5. RESERVOIR FISHERIES

Reservoir fisheries in Sri Lanka can be divided into the capture fishery and the culture fishery. The capture fishery by far outweighs the culture fishery in the total production, the man-power involved and in the extent of its distribution within the island. The culture fishery is still in its infancy. However, both these fisheries are confined to reservoirs, the capture fishery to the perennial reservoirs and the culture fishery to the seasonal tanks. In view of the different strategies of management and different social impacts, it is best that these two fisheries and related aspects are considered separately.

5.1 Reservoir fish fauna

The reservoir fish fauna, documented from commercial fish catches as well as from experimental fishing, is known to constitute 30 species, belonging to 11 families and 6 orders (Table 14). The family Cyprinidae, constituting 12 species, is best represented. Of the 30 species, only 15 are considered to be truly food fishes; of these only 6 are caught in significant numbers in any reservoir. There are also variations in the fauna between reservoirs (De Silva and Sirisena, 1987). There is some evidence that particularly some minor cyprinids such as Amblypharyngodon melettinus, B.chola, B. dorsalis, B.filamentosus, B.sarana and B.chola, form species associations, the numbers or density of one affecting and/or determininge another one.

5.2  Biology of reservoir fishes

Most biological studies are those of the commercially important O.mossambicus.

5.2.1  Biology of O.mossambicus

Fernando and Indrasena (1969) considered the density of nests of O.mossambicus to be an approximate index of its brooding intensity. These authors mapped the density of nests in the Parakrama Samudra reservoir and found that the nests were confined to areas between high and low water marks. Later studies on the nest building habits in the same reservoir (De Silva and Chandrasoma, 1980) pointed out that the nest diameter tended to increase with water depth, and nests were seldom built in water depths in excess of 1 m. Recent studies by De Silva and Sirisena (1988) on the nest building habits in the four reservoirs in the south-eastern Sri Lanka indicate that the nests are of two distinct types - small and large - and that in any one reservoir either the large or the small type is found in a specific nesting area. The overall density of nests in a breeding site varied considerably between reservoirs (Table 15), and the distances between nests were positively correlated to size.

O.mossambicus is known to breed throughout the year with possible peaks during the dry months (De Silva and Chandrasoma, 1980). Some aspects of the reproductive biology, such as the sex ratio, minimum size at maturity and fecundity are shown in Table 16. The overall relationship of fecundity (F) to body weight (W) and length (TL) for Sri Lankan reservoirs was found to be (De Silva, 1986):

F = 3.23W + 357.8 (r=0.73; p<0.001)

F = 1.52TL^2.11 (r=0.68; p<0.001).

Also included in the Table are von Bertalanffy growth parameters for certain reservoir populations. The VBGP indicate that the growth of O.mossambicus reservoir populations is comparable, if not better than that reported for similar tropical populations (Moreau et al., 1986).

The food and feeding habits of O.mossambicus in Colombo Lake, a lake which is not comparable in any way to the reservoirs of the dry zone, were studied by Abayasiri and Costa (1978); they found that blue-green algae were the main food of O.mossambicus in this highly eutrophic lake. Subsequent study by Maitipe and De Silva (1985) revealed that the food habits of adult populations of O.mossambicus in Sri Lankan reservoirs did not conform to a particular pattern. In the different seasons in certain reservoirs, the dietary habits changed from a complete detritivory habit via phytoplanktivory to a zooplanktivorous habit. Generally, detritivory was predominant in the wet season and phytoplanktivory in the dry season. This study also indicated that blue-green algae did not constitute an important component of the diet. Hofer and Schiemer (1983) found that the diet of juvenile O.mossambicus was very uniform in the Parakrama Samudra reservoir and that they fed mainly on phytoplankton and mineral sediments. The daily food consumption of the juvenile populations was estimated as 31.2% fresh material and 4.29% dry matter of body weight.

The composition, nutritional status and digestibility of the diets of adult O.mossambicus populations from 9 reservoirs were evaluated by De Silva et al. (1984), when they found that the mean percentages of organic matter, protein, total lipid and carbohydrate content were 45.71% (34.4–64.4%), 24.18% (18.53–35.15%), 7.91% (5.94–9.84%) and 22.34% (11.6–34.7%), respectively. In the ingested material of Sri Lankan reservoir O. mossambicus populations there was also a significant proportion of unaccountable organic matter, similar to that reported by Bowen (1979) for this species in Lake Valencia, Venezuela. The results indicate that the quality of ingested material was comparatively better than that reported for other populations, and that a significant portion of the ingested material was digested. Distinct trends in seasonal variations in the body condition of O.mossambicus reservoir populations were not apparent. However, the overall body condition (BC) of O.mossambicus populations were linearly related to the digestible energy : protein (PER) ratio (De Silva, 1985c), and statistically expressed as follows:

BC = 3.31 (PER) + 126.6 (r = 0.65; p<0.05).

There is evidence to believe that the abundance of O.mossambicus populations over a period of time is related to water-level fluctuations in a reservoir. De Silva (1985b) has shown that in the Parakrama Samudra reservoir the abundance, measured in terms of the yield from the commercial fishery, was related to the mean water level in the reservoir three years earlier. This evidence together with that of the influence of the fishing pressure on the mean fecundity of individual populations (De Silva, 1986) may provide clues to specific managerial measures for sustenance of O.mossambicus reservoir populations.

Reservoirs with O.mossambicus are not devoid of predators. Winkler (1983) reported that the main constituent species in the diet of piscine predatory birds were juvenile O.mossambicus, and that the daily predation by birds in the Parakrama Samudra is almost equivalent in weight to the daily yield of the commercial fishery.

5.2.2 Other exotic fish

There is very little information on the biology of the other exotics in reservoirs. Chandrasoma and De Silva (1981) studied the reproductive biology of Tilapia rendalli in Parakrama Samudra. In T.rendalli females tend to predominate in the lower length groups, and it is likely that T.rendalli also breeds throughout the year with two peaks in August-November and March-May. It is also known that T.rendalli seems to favour reservoirs with littoral vegetation (Chandrasoma, 1986). Etroplus suratensis was possibly transplanted into reservoirs and to some of the river systems as long ago as 1910 (Willey, 1910). This species is recorded in small numbers in most major reservoirs in the country. The feeding habits of this species are distinctly different in the reservoirs from those in its normal estuarine habitats (De Silva et al., 1984a): in reservoirs this fish tends to feed on vegetation/grasses while in estuaries it feeds predominantly on molluscs. The differences in feeding habits are reflected in the intestinal lengths of fish belonging to the two populations.

5.2.3 Indigenous species

The biology of most of the indigenous fish, particularly Barbus, has been mostly studied in their natural environment, and it has been reviewed by Fernando and De Silva (1984). Such fish need to be studied again especially in reservoirs, because of their fisheries potential (see Section 5.3.8). Studies of the reproductive biology of Barbus sarana in Parakrama Samudra have clearly shown that this species is a seasonal spawner, which undertakes a spawning migration out of the reservoir at the onset of rains (Chandrasoma and De Silva, 1981). De Silva et al. (1983) evaluated the changes in the moisture, protein, lipid and ash content of the body musculature in B.sarana in relation to its reproductive cycle. Based on such studies on the seasonal changes in the catch statistics, De Silva (1983a) suggested that at least the larger indigenous minor cyprinid species are unlikely to spawn in reservoirs and that they would emigrate during the spawning season to the riverine habitat.

On the basis of the feeding ecology and diurnal migrations, B.sarana, B.filamentosus, L.dussumieri and R. daniconius have been identified as litoral ‘aufwuchs’ feeders, and B.dorsalis and B. chola as zoobenthos feeders (Schiemer and Hofer, 1983). A high degree of food overlap was observed amongst the members of each group. However, differences were observed in the peak time of feeding: B.chola, which inhabits offshore areas, feeds throughout the 24 h period, whilst B. dorsalis feeds mainly during the dark hours. The detailed studies of food habits of B.filamentosus, including assimilation efficiency, have indicated that its energy demand cannot be met entirely by feeding on macrophytes, and that its digestive system is illequipped to deal with macrophytes efficiently (Hofer and Schiemer, 1983).

Newrkla and Duncan (1984) confirmed the presence of an appreciable biomass of the small freshwater clupeid Ehirava fluviatilis in the Parakrama Samudra reservoir. This species was found to feed almost exclusively on rotifers and is supposed to account for the fluctuations in the rotifer populations. These authors suggested that the species undertakes a marked diurnal vertical movements.

5.3 The reservoir capture fishery

5.3.1 The development of the fishery

Reservoirs played a pivotal role in the development and sustenance of old civilizations, but there is evidence that in ancient times fishing was prohibited. For example, inscriptions left at the earliest identifiable reservoir in the island, the Abhaya Wewa (now known as the Basawak-kulam), contain orders of King Kassapa IV prohibiting fishing in it. This prohibition is likely to have been an influence of the Buddhist way of life.

The modern capture fishery on the other hand, is a development of the second half of this century. The reasons for the lack of establishment of a reservoir and/or an inland fishery could be many. Perhaps the extensive road network in the island has ensured a ready supply of fresh seafish to the interior, and the indigenous fish fauna by itself was thought to be unable to establish sizeable populations within reservoirs to sustain a fishery. In addition, the resettlement into the dry zone area where the reservoirs are located was essentially a post-1948 phenomenon, and there was perhaps no demand for fish to the extent of initiating a commercial fishery.

The origin and the gradual development of the inland fishery - which to date is essentially the reservoir capture fishery - was attributed to the introduction of O.mossambicus (Fernando and Indrasena, 1969; Fernando, 1977; De Silva and Fernando, 1980; De Silva, 1983; Fernando and De Silva, 1984). The impact of the introduction of O.mossambicus and the consequent alleviation of the fishery is shown in Fig.12.

Fig.12. Development of the Parakrama Samudra fishery (from Fernando and De Silva, 1984).

With the gradual development of an inland fishery it is to be expected that changes in consumer preferences will occur. O.mossambicus began to be accepted, initially by the population in Polonnaruwa and Anuradhapura districts, and the fishery has gradually spread into other reservoirs. This fishery represented initially a supplement to their agricultural income. This was enhanced by the returns made by coastal fishermen who regularly migrated to reservoirs during the monsoonal seasons, when coastal fishing from traditional crafts became dangerous. The migrant fishermen with a wealth of experience behind them made their presence felt, and the message began to get across faster, with the permanent agricultural settlers around reservoirs beginning to take up fishing. It was not uncommon to have major disputes between the migrant fishermen and the residential ones in the late 1960's. By now the migratory fishing has almost completely disappeared.

In certain remote reservoirs the landings were processed on site. For example in Kaudulla reservoir in the late 1960's to early 1970's, almost the entire catch was smoked or processed into dry fish. However, with the increasing population and the consequent demand for fresh fish this practice has become obsolete. With the development of the fishery and the gradual consumer acceptance of freshwater fish, marketing of the produce became more organized; presently each reservoir has its own group of vendors. These will be discussed in detail later.

Plates II to V highlight some of the features of the fishery, such as the type of craft and marketing of the produce.

5.3.2 The fishery, gear and crafts

The reservoir capture fishery at present yields approximately 27,000 to 30,000 metric tons per annum. This yield, which is based on capture fisheries of major- and medium- scale irrigation reservoirs (as per Irrigation classification) or on perennial reservoirs (as per Fisheries classification), corresponds to 307 kg ha^-1 annum^-1. De Silva (1987) assessed the production of 18 major reservoirs and found that the production averaged 224 kg ha^-1 annum^-1. The mean production of 307 kg ha^-1 annum^-1 is rather high (Table 17) when compared to the production in other countries (also see Fernando, 1980; Petr, 1986). According to Oglesby (1985), in developing countries lake and reservoir fisheries in lowland tropics average a yield of only 80 kg ha^-1 annum^-1.

Plate II : A traditional craft

Plate II Crafts introduced under a subsidy scheme (since 1981). Note the bundle of gill nets - the common gear of the reservoir fishery.

Plate IV: A typical reservoir fishery catch: Above-from the Ridiyagama reservoir -888 ha Below - from the Lungamwehera reservoir - 3023 ha

Plate V: The change in quality of transport vehicles between 1983 (bicycle) and 1986 (motorcycle) shows the increasing profitability of reservoir fisheries especially for the middleman.

In Sri Lanka the reservoir fishery is almost exclusively a gill-net fishery. The permissible mesh size is over 3 inches (75 mm) or over (knot to knot). Generally, the amount of netting laid by a craft is limited by virtue of the size of the craft. An average fisherman (craft) generally lays 100-200 m of netting. Until recently the indigenous crafts were of two types: the dug-out canoes of about 3–4 m in length with an outrigger, or a lograft. Both types are being gradually replaced by fibre-glass canoes with an outrigger. The latter has been introduced under a subsidy scheme since 1981 and in most reservoirs all crafts are of this type.

Gill-net fishing is carried out through the night. The nets are hauled in during the early hours of the morning when the fishermen return to one or more landing sites in a reservoir. Generally two men operate a craft. However, this could go up to four in certain times in specific reservoirs, which is more an exception than the rule. On average a fisherman operates for over 250 days per year. In certain reservoirs fishermen are known to beat the water with wooden poles to drive the fish into the nets, and this method has been termed the “beating technique” (Amarasinghe and Pitcher, 1986). This technique is practiced when the number of net pieces used (length of netting) is low, and Amarasinghe and Pitcher (1986) viewed that this technique results in a higher yield.

In addition to gill-netting sporadic castnetting and angling takes place in most reservoirs. Catches from these two methods are mainly a means of supplementing the protein intake of the family. However, on certain occasions high priced species such as Ophicephalus striatus are landed and sold.

Use of motorized boats for fishing is prohibited in Sri Lankan reservoirs. Similarly, seining and use of undersized meshed gill-nets are also banned. The bottom terrain as well as numerous obstacles, such as submerged vegetation, prevent the use of any dragging gear.

5.3.3 Individual reservoir fisheries

Although the mean production from the reservoir fishery is at present around 307 kg ha^-1 annum^-1, the production in the individual reservoirs is variable. De Silva (1987) reported that the mean annual production could vary from 40 kg ha^-1 to 497 kg ha^-1. Data on the mean annual fish production and on related aspects for selected reservoirs are summarised in Table 18. The mean production ranges from 40 to 650 kg ha^-1 annum^-1; the overall mean production is 258 kg ha^-1 annum^-1. The mean yield from individual reservoirs differ vastly from each other. Such differences are not related to the extent of waterspread, depth or age of the reservoir.

In some reservoirs the production has increased 10-fold over the years (Table 18). This is unlikely to be a result of changes in abundance, but mostly the result of increasing fishing effort. In most major perennial reservoirs there has been a considerable increase in the fishing effort, resulting in a decrease in catch per unit of effort, but an increase in total yield (De Silva, 1987). For clarity and convenience the estimates of production are summarized in Table 19.

The effort, measured in terms of the number of crafts operating, in most major perennial reservoir fisheries has been on the incline over the last decade (Fig.13). The number of operable crafts in a reservoir is determined by the Ministry of Fisheries and a licencing system is enforced, a nominal annual fee being levied for each craft.

However, the number of operable crafts is not is not determined entirely by scientific reasoning. It is a common practice for fishermen to refrain from fishing or move to another reservoir when the catches decline.

Amarasinghe and Pitcher (1986) pointed out that the best measure of effort would be the number of nets utilized by each fisherman. However, obtaining such statistics is prohibitive. In such circumstances and as the craft is uniform in size and efficiency, and because the number of nets used by each craft is variable, the number of operating boats or the number of fishermen provide a reasonable and a good estimate of the effort (Amarasinghe and Pitcher, 1986).

Fig.13 Mean number of crafts operting per ha in selected reservoirs (modified after De Silva, 1987).

The overall mean effort in the reservoir fishery has increased from 0.019 boats ha^-1 in 1978 to 0.050 boats ha^-1 in 1984, nearly a three fold increase in a span of six years (Figs 13,14).

In almost all the major reservoirs the predominant species is O.mossambicus (De Silva, 1985a). Changes in the species composition have been followed since 1978 in three ancient reservoirs i.e. Rajangana, Nachchaduwa and Kalawewa-Balaluwewa, now incorporated into the Mahaweli Ganga Development Scheme. In these reservoirs in 1981 the proportion of the indigenous cyprinid Labeo dussumieri in the total fish catch has reached 41.5% in Kalawewa, 32.7% in Balaluwewa and 16.7% in Nachchaduwa (Chandrasoma, 1983). Later on Chandrasoma (1986) reported that in Kirribbanara and Udawalawe reservoirs in southeastern Sri Lanka the macrophyte feeding indigenous cichlid Etroplus suratensis and the exotic Tilapia rendalli now represent a significant part of the total catch although O.mossambicus continues to dominate, accounting for over 70% of the landings. The same author reported the shift in dominance in the Soraborawewa and Mahagama reservoirs to O. niloticus and T.rendalli respectively. Chandrasoma (1986) pointed out that Soraborawewa was the only reservoir which was stocked appreciably with O. niloticus during its filling period after drying which extended over four years. The changes reported in these reservoirs are summarized in Table 20.

Fig.14. Changes in the mean number of boata operating in the fishery between 1978 to 1984.

In summary: with the exception of Soraborawewa and Mahagama reservoirs, the fishery is dominated by O.mossambicus, which represents 60 – 100% of the catch. Some of the changes reported in the species composition seem to result from the increasing intensity of the individual fisheries. Further developments of the Soraborawewa fishery, the only reservoir in which O.mossambicus appears to have been almost totally replaced by O.niloticus through stocking, needs to be closely monitored. Replacements by O.niloticus are also known from elsewhere: S.macrochir in Lake Itasy, Madagascar (Lamarque et al., 1975) and O.esculenta in parts of Lake Victoria (Welcomme, 1967). Replacement of O.mossambicus is known only as a result of major changes in the water regimes or in the macrophyte abundance. As both O.mossambicus and O.niloticus are known to digest and assimilate blue-greens, even if not equally efficiently (de Moor et al., 1986), this is probably not the reason for the replacement of O. mossambicus in Soraborawewa reservoir.

In view of the limitations imposed on the minimum mesh size, in most fisheries in perennial reservoirs the average landing size of O.mossambicus exceeds 180 mm in total length, and 125 g in weight (De Silva, 1985a). Amarasinghe (1987) also reported changes in the monthly mean landing size in the Pimburettewa reservoir, and related these changes to the water level. The mean size was found to decrease with high water levels. These data are further updated in Table 21, where data on other species, where available, and information from a few other reservoirs are also incorporated. It should be noted that the landing size of O.mossambicus in the Lunugamwehera, a reservoir impounded only early 1986, is one of the highest.

The overall mean landing size was not significantly different (p> 0.05) between reservoirs. This can be expected from a fishery in which the gear is uniform and in which O.mossambicus populations are not stunted. The maximum landing size differed between some reservoirs. The most noteworthy aspect of the O.mossambicus fishery was the overall uniformity of the mean landing size in the different reservoirs over the period 1980 to 1986. However, there were significant variations in the mean landing size within a reservoir in different months. For example the mean landing length and weight varied between 17.6 cm (Tissawewa, January 1983) and 27.6 cm (Minneriya, April 1983), and weight between 113 (Yodawewa, August 1983) and 421 g (Udawalawe, August 1983).

Seasonal changes in the total yield, in the species composition (De Silva and Fernando, 1980) and in the effort (Amarasinghe and Pitcher, 1986) have been reported for specific reservoir fisheries. Mean monthly changes in the fishery of four reservoirs in the southeastern region of the island are shown in Fig.15. No distinct trends are evident in the yields. Absence of identifiable trends in relation to water level fluctuations and/or to rainfall pattern could be due to yield estimates representing a compilation of data over a number of years, when the yearly differences could mask the seasonal changes. Amarasinghe (1987) reported that the yield tends to increase with decreasing water level in Pimburettewa reservoir in the Northeast of the island. It is difficult to understand the underlying reasons for the higher yield during the drier months, except that it is a result of fish, in particular O.mossambicus, becoming more vulnerable to the gear as the volume of water tends to decline. On the other hand, De Silva (1983a) has shown that L.dussumieri and B.sarana, the two main indigenous species of the fishery, tend to increase in catches with the rains (Fig.16). This increase has been attributed to the tendency for these species to aggregate, with the onset of rains, for a reproductive migration into the rivers, and then the population becomes more vulnerable to the gear. Smith and Jiffry (1986) observed the spawning habits of L.dussumieri on the floodplain and in flooded streams of the Mahaweli Ganga.

Fig. 15.  Mean monthly total yield and that of O. mossambicus over the period 1981–1986 in four reservoirs of southeastern Sri Lanka.

5.3.4 Collection of statistics

Collection of basic statistical data remains a problem in most developing countries, and their accuracy may not always be high. Data collection is more difficult in a very intensive but spatially somewhat thinned-out fishery such as the reservoir fishery in Sri Lanka. At present catch data are collected by Fisheries Inspectors, who are expected to cover a number of reservoirs in a district. Fishermen, who have been granted subsidy boats are also expected to provide daily returns of the catches as part of the agreement of the subsidy.

Fig.16. Mean monthly landings of L.dussumieri and B.sarana in the Parakrama Samudra reservoir (from De Silva, 1983a).

To avoid some discrepancies in data, such as due to inadequate sampling, fishermen themselves should be more actively involved in the submission of accurate statistics, either individually or through the cooperative societies with which they are affiliated. Workshops and seminars should be used to stress the importance of accurate returns for effecting long-term management measures, and from which the fishermen will directly benefit. Such data should be supplemented by detailed analyses of specific reservoir fisheries, as well as by biological studies of economically important species.

5.3.5 Management of the fishery

The reservoir fishery of Sri Lanka has been thus far managed by virtue of imposition of a minimum mesh size of 50 mm and prohibition of seining. This practice has paid dividends in safeguarding the fish stocks and preventing collapse of the individual reservoir fisheries. However, with the increasing pressure on individual reservoir fisheries, the need for scientific management is becoming increasingly urgent. Basic questions as to when to commence fishing and what the initial fishing pressure should be in newly impounded reservoirs are being solved or attempted to be solved through intuition rather than scientific reasoning.

The first attempt to provide a model for the management of the reservoir fisheries in Sri Lanka was made by Wijeyaratne and Costa (1981), who based a predictive model on short-term yield data and the morphoedaphic index (MEI) from seven reservoirs. The validity of this model will be discussed in Section 6.4. Wijeyaratne and Amarasinghe (1984) based on longer-term yield data from nine major reservoirs derived a model on the maximum sustainable yield (MSY) (derived from a simplified version of the Schaefer Model) to MEI. They found that the MSY was related to the MEI in the following manner:

In MSY = 0.9005 In MEI + 1.9220

On the basis of this model they found that six of the reservoirs were underfished and recommended an increase in effort. They also recommended the stocking of some reservoirs with juvenile O.mossambicus. The Sri Lankan perennial reservoir fishery is essentially a capture fishery dependent on natural recruitment of the exotic cichlid O.mossambicus and supplemented by the natural recruitment of indigenous fish L.dussumieri and B.sarana, and a few other species. In such a fishery the basic management concepts to be answered are: what should be the optimal fishing effort and what other managerial measures could enhance the yield. In this context De Silva (1985a) considered the yields from 20 major reservoirs over a minimum of a five-year period and concluded that most of the reservoirs are underfished, and that the mean yield per craft (CPUE) was 5.43±2.13 mt annum^-1. Assuming this mean CPUE was sufficient to sustain a fisherman involved in an artisanal fishery from yield (Y) to effort (E) relationship viz.

Y = 4.OE - 53.98 (d.f.=18; r=0.92; p<0.001)

(where E = craft days ha^-1; Y = kg ha^-1 annum^-1) the number of boats that could be operated without a detrimental effect on the O.mossambicus fishery can be calculated for each reservoir. However, since this computation was made, the effort on each reservoir has increased substantially (Fig.14) and a reassessment of this criterion is needed.

The density of the reservoirs and the intensity of their fisheries do not make detailed management studies of individual reservoirs practical. What is needed, at least initially, is an overall management strategy, as proposed by De Silva (1985a) which could later be improved on the basis of accumulation of knowledge on individual reservoirs.

Effective management measures are also taken by fishermen through their Cooperative Societies of individual reservoirs. It is not uncommon to collectively agree to refrain from fishing for a 2–3 months period once every 2 to 3 years when the yields and the CPUE begin to decline. Such a closed season approch is practiced in Badagiriya and Pimburettewa reservoirs. The effect of the closed season on fish stocks is not known. The closed season is imposed during extremely dry years when the reservoir water level recedes beyond the average.

5.3.6 Stocking

Stocking is considered a management measure undertaken to enhance and optimize the yield of lacustrine bodies (Bhukaswan, 1980, 1983). Welcomme and Henderson (1976) listed the following pros and cons with respect to stocking a water body:

The need to stock the major perennial reservoirs in Sri Lanka has to be considered in the context of its existing fishery, and the past experience of the stocking practices in Sri Lanka itself. In addition, the biology of the species recommended for stocking and their likely behaviour in the Sri Lankan reservoirs and associated water ways, experience from the neighbouring countries with comparable climatological conditions, cost-effectiveness of the stocking practice and on the positive and/or negative influences of the species to be stocked on the indigenous flora and fauna or their environmental impact at large have to be taken in to account.

The major perennial reservoirs were stocked for the first time in 1950's when O.mossambicus, C.carpio and Osphronemus goramy were stocked. The impact of the individual species on the limited fisheries prevalent around that time was obvious and Fernando (1965) recommended abandoning the stocking of the two latter species. Fernando (1980) evaluated the stocking practices in the large lacustrine waters in Southeast Asia and concluded that the African cichlids were the most successful, and that the other species have had little impact on the fisheries of perennial reservoirs.

The stocking practices in Sri Lanka were reviewed by De Silva (1987a), who considered the practices in major reservoirs to be broadly divisible into two phases:

Phase I:  stocking with O.goramy beginning in the 1940s and culminating with O.mossambicus in the 1950s (stocking with cichlids still continues but the species utilized is the Nile tilapia, O.niloticus).

Phase II: stocking with the major Chinese carps (three species) commencing in the mid-1970s followed with two species of Indian major carps in the 1980s.

The available statistics up to 1980 indicate that nearly 9 million fry have been stocked since 1968 into the major perennial reservoirs in Sri Lanka (Table 22). This is not an excessive number and it could be construed as evidence for the lack of a well-planned stocking programme. Up to 1983 apart from the cichlids, which have been stocked, the major carps were not caught in sufficient quantity to enter into the statistics. However, occasionally dead bighead Aristichthys nobilis, weighing 20 kg, has been seen floating in one or two major reservoirs.

The reservoir fishery in Sri lanka provides a cheap source of protein for the rural poor. Therefore, all strategies adopted to increase the yield should aim at providing an animal protein source at the lowest possible price and not providing a diversity of table fish for the consumer. Sri Lanka is also blessed with a vast acreage of seasonal tanks which are proving to be ideal for extensive fish culture (Chakrabarty and Samaranayake, 1983; De Silva, 1983). The country is yet incapable of harnessing this resource effectively, primarily due to limitations in the fingerling production of the suitable species for culture in the seasonal tanks. Under these circumstances and in view of the very low return from stocking in perennial, major reservoirs stocking of the latter seems illogical.

It is possible that the carp failure resulted from the inadequate number being stocked at any one time and the stocked fish being undersized. There appears to be also biological and/or ecological reasons behind it. For example the inadequacy of the zooplankton biomass and the limited diversity may not favour a zooplankton feeding carp species. In addition the uniformity of the type of gear used in the reservoir fishery may limit the catchability of surviving major Chinese and Indian carps. It is unlikely that this limitation, due to the presence of obstructions on the bottom, will be removed.

Available data on what is claimed to be instances of planned stocking programmes in three major perennial reservoirs are summarized in Tables 23 and 24. They show that apart from the Indian major carp Labeo rohita, which is reputed to have given a return of >20% of the stocked number, the other stocked exotics have failed to have a significant impact.

The stocking of the major perennial reservoirs with Indian major carps was advocated recently by Sreenivasan and Thayaparan (1983). If one accepts that stocking becomes effective for the reasons outlined by Welcomme and Henderson (1976) then all available evidence tends to indicate that the impact of stocking of major exotic Indian and Chinese carps is likely to have little impact on the fishery. Stocking of Indian reservoirs in the state of Tamil Nadu, climatically the most comparable to that of Sri Lanka, with Indian and Chinese major carps have been only partially successful (Sreenivasan, 1984). Some species of Chinese carps are effecting the catches of rohu in Indian reservoirs (Shetty, personal communication). The yield from these reservoirs are poor compared to those in Sri Lanka inspite of stocking (Fernando, 1980; Oglesby, 1985). It is therefore incorrect to extrapolate isolated instances of success or partial success of stocking practice to Sri Lankan reservoirs. These carps are sub-tropical. None of the Indian major carp species is recorded to have established breeding populations south of 11°25'N (Sreenivasan, 1984) in the country of origin itself and it is unlikely that these species would establish naturally breeding populations in Sri Lanka which is further south than 11° 25'N and which is also unlikely to provide the complicated and little understood conditions needed for natural spawning (Sinha et al., 1974) found in large river systems such as the Ganges and the Cauvery. Establishment of naturally breeding populations either in the reservoirs or associated river systems is the key to success of stocking in fisheries that are prevalent in the major perennial reservoirs of Sri Lanka.

In some reservoirs, tilapia occurs without being stocked. In the newly impounded Maduru Oya reservoir (Table 23) the catch is dominated by O. mossambicus. Similarly Lunugamwehera, a reservoir of 3023 ha impounded in 1986 yields 132 kg ha^-1 annum^-1, of which more than 80% is O.mossambicus of 23.4 cm and 253 g in mean length and weight respectively.

In Soroborawewa reservoir (570 ha) O. mossambicus has been replaced by another cichlid O.niloticus which has established self-reproducing populations (Chandrasoma, 1986). The Ministry of Fisheries continues to stock certain reservoirs with O.niloticus, which is now considered to be a superior species with higher rate of growth and higher consumer preference in most countries of Africa and Asia (Pullin and Lowe-McConnell, 1982). The possible impact of this continued stocking on the reservoir fishery is considered later.

Considering the present fish requirements, the preference for tilapia, and especially the low cost associated with tilapia maintenance in reservoirs it is evident that there is no firm basis for continuing the stocking of the major perenni2al reservoirs of Sri Lanka with other species, especially those which are unlikely to produce naturally. However, if and when the fingerling producing capability of Sri Lanka exceeds that

needed for the more extensive and intensive culture programmes in the island - a very remote possibility indeed - the strategy of stocking the perennial reservoirs could be considered.

5.3.7 Socio-economic aspects

When compared to the agriculture sector the Fisheries sector in most countries in the region has received comparatively less attention; in most instances much less in proportion to its contribution to the country's gross national product. In such a context it is not surprising that the socio-econimic aspects have received the least attention, often giving way to scientific and technical aspects. The likely reason is that in most developing countries the upsurge in the fisheries sector is relatively recent: its modernization and associated technical inputs into proper management often take priority at the expense of the persons directly involved in the industry. This aspect was highlighted with respect to the Mahaweli Basin development, where little attention was paid to the socio-economic aspects of the fisheries sector (De Silva, 1985). However, the increasing awareness of the population that the staple, basic food such as rice has to be supplemented by good quality protein, and the increasing demand for employment, now calls for the attention of policy makers and planners to the socioeconomic aspects of the fisheries sector.

Colonization of the dry zone accelerated after independence in 1948 (Perera, 1984), and was associated with rapid development in the area. In the early period marine fish were readily available, and also the fishing skills were undeveloped amongst the colonists. These together with the preference for sea fish did not warrant an investment in the development of the inland fishery. However, with the gradual increase in the population in the dry zone and the increasing cost of sea fish the consumer habits began to change. The oil crisis in the early 1970s compelled the policy makers to develop the inland fishery further. As a result, since 1972 the public sector investment in inland fisheries has increased by nearly 800 times (Table 25). Successive Governments have given considerable attention to the development of the inland fishery, which until 1983/84 concentrated mainly at the provision of subsidized crafts and gear for the reservoir fishery.

The need for a marketing strategy of the developing reservoir fishery was first recognized by Indrasena (1965). Since then ready markets have become available and the need to process at site diminished. A marketing survey on five major reservoirs in the southeastern part of the island by Chandrasena (undated) showed that the marketing patterns differ in detail from reservoir to reservoir and that the price is dictated by traders who come regularly to the landing sites. Most traders buy their consignment from specific fishermen, and operate on push-bicycles. In some reservoirs wholesale dealers come in lorries or small vans for their daily purchasing, often visiting three to four reservoirs. It is not uncommon for a cycle vendor to move 10-20 km interior with the produce. It is rare, that a fisherman himself is engaged in trading, although over the last 4-5 years there has been an increasing trend among the fishermen of Kirribbanara Reservoir to do so.

There has been a gradual increase in the price obtained by the fisherman, as well as in the selling price by the trader. The price at which a vendor buys varies marginally from reservoir to reservoir, often the difference being less than Rs.1.00 kg^-1. The price also varies with the size of fish, bigger fish (for O.mossambicus over 300 g) selling for about 1.5 times more than the smaller fish. The indigenous fish (L. dussumieri and P.sarnana) are sold at about 60% of the price of O.mossambicus. Chandrasiri (undated) recognized that due to the lack of competition between traders they are able to keep the price fixed for a long time. The price changes only when the fishermen as an organized unit demand a higher price. Chandrasiri also reports that there is an increasing trend for largescale operators to move into the market and eliminate the cycle-vendor category.

The price per kilogram of O.mossambicus at the landing site ranges between 10 to 15 Rs., whereas the consumer has to pay at least double this price. The price to the consumer varies from season to season. In the rural markets the consumer has to pay more during the paddy harvesting season when migrant labour is utilized to work on the rice fields because of the increased demand for fresh fish to feed this labour force. However, the fisherman is not benefitting from this price increase while the middle man is.

The old prejudices, both cultural and religous, against freshwater fish have almost totally disappeared, and this has been recently (Anon., 1986a) attributed to the improved but low cost technology of introducing processed or semi-processed fish products. These cater to middle- and upper-middle income groups. It is unlikely that such activity has had a major impact on the fishery, either by improving the income of the producer or increasing, indirectly, the yields. Presently, almost all the landings in every reservoir are sold fresh on site. There are three major reasons for the increase in the demand for freshwater fish: (i) increased numbers of settlements and increasing population in the dry zone areas; (ii) the price of most marine fish has become prohibitively high over the last twenty or so years and is beyond the reach of even the above-average income households in the dry-zone villages; and (iii) over the last two decades the dryzone villagers acquired a ‘taste’ for O.mossambicus and developed methods of preparation to the Sinhala culinary taste.

Chandrasiri (undated) strongly recommended that the marketing of the reservoir fishery produce is channelled through a cooperative system or through an equivalent mechanism which is able to legally dictate a guaranteed price to the producer, and which changes from time to time according to market fluctuations. This suggestion is worthy of consideration but evolving and executing a price control mechanism for a fishery which is scattered is an uphill task.

At most reservoirs, with the intervention of the Ministry of Fisheries, Fishermen Cooperative Societies have been formed. Almost all the licenced fishermen are members of such a society. The degree to which a Society becomes effective varies considerably from reservoir to reservoir. In general, a society could decide to refrain from fishing activity for a specified period of time, and even decide on the number of operated crafts during a season. Amarasinghe (1988) believes that effective cooperative societies have a major role to play in the management of the fisheries, and quotes the example of the high yielding Pimburettewa reservoir (834 ha, Table 18). It is possible for a group of cooperative societies to federate and deal with the marketing of gear, for which at present no proper provision is made. Most fisherman have to travel considerable distances to obtain gear replacements, repair kits etc.

As mentioned earlier, most of the early reservoir fisherman began to be engaged in fishing activity as a means of supplementing their agricultural income. Chandrasiri (undated) believes that the second and third generations of the original settlers in the irrigation schemes have now taken to fishing as the sole mean of income, due to non-availability of land for the rapidly expanding population. This author surveyed the age-structure, educational level, and other associated features of the community, and found that the literacy rate of the fishing community was lower than the national average and that the most acute problem facing the community is the acute shortage of land and proper housing.

5.3.8 Future developments

The future developments of the fishery of the major perennial reservoirs should concentrate on optimizing the yield, and on improving the socioeconomic conditions of the fishing community.

5.3.7.1 Optimization of the yield

There are several avenues for optimization of the yield in reservoirs. Not all these strategies are applicable to all reservoir fisheries as they have to be considered in the context of the conditions of each fishery and the nature of the existing fish communities (Fernando and Holcik, 1982).

(i) New introductions

In tropical Asia most reservoir fisheries are dependent on introduced species or transplants, as the yields from indigenous species are low to moderate. There are some exceptions, such as the Ubolratana reservoir in Thailand (Bhukaswan, 1985; Fernando, 1980).

One of the earliest strategies advocated for optimization of the yield in Sri Lankan reservoirs was the introduction of both herbivorous and carnivorous species, particularly into the deeper reservoirs in the mid- altitude and the hill country (Fernando, 1971), which are very few in number and are of small surface area. Later on, Fernando (1980) considered it to be inadvisable to introduce carnivores into the reservoirs in South East Asia. It is often pointed out that the Sri Lankan indigenous fauna lacks a true zooplankton feeder and that it might be useful to introduce freshwater clupeids such as Limnothrissa miodon and Stolothrissa tanganicae or their equivalents, in view of the success of their introduction into Lake Kariba, Africa (Petr and Kapetsky, 1983). At least some Sri Lankan reservoirs do contain a small clupeid Ehirava fluviatilis which is an exclusive zooplankton feeder (Newrkla and Duncan, 1984). The estuarine, zooplankton feeding halfbeak, (Hyporamphus gaimardi) seasonally colonizes most of the dry zone reservoirs (De Silva, 1985). However, zooplankton species diversity as well as the proportion of zooplankton in relation to the phytoplankton in Sri Lankan reservoirs are considered to be low and therefore cannot support dense stocks of zooplankton-feeding fish.

In the Sri Lankan context, and in view of the increasing concern about recent (Barrel et al., 1985) and proposed introductions (McKaye et al., 1985) elsewhere, there are no valid reasons to believe that introduction of a zooplankton feeding fish into Sri Lankan reservoirs would have a positive impact on the fishery.

(ii) Exploitation of non-conventional resources

The fishery on the major perennial reservoirs in Sri Lankan is a mono-specific fishery, in which the gear and the craft have not changed (De Silva, 1983; De Silva, 1985). The likelihood of the presence of appreciable stocks of indigenous minor cyprinids in Sri Lanka reservoirs was reported by Schiemer and Hofer (1983), who based this opinion on their studies in the Parakrama Samudra. De Silva (1985) suggested that minor cyprinids, in particular Barbus chola, B.dorsalis, B. filamentosus, Rasbora daniconius, Amblypharyngodon melettinus, could be commercially fished without harming the existing fishery for O.mossambicus. Preliminary studies based on five reservoirs have proven that a subsidiary gillnet fishery, using mesh sizes 15 and 30 mm, could be introduced concurrently (De Silva and Sirisena, 1987). It is estimated that this fishery could sustain an annual yield three times that of the (Fig.17) existing fishery (De Silva and Sirisena, 1988a). By tapping this resource the yield from reservoirs could be increased by a minimum of 100%. However, it is expected that the fishing community will not readily take-up to this fishery and it would be a few years before the targets are fully realized. There are many reasons for this: the fishery needs to be strictly managed as indiscriminate use of small-meshed gillnets would be detrimental to the existing fishery; this requires the establishment of the supervisory infrastructure. Secondly, the fisherman have to be convinced of its potential, which would take considerable time. Also the number of fishing licences to be issued must be based on careful evaluation of stock assessment data. Moreover, the species from this fishery are unlikely to be accepted for direct human consumption. Processing techniques have to be developed to make them attractive for human consumption. An alternative and a more viable option would be to use this resource as a base for fish meal for the rapidly expanding aquaculture industry or for animal husbandry. In the latter case the producers would be able to solar dry the fish and introduce a system for the regular collection of this produce, at a suitable price.

The present evidence on the indigenous minor cyprinid resources in Sri Lankan reservoirs refutes the views expressed by Fernando (1980) with respect to indigenous fish species and their impact on the reservoir fisheries in tropical South East Asia. Poor knowledge of the stocks of small size fish, possibly also due to the lack of attention paid to them by scientists and not the shortage of a resource, could have prevented a full scale development of a fishery for such fish in the reservoirs of South East Asia. The minor-cyprinid community as well as the exotic cichlid O.mossambicus have coexisted for over three decades in the Sri Lankan reservoirs. Exploitation of the latter may have little adverse effect on O.mossambicus, but more work is needed to confirm this suggestion.

Eels present another non-conventional resource with potential for exploitation. Two species of eel (Anguilla nebulosa and A.bicolor) occur in the reservoirs in Sri Lanka (Table 14). Initial studies on five southern reservoirs have indicated that eel stocks differ from reservoir to reservoir. Experimental yield of 2.57 eels per unit of fyke net, approximating 0.61 kg, was obtained (Wickstrom and Enderlein, pers. comm.). Eels are not readily accepted for human consumption by the general population. The results of preliminary studies show potential for a small-scale fishery in selected reservoirs. For such a fishery sound marketing strategies will need to be developed in conjunction with the exploitation of these non-conventional resources.

Fig.17 The mean percentage of minor cyprinids and O.mossambicus, by number and weight, caught per net in five southern reservoirs (data from Sirisena and De Silva, 1988a; A.m- A.meletti nus, B.c.-B.chola, B.d-B.dorsalis, B.f.-B. filamentosus, R.d-R.daniconius, O.m.-O. mossambicus).

The third non-conventional resource that has remained underutilized in Sri Lankan reservoirs, as elsewhere in South East Asia, is the crustacean resource. Furst and Fjalling (pers.comm.) commented on the relatively rich crustacean/shrimp fauna in their preliminary investigations of this resources in five southern reservoirs (Table 26). But the Badagiriya reservoir, one of the most productive reservoirs in the island (see Tables 18 and 19), has few crustaceans, and Furst and Fjalling indicated that the high density of eels in this reservoir could be responsible for this paucity. A similar situation is known for some Swedish lakes (Svardson, 1972) where the presence of eels is known to greatly reduce the crustacean population. This suggests that both resources cannot be expected to reach a high yield in the same reservoir.

(iii) Water regime management

The influence of water level on fish stock abundance has been assessed by a number of authors (Welcomme, 1970; Martin et al., 1981; Beam, 1983; Miranda et al., 1984; Gaboury and Patalas, 1984; Henderson, 1985; De Silva, 1985b). The draw-down regimes may influence the fishery in a reservoir in numerous ways (Bernacsek, 1984).

In Sri Lankan perennial reservoirs the overall yield is known to fluctuate significantly through the year, generally being low during the months of very low water, as well as during months of very high water levels. The potential area available for nest-building for O.mossambicus is largely determined by such fluctuations which also affect fry survival. In reservoirs, sudden and significant changes in the water level are likely to strand fry and young in pools exposing them to severe thermal strains and predatory pressure. The yields of indigenous cyprinids L.dussumieri and B.sarana are known to be higher during the rainy months, possibly a reflection of greater vulnerability to the gear during their reproductive migration into the rivers.

Our understanding of the effect of water level fluctuations on the abundance of reservoir fish, particularly on O.mossambicus, is still not sufficient to allow fishing activities to be coordinated in relation to the water-regime management. Due to the recent river basin development the water regimes in a number of ancient reservoirs are likely to be affected, especially in the Mahaweli Ganga and Kirindi Oya catchments. As a result of the Mahaweli Ganga diversion changes have taken place in the ancient reservoirs Parakrama Samudra, Minneriya, Kalawewa and some others; as a result of the Kirindi Oya development and the creation of the Lunugamwehera Reservoir the water regimes of the Tissawewa, Yodawewa, Weerawila Reservoir and several others are also being affected.

In most of the above instance the water level fluctuations are likely to be stabilized and the flushing rate increased as a result of these diversions. Stabilization of the fluctuations may affect the nesting of the major fish species, especially O. mossambicus through the limitation of suitable areas and depth for nest building (De Silva and Sirisena, 1988). On the contrary, draw-downs of high magnitudes are known to adversely affect the reproductive biology and recruitment of tilapia stocks in reservoirs (Ruwet, 1961). Higher flushing rate may lower the overall productivity of the reservoirs. It is therefore still rather difficult to predict and quantify the overall effects of water level fluctuations on the respective fisheries.

A rapid progress in housing and other amenities available to the fishing communities at the major reservoirs are likely to occur in the next decade. Land allotments in the vicinity of the reservoirs to persons involved in fishing will be an official recognition of the community and will lead to better organization and management of the individual cooperatives and therefore the fishery. A consequent development of the marketing procedures and facilities are also likely to result.

It is envisaged that with the increasing emphasis which the Government (Anon., 1980) is putting on the inland fishery a guaranteed price structure for inland fish would come into effect in the near future. The introduction of a guaranteed price will further enhance proper management of the fishery of individual reservoirs through their cooperative societies, and lead to activities such as regular collection of statistical data, enforcement of closed seasons when needed, and to introduction of a subsidiary small mesh size gillnet fishery whenever applicable (see also Section 5.3.8).

In view of the increasing emphasis on the development of semi-intensive and intensive aquaculture programmes the demand for fingerlings of culturable species over the next decade is bound to rise almost exponentially. A conservative estimate of the fry requirement for such programmes by 1990 is expected to be around 45-60×10⁶ (for details see Section 5.4). Consequently, the demand for suitable land for fish pond construction, for large scale fingerling production will also increase. Competition for land between the inland fisheries sector and other sectors, primarily the agricultural sector, is already increasing. In view of this the major perennial reservoirs are likely to play a pivotal role in fish production. The enclosed bays and covers of the reservoirs in the vicinity of hatcheries could be utilized for rearing fry to fingerlings, a practice recently commenced by the Ministry of Fisheries (Plate VI) on an experimental basis. It is unlikely that this will have any negative impact on the existing fishery. The reservoir fishing community could be effectively engaged in the care of such facilities, giving them a sense of involvement in developing new activities, and also provide a small economic return to the community. Such involvement would not involve the introduction of an apparently competing force in the eyes of the established community, an important psychological consideration in a developing country.

5.4 Reservoir culture fisheries

Sri Lanka, unlike most other countries in South Asia, lacks a tradition of fish culture. Intensive culture as found elsewhere in South East Asia at the small-scale farmer level does not yet exist. Sri Lanka has nearly 30,000 ha of seasonal tanks (Table 8), which for most purposes are comparable to fishponds: they are small in size, they dry up for 3-4 months of the year, and they are highly productive. The potential of these water bodies for fisheries development was first pointed out by Mendis (1965). It was not until the late 1980's however, that his suggestion began to be realized on an experimental scale. Plans have been drawn up for the use of seasonal tanks for fish production on a large scale (Thayaparan, 1982).

Plate VI A pen utilized for rearing fry to fingerling stage of Indian major carps (Udawalawe reservoir).

5.4.1 Seasonal tank aquaculture programmes

The programme of utilization of the seasonal or village tanks (=reservoirs) on a large scale was drawn up in 1983 by a planning mission of the FAO Aquaculture Development and Coordination Programme, when the mission gave top priority to the culture of fish in these water bodies. The reasons for giving top priority for this extensive culture programme were:

An experimental project, funded by the UNDP, was carried out between 1981-83 to evaluate the feasibility of the seasonal tank programme. Based on these studies the Asian Development Bank came forward to finance the Aquaculture Development Project for Sri Lanka, of which the main activity was the utilization of the seasonal tanks for fish culture. Under this programme, commencing in 1984, it is envisaged to utilize 10,000 ha of seasonal tanks for fish production over a 5-year period (Table 27).

The programme also involves the strengthening of the fish breeding stations, primarily to increase their fingerling producing capacities, training of manpower and strengthening and development of the infra structure of the extension services.

5.4.1.1 Strategies

(i) Preparation of the water bodies

The seasonal tanks usually dry up completely between July-September and fill during the north-east monsoonal rains by the following December-January. As they fill up, the reservoirs are colonised by indigenous carnivorous fish species with accessory respiratory organs, such as Anabas testudineus, and Macrones sp. Small puddles in the reservoirs which do not dry up completely may continue to harbour some of these species. In the earliest strategies adopted and when the number of reservoirs stocked were small, these ‘nuisance’ species were eliminated by spreading a biodegradable toxicant such as bleaching powder. However, with expansion of activities this practice has become difficult to execute and expensive. The likely purpose of this strategy was to enhance the survival of the stocked fingerlings. However, it is believed that an almost equal return could be obtained by stocking larger fingerlings. Chakrabarty and Samaranayake (1983) suggested that only those reservoirs which retain water for 6-8 months, of which a depth of 1 m of water is found for a minimum of six-months should be selected for fish stocking.

Seasonal tanks come under the jurisdiction of the Department of Agrarian Services, and the day to day water management is carried out by the Village Cultivating Committee. Over the years, the liason between the Ministry of Fisheries and the Agrarian Services has improved, at least in certain Administrative Districts, and this has made it easier to use the suitable reservoirs for fish culture. For example, with the concurrence of the Agrarian Services, the area around the sluices has been protected with netting, to prevent fish escaping when water is released to rice paddies. It is envisaged to utilize a proportion of the annual quantum of profits from the culture operations for improvements of the reservoirs, such as for strengthening the earthern bunds, and for construction of barriers to prevent fish escape under unusual flood conditions.

(ii) Species cultured; yields

The species suitable for culture in the seasonal tanks should reach a marketable size in 6 to 8 months, and they should be able to utilize the natural food resources available in the tanks. In view of the paucity of such species in the indigenous fauna, perhaps with the exception of Labeo dussumieri and Barbus sarana, suitable exotic species have to be utilized. Unlike in the fishery of the major perennial reservoirs it is to be expected that cichlids will not be desirable for culture operations in the seasonal reservoirs because of their tendency to reproduce early in life in confined waters. The obvious choice of species becomes therefore restricted to the Indian major and Chinese carps, specifically those species which have been successfully artificially spawned in Sri Lanka, i.e. the grass carp, bighead carp, silver carp (Weerakoon, 1979), catla, rohu and mrigal (Balasuriya et al., 1983).

The yield from seasonal tank culture trials until 1983 and the performance of individual species were assessed by De Silva (1987; and Tables 28 and 29). It is evident that the overall yield was very variable between years and between reservoirs as well in a reservoir. The potential of these reservoirs for fish production is also evident from the data. The yield was not related to the stocking density. The Ministry of Fisheries has arrived at 2000-2500 fingerlings ha^-1 as a suitable stocking density for seasonal tanks, to be stocked approximately in equal proportions of grass carp, bighead carp, rohu, catla, mrigal and common carp, envisaginh the complete utilization of the possible available niches in the water body, as in a composite fish culture operation (Chakrabarty, 1982). The results of preliminary trials have clearly shown that the indigenous L.dussumieri is unsuitable for stocking, in view of its very low survival and the small size at harvesting. The most discouraging aspect of the culture operation is the extreme range in length and weight of all species at harvesting.

Chakrabarty and Samaranayake (1983) suggested that at a stocking density of 2000 fingerlings ha^-1 an average yield of 0.75-1.0 t ha^-1 could be obtained, provided that the selection of the reservoirs and the post-stocking management is carried out well.

(iii) Management

The involvement of the village in the culture operations is recognized as the key to the success of the operation. This has been achieved by forming small cooperatives for the management of each reservoir, the incentive being the profits, which are to be shared by the members. One of the most important aspects of the management is the prevention of poaching, and staggered harvesting. Staggered harvesting offers two major advantages: (i) prevents the flooding of the market over a very short-period, resulting in a consequent drop in the prices, and (ii) reduces the size-range of the harvested fish, making it possible for the slow-growing individuals to accelerate their growth and thereby minimise the size-range at harvesting.

It is reported that in instances where the trials have been successful the respective societies have invested in installing fixtures which reduce predation by birds by obstructing the paths of diving.

Plate VII: Receding water level in a seasonal tank during the dry period.

Plate VIII: A harvest from a seasonal tank (by Courtesy of the Inland Fisheries Division, Ministry of Fisheries, Sri Lanka).

(iv) Socio economic impact

It is expected that the seasonal tank programme, when properly conducted, will have a profound impact on the society. By being located in the midst of the villages in the dry zone, where the elements are harsh and the living conditions are poor the utilization of the seasonal reservoirs for fish production through the involvement of the villagers will provide a source of much needed animal protein and the excess produce will provide an extra source of income. The indirect benefits to the community will come from the management and the care of the reservoir itself, which in turn will affect the agricultural practices in the village. The seasonal tank programme is also bound to create opportunities for middlemen in the marketing and processing of fish. Most of all this would provide an activity in which the whole village would have to act as one unit, if they were to reap the maximum benefits. Such unity in the village undoubtedly has a very positive social impact on the population.

In one village, in which the seasonal tank programme has been successful for the last five years, the average household income has risen above that of the average peasant farmer, who was hitherto entirely dependent on the agricultural income (Anon., 1986a). The income from fishing is obtained before the crop harvest, and this minimises borrowing which is otherwise a common practice before grain harvest. Fish consumption has increased as a result of the seasonal tank programme. For example, in the village Thunkama up to 18.36 kg annum^-1 per head has been consumed which is about 3.28 kg annum^-1 per head above the national average (Anon., 1986a).

Any fish-culture programme, or for that matter any food production programme, of the magnitude of the seasonal tank programme of Sri Lanka requires specific conditions to become successful. Some of these conditions can be fulfilled with appropriate inputs at the correct juncture either in space or time: but some others such as the time and amount of rainfall, which is crucial to the success of the seasonal tank programme, is beyond the control of man. It is important to identify these prerequisites for success.

(i) Fingerling production

The successful implementation of the use of seasonal reservoirs for fish culture is indicated in the following diagram.

diagram 1

:-North-east -: monsoonal rains, reservoirs filling up INPUT I

:-Stocking -: INPUT II

:- Growth Period -: INPUT III

:- Drying Up -: INPUT IV

:- Harvesting-: INPUT V

Of the five “inputs” two are climatic and beyond the control of man. What is important is that stocking (i.e. Input II) is coordinated with the filling of the reservoirs, provided the climatic elements follow the expected general pattern. This means to ensure the success of this programme, by 1990 a total of 20 million fingerlings should be available during each stocking season (November to January).

To have 20 million fingerlings available during these three months, the ‘peak’ spawning period of the required species would have to be in August-September, when the water availability at most of the hatcheries is not optimal. The recommended artificial spawning periods for species to be stocked are summarized in Table 30.

Ideally, the peak breeding should be around July-August for the fish to reach fingerling size by November to be ready for stocking. Most of the Chinese and Indian major carps could be spawned more than once a year, and timing of breeding to suit the stocking time should not be a problem. The major constraint is the limited space for bringing up the fry to fingerling stage. To overcome this plans have to be made for using bays, coves or net-cages in major perennial reservoirs and to use suitable diets for each of the species to enhance this phase of growth under high stocking densities.

(ii) Fish feed

The large number of fry and fingerling will require a ready and sufficient supply of good quality feed for growing fish from fry to fingerling, both for the Seasonal Tank Aquaculture Programme and for the cage culture operations.

With intensification of fish culture the need to develop an appropriate feed technology is essential and remains one area in which major headway has not yet been made in Asia (Cho et al., 1985). In Sri Lanka fish meal is an expensive and rare commodity and fish feed industry is still to be established. In order to make the fish culture programmes successful there is an urgent need for development of low-cost feeds, primarily using locally available ingredients.

(iii) Minimising the effects of freak floods

Post-stocking flash floods may result in drastic reduction in the final yield. Although floods cannot be prevented their effect on the seasonal tank programme could be minimised with the incorporation or modification of proper fish barriers at the spills. Such barriers in the form of netting could prevent the escape of most of the fish and minimise the impact of such floods.

(iv) Harvesting and yields

When the programme is fully successful, it is to be expected that nearly 10,000 t of fish could be harve- sted within a three to four month period of March-June, much of it becoming available towards the end of this period. This yield would be in addition to that produced from the reservoir capture fishery. In view of the concentration of seasonal reservoirs primarily in five administrative districts (AnuradhaPura, Hambontota, Kurunegala, Polonnaruwa, Moneragala - see also Figs 6 and 7), which also have substantial capture fisheries, there is likely to be a significant increase in the availability of freshwater fish, much above the levels needed in these areas.

When the production increases due to regular stocking the middleman is likely to exploit the situation of excess produce which has to be transported considerable distances into areas where freshwater fish is normally not acceptable. A plausible solution to this problem would be to process the excess produce into smoked or sun-dried fish, which would not only remove the strain of marketing fresh fish but provide a means of guaranteeing a reasonable price to the producer. Dry fish always command a higher price, for some species well in excess of the corresponding lost in weight due to drying. It is not known yet whether all species cultured in the seasonal tanks would be suitable for smoking or drying. This research should receive top priority.

The suggested processing parallels the processing of tuna (Katsuwonus pelamis) in June-August on the southern coasts when landings reach a peak. During this period governmental organisations are unable to handle all the landings and hence the surplus fish is processed into dry-fish or ‘maldive fish’ (‘maldive fish’ is salted and sun dried fillet of tuna, and is used to flavour curries and coconut salad). A comparable situation has to be envisaged with the expected large harvests from seasonal tanks. It is not too early to train the villagers in the art of processing the produce and getting them ready to deal with such harvests.

(v) Stocking Rates

The potential yield from each seasonal tank is likely to be determined to a large extent by the stocking rate and the productivity of the tank. As evident from Tables 28 and 29 the yields are very variable between tanks and so is the performance of each species. Attempts have to be made to correlate the potential productivity of the seasonal tanks with the overall stocking density and species density. In other words the stocking rates for each water body have to be determined on the basis of data obtained over a number of years and a dynamic model formulated for the stocking rates. The present data do not provide a sufficiently broad base to make such an attempt.

The extension arm of the seasonal tank programme is being gradually strengthened, and stocking targets have been achieved within reasonable limits. It would be desirable to accumulate data on the limnology and productivity of the stocked tanks, which would enable to develop models based on limnological parameters and fish yields. Such models would, apart from helping to adopt measures to optimize the yield of individual water bodies, also make a unique contribution to fisheries science in general and also assist in better understanding of the link between limnology and fisheries biology.

(vi) Genetic Pool

The reservoir stocking programme will have to depend on the artificial breeding of 6 to 7 exotic species from brood stocks maintained, reared and cared for at a limited number of fish breeding stations. Careful management is needed to avoid degeneration of brrodstocks and to minimise disease problems. The obvious solution would be to replace the brood stocks regularly with stocks from the country of origin, but this is costly and also increases the risk of introducing new parasites and diseases. In most cases brood stocks thus imported are also the result of generations of inbreeding in the confines of fisheries stations. It has been recently shown that the performance of progeny from wild-caught major Chinese carps is far superior to that from broodstocks in established breeding stations (Sifa, 1985). The seasonal programme itself may provide a desirable way of halting, or at least minimizing the decline of the brood stocks. It could be appropriate to select for size, condition, coloration and other desirable characteristics of individuals at each harvest to supplement the brood stock. Although such a replenish -ment is not entirely equivalent to that of wild-caught fish, this method selects individuals which have been exposed to a comparatively harsher environment than in a breeding station and could provide a reasonable compromise enabling the vitality of the broodstocks to be maintained.

Although the fish culture programme in seasonal reservoirs is bound to be successful and to have a high degree of social impact, its cost-effectiveness has to be assessed as the project progresses from year to year and attempts should be made to recover the inputs of the Government into the programme. The main input by the Government, which will be a recurrent cost, will be the cost of production of fingerlings. The cost evaluations will need to take into account among others the salaries of personnel and cost of infrastructure amenities.

The management societies and/or fisheries cooperatives of each seasonal tank should be made to purchase the fingerlings. It may be necessary to subsidise the cost of fingerlings during the first few years of operation but not after a tank has yielded two consecutive good harvests. As an alternative a levy could be made on the quantity of fish produced. The latter may be more acceptable to the management committees because in a year of poor harvest they would not have to borrow for the purchase of fingerlings for the next year.

5.4.2 Culture fishery in major perennial reservoirs

As mentioned earlier (5.3.7.3), major perennial reservoirs are likely to be increasingly used for rearing fry to fingerling of species for the seasonal tank aquacualture programme. This practice is not considered as a strict aquacultural operation in the perennial reservoirs. Based on the results of the experimental cage culture of Oreochromis niloticus in selected major reservoirs (Wannigama and Weerakoon, 1982) one can assume that a significant number of people living around major reservoirs will in the near future take up cage-culture as a supplementary source of income.

In view of the capital investment that has to be made into cage culture, in the initial stages it will be necessary for the Government to consider providing subsidies for the construction of cages, and providing the necessary fingerlings. The experimental systems have shown that a stocking density of 200 m^-3 is the most profitable and that by providing a diet of 20% protein content, a profit of Rs.1000–3000 could be obtained at the end of a six-month growing period.

The families which are likely to be involved in cage culture are those engaged in the reservoir capture fishery. Such involvement will eliminate disputes and will provide a supplementary income to the fisherman, especially during the lean months of the capture fishery. It is expected that increased involvement of people in cage culture will also lead to a steady market price.