972/73 to 1978/79 but very few were caught. Being indigenous, they should have given high production without stocking if they could breed and survive in the altered environment. Etroplus suratensis was not a success in this reservoir. 123 030 were stocked between 1953/54 to 1960/61 but only stray specimens were caught. Inadvertent entrants like Oreochromis (= tilapia) and Rhinomugil corsula have made an appearance in recent years. L. bata and P. sarana have been landed in noticeable quantities in the last four years because of the use of small-meshed gillnets. These native species are of very low economic value. Fish production ranged from 12.28 to 43.51 kg/ha with an average of 24.38 kg/ha.
Sathanur Reservoir, Tamilnadu
This hardwater impoundment is the second of a cascade series on the Ponniar River, which had practically no indigenous fishery of importance except for L. fimbriatus and W. attu and, to a lesser extent, Notopterus notopterus. The first introduction was made as soon as the dam was closed in 1957. Oreochromis mossambicus and a few catla were planted but the main species were 2.03 million individuals of C. cirrhosa, L. kontius, C. reba and also L. fimbriatus from the Cauvery system. Although recaptures were not high the cost of stocking was recouped by the sale of catches. Large-scale stocking with 50 000 catlas was done in 1969/70 but this species was already prominent in the catches. Catla was not subsequently stocked except for 2 000 in 1976/77. The first stocking was evidently sufficient as yields of catla increased until it became the single dominant species, forming over 80 percent of the weight of the fish landed. It is quite obvious that a very successful breeding population was established and that the ecological conditions favoured the spawning of catla. Mrigal (50 000) and rohu (12 300) were stocked in 1968/69 and in succeeding years. Labeo fimbriatus initially dominant at 36 percent of the catch in 1964/65 was reduced to less than 1 percent in subsequent years. During the earlier years, tilapias were also stocked but failed to establish a stable dominant position. Rhinomugil corsula, which escaped from the upstream Krishnagiri Reservoir, were observed in the shallow margins but were not caught in the large meshed nets nor formed an important fishery (as opposed to some other reservoirs like Krishnagiri, Vaigai, Uppar, etc.). Rohu, mrigal, L. fimbriatus, C. cirrhosa and L. calbasu were not being caught in adequate numbers (less than 10 recaptured) to make stocking economical (a yearly stocking of between 0.5 and 1 million fingerlings). This failure was due rather to the use of large meshed gillnets to selectively fish for catla which fetched more money than the other carps than to any failure in survival. When fishermen used smaller meshed gillnets, catla catches dropped to about 50 percent while the proportion of mrigal, rohu, C. cirrhosa and L. fimbriatus rose significantly. This exemplifies appropriate management practices as all stocked species are now being recaptured except for C. carpio. For obvious reasons, a multi-species fishery is preferable, since “collapse” of a mono-species fishery will also ruin the total catch. Fish yield has been in the range of 83.07 to 170.86 kg/ha/year (average 126.02 kg/ha).
Tilapia was stocked along with the heterogenous Cauvery carp community as soon as the dam was closed in 1957. Common carp and mirror carp were also stocked. Tilapia increased from nil in 1958 to 16 percent in 1964/65, reaching a peak of 90 percent in 1973/74 and has since maintained this level. In fact, this reservoir maintained an almost mono-specific tilapia fishery, despite continuous heavy stocking with carps. The indigenous P. dubius, which contributed up to 55 percent of the catch until 1964/65, dwindled and disappeared. P. carnaticus had a similar fate. Due to very low recapture rates, stocking with catla, rohu, mrigal and L. calbasu seemed uneconomical. The common carp fared better, with its high primary productivity and permanent bloom of bluegreens (mainly Microcystis). This reservoir should be ideal for silver carp and also the catla species from Rihand, which feeds on Microcystis. The average weight of tilapia was 1.0 kg initially but has now dropped to 300 g. Intensive removal of tilapia and stocking of herbivore carps would maintain the yield and fetch more money. Good fish production was achieved - 75.64–163.54 kg/ha (average 109.65 kg/ha).
Other reservoirs in Tamilnadu
Krishnagiri Reservoir: This reservoir upstream of Sathanur Dam has the same area but is much shallower. The indigenous species were uneconomic: P. sarana, Ompok bimaculatus, M. vittatus, Notopterus sp., murrels, etc. A year after closure (1957/58) catla, rohu, mrigal, L. fimbriatus, C. cirrhosa, C. reba, C. carpio, L. calbasu, P. dubius, L. kontius, Rhinomugil corsula and Oreochromis mossambicus were stocked. The stocking rate was high, 420–1 248/ha/year (average 870) and until 1964/65 major carps constituted only 0.3 to 0.6 percent of fingerlings stocked. Later the proportion of carps was raised to 50 percent. Initially, the major carps contributed less than 1 percent of the catch but from 1965/66 they increased and reached 50 percent of production by 1972/73. This was a healthy trend, indicating the success of proper stocking. But the recapture rate did not correspond to the intensity of stocking. Introduction of R. corsula, upset the balance by its dominating the fishery -it yielded 46.6 percent in 1969–70 but was reduced to 8 percent in 1972/73 by intensive fishing. The present yield is only about 10.6 kg/ha/year with a range of 8.61 to 13.00 kg/ha.
Vaigai Reservoir: This is also a shallow reservoir but turbid with silt for the major part of the year. The stocking rate ranged from 160 to 560 average 370 fish/ha/year. Major carps formed 7–30 percent of fingerlings stocked. During certain years, catla, mrigal and L. fimbriatus showed good production and the recovery of stocked fish was satisfactory. For many years, tilapia dominated the catches, when its abundance was inversely proportional to carps. In later years R. corsula also appeared and this had a depressing effect on tilapia yields (as in Krishnagiri). Heavy stocking, as in 1976/77 when 867 200 fingerlings were planted, is wasteful as recovery was insignificant. When the production was highest (45 800 kg) 99 percent of the yield was from tilapia. When the major carp Catla dominated the catches, (45.82 percent in 1967/68, 41.27 percent in 1968/69 and 39.77 percent in 1969/70) the yield of tilapia was less than 10 percent. Mullets, then appeared in the catches - 1.26, 19.98 and 14.72 percent, respectively. During the years 1969/70 to 1971/72 mrigal and C. cirrhosa together contributed to 25.29, 55.44 and 69.26 percent, respectively of total catch. Similarly in 1973/74 they accounted for 58.79 percent of the yield. It is clear that with correct stocking, recapture would be assured. During 1978/79, carps yielded 30 percent of the catch. The average yield is only 6.9 kg/ha, with a range of 2.16–18.8 kg/ha.
Tirumoorthy Reservoir: This is a moderately productive small reservoir at the tail end of the Parmabikulam Aliyar system. Stocking was done as soon as the dam was completed (1966/67). The stocking rate ranged from 260 to 970/ha/year. Only 4 200 tilapia were stocked that year but in two years, 14 500 kg per annum was produced with an average size of 1.6 kg each, which is a commendable size for O. mossambicus. After reaching a peak of 25 000 kg in 1970/71 the catches tapered off (CPUE also decreased) coincident with a drastic reduction in the average size of tilapia to 0.5 kg. The growth of carp was good, though recaptures were poor. Labeo fimbriatus, mrigal and common carp, however, contributed 20–30 percent of the yield (47 percent in 1970/71). Proper stocking of these species will improve their recapture rates. In the absence of predators, herbivore stocking will also give higher yields. The average yield now is 39 kg/ha/year but higher yields (54.72 kg/ha) have also been obtained.
Aliyar Reservoir: This is in close proximity to Tirumoorthy but with a fishery dominated by the indigenous P. dubius and L. bata (which declined in succeeding years). High stocking rates were adopted (310 to 1 270 per annum/ha, average 700). Tilapia did not make any headway unlike in the nearby Tirumoorthy or Amaravathy reservoirs. P. dubius still accounts for over 50 percent of production. Among the stocked species, mrigal, L. fimbriatus and common carp have been recaptured in modest quantities not commensurate with the heavy planting. Increase in fishing effort and the use of proper gear could possibly capture more of these. The average yield in the last decade was 18 kg/ha but lately it has reached 37 kg/ha.
Uppar Reservoir: This small shallow reservoir has been giving a high production of introduced tilapia reaching a peak of 120 350 kg in 1979/80 (over 270 kg/ha). From 1971/72 to 1975/76, tilapia contributed 63.08–87.1 percent of the yield, common carp ranking second and L. fimbriatus third. Rohu and mrigal were not recaptured in noticeable quantities. From 1975/76 mullets (escapees from Tirumoorthy Reservoir upstream) began appearing to yield 10.65 percent of the catch. By 1981/82, they attained 46.25 percent (26 175 kg), when tilapia fell to 27.4 percent. This inverse relationship between mullet and tilapia abundance has been noted frequently - in Vaigai, Tirumoorthy, Krishnagiri, Uppar Reservoirs. The mullets always move in groups in the littoral zones and may be interfering with the nesting of tilapia, which build breeding pits in the shallow margins. Recently there has been a satisfactory recovery of the stocked major carps (catla 6.37 percent, common carp 10.18 percent, mrigal 2.64 percent of total yields) and with proper management this increases further.
Manjalar Reservoir, Tamilnadu: This small reservoir hosts a predominantly tilapia fishery - over 80 percent of production. This species is well distributed all over Madurai district. Repeated heavy stocking of major carps and common carps gave disappointingly low recaptures. Until the tilapia population is controlled, the intensity of carp stocking has to be reduced to avoid waste of carp seed and money. Production ranged from 85.08 to 187.41 kg/ha/year (average 123.95 kg/ha).
Reservoirs outside Tamilnadu
Gandhisagar Reservoir, Madhya Pradesh: Stocking and harvest have been thoroughly documented by Dubey and Chatterjee (1977). Catla and rohu, which were not native to these waters, were stocked in 1959/60. Mrigal and L. calbasu were present even before stocking but L. calbasu declined from 25 percent of the catch in 1963/64 to 7.55 percent in 1975/76 while mrigal was reduced from a high 55 percent in 1963/64 to a low 4.83 percent in 1975/76. The mahseer, Tor tor, also decreased but continued to maintain a limited population and has not been eliminated as it has in southern Indian reservoirs.
The stocking rate was low compared to many southern Indian reservoirs (0.12 to 12.1 per ha/year). In the course of 16 years, a total of 5.3 million fingerlings were planted but quality-wise, they were made up of fast-growing catla, rohu, mrigal and also common carp. Of these, 2 million were catla, 1.27 million rohu and 1.10 million mrigal. The yield of catla rose from 2.3 percent of the catch in 1964/65 to over 60 pecent from 1969/70 and it is clear that there is a natural repopulation of this species. Intensive stocking is thus not warranted. Rohu contributes less than 1 percent of production in most years, despite heavy stocking. Obviously a breeding population is not established and the cost of stocking this species is not recouped. Mrigal is decreasing in the catches (from 55 to 4.83 percent between 1963/64 to 1975/76) and there is only a marginal recovery of the cost of stocking this species. L. calbasu is another species which is dwindling in catches (from 25 to 7.55 percent in the same period). It is not known why this indigenous species should decrease while in Bhavanisagar, where it was transplanted, it became the single predominant species. Common carp also failed to establish itself. The average weight of fish landed (Choudhury, 1977) are as follows: catla, 9.05 kg; rohu 3.29 kg; mrigal 1.98 kg; L. calbasu 1.24 kg, etc. It is possible that large-meshed nets are being used to capture catla, which fetches more money and thus, mrigal, L. calbasu, etc., are missed. The cost of stocking is insignificant in relation to the income from fish produced. But the fish yield was still low - 0.5 to 10.56 kg/ha/year.
Govindsagar, H.P.: This is a deep, comparatively cool reservoir. The average number stocked was 584 000 a year from 1962 to 1973 and 508 000 from 1973 to 1976, i.e., a rate of 34.6 to 30.1 fingerlings/ha/year. The predominant species stocked was common carp. A total of 2 255 029 common carp and 169 000 major carps were transplanted in the three years from 1977 to 1980. Catla formed 29.61 percent of the catch in 1976/77 but decreased to 12.46 percent by 1980, may be due to competition with silver carp. The population of rohu was stable - 17.55 to 25.81 percent and mrigal contributed 5.85 to 16.45 percent of the catch. Over the years mahseer declined and L. dero, the native species, shows signs of decline.
In years when natural spawning is very successful due to favourable weather, any fish stocked make a negligible contribution to the total stock (Watt, 1968). Common carp contributed the major part of the production in this reservoir. From 81 000 kg in 1975/76 (14.83 percent) it rose to 228 860 kg (38.15 percent) in the first half of 1980/81 (ICAR, 1980). This high production was not only due to stocking as natural repopulation has also contributed. Catla, rohu and mrigal have also been yielding a sizeable catch without much stocking and seem to have established self-sustaining populations. Furthermore, a few hundred silver carp that escaped from fishfarm ponds into the reservoir established a self-generating population which contributed (first half) 75 270 kg (12.55 percent of catch) by 1980/81. This is the only reservoir in India where such large self-propagating populations of common carp and silver carp have been recorded.
It appears that the cooler climate and possibly hydrographic features are conducive to the breeding and repopulation in Govindsagar. The yield ranged from 28.1 to 44.0 kg/ha/year, which is much better than in most other Indian reservoirs. What is more, all the species are of economic importance and hence in terms of revenue, high income species.
Tungabhadra Resevoir, A.P.: The introduced gangetic species fared very badly in this reservoir, contributing between 0.1 to 2.4 percent of production up to 1969 (David and Rajagopal, 1969) and the situation does not seem to have improved. The uneconomic P. kolus yielded over 50 percent of the catch while the catfish M. seenghala yielded 44.8 percent. The reservoir fisheries have not improved in spite of stocking with valuable species from an induced fish breeding station. The Microcystis- feeding catla from Rihand may be transplanted here.
Nagarjunasagar Reservoir, A.P.: Stocking rates have been low in this large reservoir, ranging from 50 100 (1970/71) to 703 700 (1976/77) fingerlings and 312 000 during 1978/79. Catla, rohu, mrigal and common carp were stocked at a range of 105 to 35.8 fingerlings/ha/year. After yielding 20.25 percent of total catch during 1972/73, catla virtually disappeared despite stocking. Rohu and mrigal fared worse, not contributing even 1 percent to the production. Common carp failed to register in the catches despite annual stocking. Labeo fimbriatus, however, contributed to production without adequate stocking but is decreasing slightly now. Very significant increases in abundance of the indigenous, unstocked catfishes, M. tor, Pangasius spp. and Silonia childreni which contributed as much as 70.86 percent of catch in some years might depress the carp population. Selective fishing and removal of catfish may prove helpful in increasing the carp fishery and consequently the total production. The yield ranged from 32 320 kg in 1971/72 to 190 770 kg in 1979/80 (i.e., 1.75 to 10.35 kg/year) which is poor.
Rihand Reservoir: This has a predominantly catla fishery, catla comprising over 90 percent of the catch between 1971 and 1980. Stocking rates have been low - 8 703–2 265 000 fingerlings a year (0.19–48.7 fingerlings/ha/year) but have enabled a spawning population of catla to be established and give adequate returns. Production has ranged from 24 870–328 820 kg per year during 1971/72 to 1979/80 (0.53 to 7.06 kg/ha/year). The average yield is still low - 3.82 kg/ha. A fishery for mrigal, rohu and L. calbasu should be established through stocking with these species.
Vallabhsagar (Ukai), Gujarat: From 1977 to 1980 2.5 million catla, rohu and mrigal were stocked. The catch composition indicates that economic major carps are predominant, accounting for 80–88 percent of the catch. Catla yielded 16.55 to 20.55 percent, rohu 36–57.85 percent, mrigal 13.25–14.85 percent and Tor tor 6.55–7.91 percent.
The yield as such is not high, and works out to 2.29–3.98 kg/ha/year. But with such a predominance of herbivores and absence of predatory catfishes, production should be much more.
Damodar Valley Corporation (DVC) Reservoirs: These include Konar, Tilaiya, Maithon and Panchet, in all of which catla was the predominant although mrigal and rohu also contributed slightly. Stocking of 109 000 common carp in Konar made no impact but between 1957 and 1968 550 000 major carps were planted in this reservoir. There was evidence of breeding and recruitment of both rohu and mrigal (Jhingran and Natarajan, 1979) but while mrigal showed a spectacular increase in production from 1.3 to 38.78 percent (from 1963 to 1968) rohu did not make any significant contribution. Catla too established a breeding population although its contribution to the catch decreased from 97.22 to 33.38 percent in the same period. L. calbasu, however, increased from 1.07 percent of the catch in 1963/64 to 17.85 percent in 1964/64, 26.24 percent in 1965/66 and 16.31 percent in 1967/68. Total production was, however, low with an average of 11 830 kg/year.
Tilaiya Reservoir was stocked with 65.75 million fish seed between 1952 to 1968. Catla was the only successful species, yielding 88–89 percent of total catch during the years 1964–68. The total production of 24 000 kg/year was low.
In Maithon Reservoir, 4 862 000 fingerlings were stocked between 1958 and 1968. But this was not related to recapture or yield. Common carp was stocked in adequate numbers: 836 460, but a solitary 2-kg fish only was recaptured. In this reservoir, catla was the major species rising from 26.13 percent in 1961 to 80.06 percent in 1964/65 and 66.73 percent in 1967/68. Mrigal decreased from 51.06 percent in 1961 to 5.58 percent in 1964/65 but recovered subsequently to about 20 percent. Catfishes were of no significance.
Panchet Reservoir was stocked with very few fingerlings. The major carps yielded 86–92.4 percent of the catch. L. calbasu contributed 3.13–8.16 percent. There was a balanced distribution of species. From 1960 to 1967/68, yield of catla ranged from 20.4–45.28 percent, mrigal from 50.26-43.13 percent, rohu from 21.76-10.19 percent and L. calbasu from 7.58-3.13 percent. This was the one reservoir in the DVC group to have a significant population of Wallago, which accounted for 8.91 percent of the catch in 1967/68. In others, catfishes were insignificant. The yield was very poor-4 604 kg in 1960 but slightly better with a respectable 22 437 kg in 1967/68.
In all the DVC reservoirs, stocking has not contributed much to the production. Self-regenerating populations of catla and mrigal seem to have been established. Rohu and L. calbasu, however, may need support stocking to maintain their numbers.
Introductions of new fish species into impoundments play a very important role in increasing the productivity of the fisheries although unwise introductions can lead to undesirable consequences. Stocking, however, is more difficult to evaluate and always related to recaptures. The acute shortage of fish seed requires that existing resources should not be frittered away in indiscriminate, excessive or incompatible stocking in reservoirs, when they could as well have been stocked in ponds to produce more fish. For example, the stocking of the “Labeo and Cirrhina complex” (or Cauvery carps in Tamilnadu) used to be a ritual in southern India. These consisted of uneconomic species rather than major carps and the expense of stocking was never reflected in the harvest.
For instance, not even half the cost of the fingerlings stocked has been recovered through the sale of fish (all species caught) in 22 years in Manimuthar. In this Tamilnadu reservoir the phytoplankton is dominated by bluegreen algae and herbivorous species would do well. Catla, when stocked, has yielded moderate catches. It is being spawned artificially every year at the dam site and adequate numbers of fingerlings are available. Krishnagiri is another instance where for many years cost of stocking was more than double the return from the sale of fish. Elimination of trash fish, mullets, etc., and stocking of catla would rectify this situation. Stocking with common carp was not a success because of the absence of an adequate bottom fauna.
A classic example of successful anticipatory stocking is that done by Sunderaraj (1942) in Stanley Reservoir, where he introduced catla from the Gangetic system into the Cauvery system. Probably this is the earliest transplant of fish species within India, which profited the fishermen in the reservoir. Later, introductions of L. rohita, L. calbasu and mrigal also sustained a fishery and were economically successful. However, target-oriented stocking with species that are not recaptured was wasteful even in this reservoir. Insofar as L. rohita and L. calbasu are already established and contribute substantially to the catches, it is unnecessary to stock L. fimbriatus, L. kontius, etc., thereby incurring heavy expenditure. The major part of seed stocked was of uneconomic but cheaper varieties (costing less than I.Rs. 50 per 1 000) and there is a good margin of profit since in 1975/76 the average cost of fish sold rose to over I.Rs. 7 per kg.
Profitability has also been achieved in Bhavanisagar Reservoir by discriminatory stocking in recent years. Earlier fish seed was wasted because the species stocked were not recaptured. Despite this, fish production was profitable because 35 percent of the catch was of medium value catfishes having great local demand and over 40 percent of the catch was of high value carps which commanded high prices in Calcutta. In recent years the capture of P. sarana by the use of small-meshed nets has had a tendency to depress the market (in 1981/82, nearly 15 500 kg of this fish, whose keeping quality and transportability is poor, were caught).
Collection of natural spawn may indicate a trend to repopulation but not always. Species like L. fimbriatus and catla are noted in significant numbers in samples of natural spawn but were not reflected in catches, maybe due to poor survival. On the other hand, L. calbasu formed only 0.82 to 3.1 percent of spawn collected but contributed 35–40 percent of commercial landings. Research biologists monitor the size frequency, maturity and other aspects of the catch and thus enabled the reduction in stocking intensity and the choice of proper species for stocking. Support to catla, rohu, mrigal populations through stocking is needed in this reservoir.
The earliest record of stocking catla, rohu, mrigal in Sathanur Reservoir was November/December 1958. From 1958 to 1964 only 3 649 catla (adults) were stocked. But this was enough to establish a breeding population and provide a very profitable fishery for over two decades. Because of the pre-eminent position occupied by catla in the catches and its high unit value the income from this reservoir was high. The initial error of overlooking the production potential of stocked mrigal, rohu, L. calbasu, C. cirrhosa, etc., was rectified by operating suitable gear so that these were also captured. This reservoir gave maximum profits with minimum stocking. Planting of common carp, L. kontius, etc., has been stopped or reduced drastically.
There is no relationship between stocking and harvesting especially in reservoirs dominated by either O. mossambicus or R. corsula. Gophen et al. (1983) state that the influence of stocking on the catch of Oreochromis is not clear. Two outstanding examples are Uppar and Manjalar reservoirs in Tamilnadu. In the latter, as many as 3 848 253 fingerlings of carps were stocked between 1968 and 1977 but over 80 percent of fish yield was tilapia, less than 0.25 percent of carp being recaptured. For seven years (1968–75) 2.581 million fingerlings were planted at a cost of I.Rs. 258 100, while the total income from fish sales (90 percent were non-stocked species) was only I.Rs. 68 578. Thus, in this reservoir, until the population of Oreochromis is reduced (the average size is decreasing) stocking of carp has to be only on a low key. In Uppar Reservoir, O. mossambicus was dominant until Rhinomugil corsula began appearing in the catches. From 83 percent of total catch in 1971–72, Oreochromis was reduced to 64 percent in 1975/76 when 10 percent of production was of mullets. By 1980/81 the picture changed completely. Stocking of L. fimbriatus seemed to have some impact - it accounted for 9, 22.2 and 14.1 percent of total catch respectively during 1973/74, 1974/75 and 1975/76. Common carp stocking had some positive effect though stocking with larger fingerlings could give better results. Thus, without stocking, mullets and Oreochromis could be expected to provide a sizeable catch while, with selective stocking of L. fimbriatus and common carp, more of these species could be landed, making the stocking process economical.
Aliyar Reservoir presents a peculiar pattern. In spite of massive stocking of non-native species, production was dominated by the indigenous P. dubius (27–70 percent) until 1975/76. L. bata and mahseer - also native to this system - have shown signs of depletion. Success of proper stocking is evident from the fact that in very recent years (e.g., 1980/81 and 1981/82) stocked C. mrigala, common carp, L. fimbriatus, rohu and catla are contributing to over 90 percent of the catches and the value of the catch exceeded the cost of stocking.
It will be uneconomical to stock major carps in Amaravathy Reservoir till the Oreochromis population is reduced. Since the average size of Oreochromis has fallen, it is necessary to fish the species out intensively and introduce common carp and even silver carp which, with the dense bloom of phytoplankton in this lake, will give high production. Despite the high productivity, yield and catch per unit effort in this reservoir, fishermen were abandoning it in favour of Bhavanisagar because the producer earns 4–5 times less per unit weight from tilapia than from the carp fisheries in Bhavanisagar or Sathanur.
Rihand Reservoir in northern India resembles Sathanur Dam in that a good yield of very high value catla is obtained with minimal stocking. Here, only marginal stocking with other major carps needs to be carried out. Major carps formed only 18.9–27.9 percent of total fingerlings collected in 1975–77 but the commercial catch consisted of 88.93–99.08 percent. Other species are not cropped. Mrigal formed 34–55.6 percent of the spawn collected but did not appear in the catches, maybe due to low survival or due to use of over-large meshed nets. Breeding of rohu and L. calabasu was meagre. The yield is therefore low, 0.53–5.08 kg/ha/year.
Govindsagar is one of the best examples (similar to Vallabhsagar) of successful colonization of the reservoir by all the valuable species introduced -catla, rohu, mrigal and common carp. A very striking but rare phenomenon is the production of silver carp in commercial quantities from a stocking of a mere 100 fingerlings. The total yield ranged from 474–735 t (28.1–43.58 kg/ha/year) which, for a reservoir of this size (16 867 ha) and depth (67 m), is appreciable.
Gandhisagar, the largest reservoir in India (66 000 ha) also developed a predominantly catla fishery. This species is being continuously stocked although this does not seem to be necessary. Rohu does not appear in catches at a level commensurate with its stocking rate and is obviously uneconomical. Mrigal has fared only slightly better where stocking cost is marginally recouped by value of catch. Common carp stocking is a waste. Labeo calbasu and Tor tor occur in noticeable quantities but the former may need stocking support and the latter breeding facilities.
In Bundh Beretta in Rajasthan a yield of 94 kg/ha, 80 percent of which are high value carps, was obtained by stocking 100 000 catla, rohu and mrigal fingerlings a year (164/ha). This type of stocking and recapture is very profitable.
Nagarjunasagar in Andhra Pradesh has been an unprofitable reservoir. Stocking rates for this large reservoir have been inadequate. In a large reservoir, unless an adequate breeding population is established, one cannot expect the successful establishment of a species, especially an annual spawner. The stocking rate for seven years from 1970 to 1977 ranged from 1.44 to 9.87 individuals/ha/year, all major carps excep L. fimbriatus being stocked as adults. Common carp would not succeed with a large resident population of predatory fish. The fish yield has been very poor -2.87 kg/ha in 1973/74.
Jhingran and Natarajan (1979) made an assessment of the fisheries of DVC reservoirs in relation to stocking. In spite of moderate to heavy stocking and natural breeding, production was low, ranging from 11.8 kg/ha in Konar to 24.0 kg/ha in Tilaiya.
SOME SUGGESTIONS FOR STOCKING RESERVOIRS AND ITS ECONOMICS
Introduction of desirable species should start prior to impoundment. Species which breed at very frequent intervals and are likely to stunt should be avoided even though they may produce a higher biomass. Normally the edible portion of such fish is low and other monetary value less than that of larger species. Reservoir stocking may be considered as “ranching”. Initially, adequate stocking with fast-growing species is necessary to establish a breeding population. Christie and Regier (1973) stated that a year-class should be sufficiently abundant numerically to make a significant contribution to spawning. Muncy (1978) also felt that “spawning stocks should not be depressed to a level where reproductory response is hindered”. Stocking of adults or even breeders is preferred where feasible. While native species should be conserved as far as possible, new introductions of superior value are needed to fill the new niches created in the reservoirs. If massive stocking does not result in the stocked species appearing in the catches, test cage-rearing may be carried out and after observing survival, further decisions on continuance of stocking may be taken.
Neither Labeo fimbriatus nor common carp have given economic yields except in one or two reservoirs. The former is essentially a browser and the latter a bottom feeder. Even where it was a native, L. fimbriatus did not succeed after impounding the river. Hence these species may be substituted by L. rohita and L. calbasu. Mrigal is also a suitable candidate for reservoir stocking. Catla should be stocked in reservoirs for good profits and has done well when adequate numbers were used. So far, Chanos was a total failure. On one or two occasions massive stocking was done in Stanley Reservoir but no recaptures were made. Over 10 000 Chanos were stocked in Amaravathy but only less than a dozen were recaptured (each weighing 5 kg) despite the good phytoplankton in the reservoir. Cage culture of Chanos in plankton-rich reservoirs is, however, worth trying. Etroplus suratensis is capable of breeding, repopulating and contributing to the catches - as in Stanley Reservoir and in smaller ones like Vidur, Poondi, etc., in Tamilnadu. Silver carp could be stocked, preferably where catla does not succeed. Grass carp is not of much utility in reservoirs unless they are needed to control submerged weeds. Furthermore, they are notorious carriers of parasites and diseases (Rosenthal, 1976) and introductions of the species into cyprinid-rich communities should be viewed with caution.
The role of predatory species in reservoirs is complex. Murrels have not occurred in any quantity in reservoirs but catfish are common in many and, in fact, their population increased in some cases. Total yield in reservoirs is reduced by excessive numbers of catfish. Thus, with the same degree of primary production, higher total fish yield was achieved in Sathanur, where catfish formed 5 percent of the population than in Mettur and Bhavanisagar, where over 35 percent of the population were catfish (Sreenivasan, 1972). However, they do play a useful role in converting non-catchable trash fish into higher quality flesh. Predatory catfish and Notopterus also check the population expansion of Oreochromis, where the latter are present.
Stocking rates cannot be prescribed as a general formula, as is done for ponds. This would depend on the size of the reservoir, its biota, the species to be stocked and their availability, the presence of predators, the productivity and carrying capacity and, most important, the breeding success or annual recruitment of indigenous and transplanted species. In the initial years, however, 250–500 fingerlings/ha/year may be stocked.
By contrast, Chinese reservoirs are stocked at very heavy rates - for example, 1 429/ha/year in a 7 000-ha reservoir, 2 727/ha/year in a 1 467-ha reservoir and 75 000/ha/year in a 400-ha reservoir. Stocking of such magnitude is not practised elsewhere, but recapture rates are reported to be up to 20 percent because of the large size of fingerlings (16–17 cm) at stocking (ADCP, 1979). Song (1980) indicated the stocking rates for Chinese reservoirs, established on the basis of the fertility of waters, to range from 1 200 for sterile waters to 3 000 for fertile waters. Interestingly, he has indicated that mrigal has a better growth than either silver carp or bighead carp.
Economically, a 10 percent recapture rate would meet the cost of stocking, gross income and profits rise with increases in recapture rate. For instance, if 100 fingerlings cost I.Rs. 10 and if 10 marketable fish are recovered (with an average weight of 1.5 kg each), 15 kg of fish valued at a modest I.Rs. 75 would be obtained. Return rates on stocking in terms of number of fry needed to produce 1 kg of fish are given in Table 2.
In working out the economics of stocking, we have to take into account the recapture of stocked species and avoid stocking species which do not contribute to at least 10 percent of the catch nor those which breed in profusion and sustain their own population. Small reservoirs of the “put-and-take” category must be stocked with suitable compatible species at rates higher than those for larger reservoirs where natural replenishment of stock occurs. In “put-and-take” reservoirs, the yield must be correctly monitored and stocking done on the basis of survival, growth and recapture in the previous year.
The profitability of reservoir fisheries is exemplified by a couple of examples from Tamilnadu-during 1981/82, there were gross earnings of I.Rs. 1.3 million and net profit of I.Rs. 0.5 million from Sathanur Dam, while it was I.Rs. 2.5 million gross and I.Rs. 0.38 million net in Bhavanisagar. It would be apt to close this account with a statement from Baluyut (1982): “The fact is that it (stocking) remains the only successful means of maximizing fish production in reservoirs in S.E. Asia”. This applies to other parts of Asia too.
ADCP, 1979 Aquaculture development in China. Report on an FAO/UNDP study tour to the People's Republic of China, led by T.V.R. Pillay, Aquaculture Development and Coordination Programme, FAO, 2 May–1 June, 1978, Rome, FAO/UNDP, ADCP/REP/79/10:65 p.
Baluyut, E.A., 1982 A review of inland water capture fisheries in Southeast Asia with special reference to stocking. FAO Fish.Rep., (288):13–57
Chacko, P.I. and P. Dinamani, 1949 Preliminary report on the fishery resources of Bhavani River in relation to Lower Bhavani project. Proc.Indian Sci.Congr., 36(3):196
Choudhury, D.K., 1977 Case study of Gandhisagar reservoir fisheries (M.P.). CIFE Newsl., Bombay, 1977:32 p. (mimeo)
Christie, W.J. and H.A. Regier, 1973 Temperature as a major factor influencing the success of fish. Rapp.P.-V.Reun.CIEM, 164:208–18
David, A. and K.V. Rajagopal, 1969 On some aspects of the fish population of Tungabhadra Reservoir. In Seminar on the ecology and fisheries of Freshwater reservoirs, Barrackpore, 1969. Delhi, ICAR
Dubey, G.P. and S.N. Chatterjee, 1977 Case study of Gandhisagar reservoir. Madhya Pradesh, India. Proc.IPFC, 17(3):232–45
Gophen, M., R.W. Drenne and G.L. Vinyard, 1983 Cichlid stocking and the decline of the Galilee Saint Peter's fish (Sarotherodon galilaeus) in Lake Kinneret, Israel. Can.J.Fish.Aquat.Sci., 40(7):983–8
ICAR, 1980 Sixth Workshop on All-India Coordinated Research Project on the ecology and fisheries of reservoirs. Proceedings. Simla, November 1980. Delhi, ICAR
Jhingran, V.G. and A.V. Natarajan, 1979 Improvement of fishery resources in inland waters through stocking. In Advances in aquaculture, edited by T.V.R. Pillay and W.A. Dill. Farnham, Surrey. Fishing News Books Ltd., for FAO, pp. 532–41
Muncy, R.J., 1978 An evaluation of the Zambian Kafue river flood plain fishery. CIFA Tech.Pap./Doc.Tech.CPCA, (5):155–65
Rosenthal, H., 1976 Implications of transplantations to aquaculture and ecosystems. Paper presented at the FAO Technical Conference on Aquaculture, Kyoto, Japan, 26 May–2 June 1976. Rome,FAO,FIR/AQ/Conf./76/E.67 (mimeo)
Song, Z., 1980 Manual of small-scale reservoir fish culture. FAO Fish.Circ., (727):18 p.
Sreenivasan, A., 1972 Energy transformations through primary production and fish yield in some tropical freshwater impoundments and ponds. In Proceedings of the IBP-Unesco Symposium on Productivity problems in freshwaters, Kazimierz Dolny, Poland, 6–12 May 1970, edited by Z. Kajak and A. Hillbricht-Ilkowska. Warsaw, PWN Polish Scientific Publishers, pp.505–14
Sreenivasan, A., 1976 Fish production and fish population changes in some South Indian reservoirs. Indian J.Fish., 23:134–52
Sreenivasan, A. and F. Chandrasekaran, 1980 Development of reservoir fisheries in Tamilnadu: case studies. In Proceedings of the Workshop on Reservoir fisheries for rural development, 8–9 April 1980. Bombay, CIFE
Sunderaraj, B., 1942 Dams and fisheries: Mettur and its lessons for India. Proc.Indian Acad.Sci., 14B:341–58
Watt, K.E.F., 1968 Ecology and resource management. London, McGraw Hill, 450 p.
Table 1 Physical features of the reservoirs
|Name of reservoir||Area (ha) at FSL||Volume m3 × 106 at FSL||Location||Year of closing||Depth (m) max.|
|Stanley (Mettur)||14 690||2 646.0||11°50'N||1937||37.5|
|Govindsagar||16 867||9 400.6||31°25'N||1963||67.0|
|Gandhisagar||66 000||7 744.0||24°00'N||1960||49.5|
|Vallabhsagar (Ukai)||60 095||7 095.5||21°15'N||1971||81.0|
|Rihand||46 538||12 093.0||21°04'N||1962||24.1|
Table 2 Yield in relation to total stocking
|Reservoir||Fingerlings stocked to yield 1 kg fish||No. of years||Remarks (species captured)|
|Rihand||0.72||5||79% self regenerating catla|
|Gandhisagar||1.38||16||Self-populating valuable carps|
|Nagarjunasagar||2.14||7||Non-stocked catfish and trashfish|
|Gomukhi, Tamilnadu||2.50||5||Mainly non-stocked species|
|Uppar, Tamilnadu||2.68||7||Mainly tilapia and mullets, very few stocked species|
|Sathanur, Tamilnadu||4.08||15||Mainly catla (sparse stocking)|
|Amaravathy, Tamilnadu||4.16||6||Mainly tilapia (non-stocked)|
|Bhavanisagar, Tamilnadu||5.85||10||Self-populating catfish, L. calbasu and stocked carps|
|Stanley, Tamilnadu||5.95||18||Self-populating carps and catfish|
|Vallabhsagar (Gujarat)||8.1||5||Stocked species (repopulating)|
|Periachari, Tamilnadu||19.5||16||Non-stocked species|
|Manjalar, Tamilnadu||28.0||7||80% tilapia|
|Manimuthar||66.8||22||Mainly non-stocked species|
|Vaigai, Tamilnadu||70.50||10||Mainly non-stocked species|
|Pechiparai, Tamilnadu||110.0||14||Non-stocked species|
Department of Zoology, University of Kelaniya
Kelaniya, Sri Lanka
Stocking of juvenile fish in lakes, reservoirs and lagoons is now being carried out in most countries of the Indo-Pacific region with the objective of increasing the fish production. However, in most cases stocking has been carried out haphazardly. Hence, there is a great need for the quantification of the amount of fish juveniles that is needed to be stocked in these water bodies. A method to estimate the number of juveniles needed to be stocked in a particular water body was suggested by Welcomme (1977). Wijeyaratne and Costa (1981) modified this method to estimate the stocking rates of juvenile Oerochromis mossambicus in some inland reservoirs of Sri Lanka. In estimating these numbers, however, several assumptions have to be made in the calculations.
METHOD OF ESTIMATING THE AMOUNT OF FISH JUVENILES NEEDED TO BE STOCKED IN A WATER BODY
Using the instantaneous total mortality coefficient of fish populations, the following equation can be derived to estimate the amount of fish juveniles needed to be stocked in a particular water body:
where S = the amount of fish juveniles needed to be stocked
Pi = the maximum possible sustainable yield of fish from the water body (potential yield)
P = present maximum sustainable yield
W = mean weight of the stocked fish at capture
z = instantaneous total mortality coefficient of stocked fish
ti = age of stocked fish at capture in years
to = age of stocked fish at the time of stocking in years.
If fish are harvested one years after stocking, then ti-to = l.
PROBLEMS ASSOCIATED WITH THE DETERMINATION OF THE AMOUNT OF FISH JUVENILES NEEDED TO BE STOCKED IN A WATER BODY
Many parameters of the above equation cannot be precisely determined in most instances and, therefore demand that certain assumptions be made.
Present maximum sustainable yield fo the water body (P)
Present maximum sustainable yield of the water body can be determined by the surplus yield model of Schaefer (1954). As described by Pauly (1980) catch and effort statistics for at least eight years are needed in this analysis. Fish catch per unit effort has to be determined and should be plotted against the fishing effort of each year. The intercept (a) and the slope (-b) has to be calculated by simple linear regression analysis. Then,
Maximum Sustainable Yield =
and the fishing effort needed to obtain MSY =
However, the determination of fishing effort is a difficult task due to the use of various mesh sizes and different types of gear. In addition, the skill of fishermen, time of fishing and the amount of time spent in fishing may also vary considerably. Furthermore, some fish species may be more vulnerable to a particular type of gear and similarly the time of operation may also affect the total catch of certain fishes. With certain assumptions, however, the number of craft operated or the number of fishermen involved in the fishing operation can be used in most instances to determine the amount of fishing effort involved.
If a fishing craft is used as a unit of effort it has to be assumed that:
On determining the fishing effort, the present maximum sustainable yield of the water body can be calculated using the method described by Pauly (1980).
Maximum possible sustainable yield (potential yield) of a water body (Pi)
Even though the present maximum sustainable yield (P) could be determined using the assumptions described above, the estimation of the potential yield of a water body still remains very difficult. When P is estimated, the existing fishing effort of the water body can be adjusted to the desired level to obtain that amount of yield which is the maximum yield which can be obtained without adversely affecting the existing fish population in that particular water body. However, the potential fish productivity can be higher than the present level of maximum sustainable yield depending on the nutrient content of the water body. Due to small or insufficient population sizes of fish existing in the water body the maximum sustainable fish yield may be less than the potential level. As indicated, precise determination of potential fish yield of a water body may be difficult but with several assumptions this can be roughly estimated.
By taking into consideration the conversion efficiency of net primary productivity into fish yield, the potential fish yield of a water body can be estimated from primary productivity data. However, this conversion efficiency may vary considerably according to the trophic level of the fish species present in the water body. As stated by Nikolskii (1963) when the end product is closer to the first link of the food chain, the yield from that water body is higher. In India this has been observed to range from 0.06 to 0.97 percent (Sreenivasan, 1972) where the lower values have been obtained when the majority of the fish were those of higher trophic levels. In experimental fish ponds in Malacca, Malaysia, conversion efficiency of net primary productivity to fish production has been estimated to be 1.02–1.79 percent (Prowse, 1972). The fish species which have been stocked in these ponds were male hybrid Tilapia (Oreochromis sp.), Ctenopharyngodon idella and Puntius gonionotus, all of which are primary consumers. However, in natural ecosystems total fish productivity is not harvested by man and only a portion is taken as yield. Therefore, conversion efficiency of net primary productivity to fish yield may be less than 1.02–1.79 percent in natural ecosystems. If herbivorous fish are to be stocked, a hypothetical conversion efficiency of about 1 percent can be assumed and the potential fish yield can be estimated from primary productivity data. The following conversion factors can be used in these calculations.
1 g of C = 10 g of wet weight of fish (Rodhe, 1958)
0.375 g of C = 1 g of oxygen (photosynthetic) (Sreenivasan, 1972)
1 g of oxygen (photosynthetic) = 3 600 Cal (Odum and Smalley, 1959)
Recently the morpho-edaphic index, which is a measure of ionic content in relation to depth has been used to determine potential fish yield of water bodies with some limitations. This is calculated as follows:
It has been found that MEI is closely related to fish yield of inland water bodies by Ryder (1965) for Canadian lakes, Henderson and Welcomme (1974) for water bodies in Africa and by Wijeyaratne and Costa (1981) for inland reservoirs in Sri Lanka. In this approach maximum sustainable yields for several water bodies are estimated and the statistical relationship between these and morpho-edaphic indices is then determined.
There are also limitations in the use of MEI in the determination of potential yields of fish. This method could be utilized only for water bodies when the temperature and the water level are more or less constant throughout the year and where the ionic composition of water is dominated by bicarbonate and carbonate ions. Also these water bodies should contain a sufficient population size of fish and should not require stocking. It is assumed that the maximum sustainable yields of these water bodies are equal to their potential yields. In such an analysis 8–10 fully exploited lakes or reservoirs having a high intensity of fishing can be used. The relationship between MEI and maximum sustainable yield is then calculated. Using this relationship the potential yield of any water body having the above mentioned characteristics can be estimated in that area (Wijeyaratne and Amarasinghe, 1982).
Instantaneous mortality rate (z) and mean weight at capture (W)
Since the survival rates and growth rates of fish vary with different habitats, the precise determination of these two factors is also difficult. These would have to be assumed or should be determined by a trial experiment. Since these could vary from one habitat to another these would have to be calculated separately for each reservoir or lake.
It is, therefore, to be concluded that it is still not possible to estimate precisely the number of fish juveniles needed to be stocked in a particular waterbody without making several assumptions. However, most of the assumptions made as, for example, with regard to fishing effort, potential yield, etc., appear to be not unrealistic.
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Wijeyaratne, M.J.S. and H.H. Costa, 1981 Stocking rate estimations of Tilapia mossambica fingerlings for some inland reservoirs of Sri Lanka. Int.Rev.Gesamt.Hydrobiol., 66(3):327–33