REPORT OF THE JOINT WORKSHOP ON THE ROLE OF STOCKING AND INTRODUCTIONS IN THE IMPROVEMENT OF PRODUCTION OF LAKES AND RESERVOIRS

INTRODUCTIONS

The difference between introductions - as the one time transfer of species new to a region or waterbody, and stocking - as the repeated transfer of native or established fish into a waterbody - was clarified. Similar work on stocking and introductions by FAO through other regional bodies was also described. Participants then briefly summarized the introductions that had taken place in their own countries (Table 1). The oldest introductions in the region were those of European temperate species made late last century and early in this one. However, since 1940 transfer of tropical species, principally tilapias and Indian and Chinese carps have been preferred.

Introductions have usually been made intentionally but accidental appearances of exotic species have also been recorded. Both lakes and reservoirs have been included in introduction programmes with varying success, although usually increases in total production have been recorded. Some harmful impacts on native species have been observed but overall the effects have been successful. A possible exception to this is Oreochromis mossambicus, which because of its free proliferation and stunting, finds little favour in some areas. Cyprinus carpio has, on the other hand, contributed considerably to local capture and culture fisheries.

Concern was expressed on the risks of a species introduced into one country spreading to another, and it was felt that some measure between some countries concerned might be advisable where there is a risk of an introduction spreading from one country to another.

Most of the countries have legislation which may suffice to prevent entry of dangerous species but others have reasonably well established laws prohibiting or controlling new introductions. Sri Lanka, Thailand, Indonesia and Hong Kong have quarantine regulations which may avoid the spread of diseases but enforcement is difficult and even the country with the most strict regulations has witnessed the recent appearance of new undesirable species.

STOCKING

Stocking practice

In discussing management of reservoirs, objectives of the various countries differ. Some nations give priority to large-size fish of good market value, whereas others manage their fisheries for maximum production. This is reflected in stocking practice and in the preferred species. It also indicates that the development of a standardized stocking strategy for the region as a whole is not possible.

India has followed a policy of stocking with major Indian carps supplemented with some Chinese carps. The present stocking strategies are dominated by marketing preferences and by the short food chains of such fish. Major carps of northern origin have, therefore, been introduced into the southern water. Because of high market values of the carps the stocking cost is usually recouped despite relatively low levels of yield. Stocking strategies on the islands are better defined in populous countries such as the Philippines and Sri Lanka, where emphasis is placed more on high yields. Similar trends have been noted in small island nations of the Pacific which are also characterized by a lack of suitable endemic lacustrine fish.

It was concluded that there is a large diversity of criteria which have to be taken into consideration for establishing stocking strategies. Such criteria are difficult to unify into a common policy at the present. As the first step a collective effort should be made to identify species already tested in various countries, which have proved to be of value for stocking into reservoirs. It was, therefore, recommended that an inventory of existing and potential species for introduction be prepared for the region. This would include information on the biology of the species, its ecological requirements, current experiments with its stocking both positive and negative, interactions with other species, long-term changes in its stocks, notes on its aquaculture use, etc. The Secretariat will prepare a standardized questionnaire which will be sent to fisheries departments and individuals. Coordination, editing and publishing would also be done by FAO in collaboration with Mr. M. Mackinnon of Queensland State Fisheries, Australia.

Main species of fish introduced into lakes and reservoirs of IPFC countries represented at the Working Party

Several approaches to the determination of optimal numbers of fish seed needed for stocking into lakes and reservoirs based upon simple indices in particular, seem suitable for structures where large numbers of water bodies are to be stocked. However, the use of such formulae is limited by the lack of information at present available to enable the several assumptions inherent in such formulae to be validated. Further investigations are needed at the experimental level and a wider data base is required for a better evaluation of MEI. It was therefore recommended that studies be carried out on a regional basis to elaborate simple models for predicting fish yields from Southeast Asian dams, reservoirs and lakes and to determine their validity. Dr. Dwivedi, subject to official approval, offered the services of the Central Institute of Fisheries Education, India, to collect and interpret information from the region in collaboration with the Secretariat. Several participants (Malaysia, Sri Lanka, SEAFDEC) expressed their willingness to collaborate in this exercise.

Environmental aspects of stocking

The paper introducing the topic “Environmental aspects associated with stocking and introductions”, highlighted the problems arising from attempts to establish unified stocking strategies not only for the IPFC region but also for one country. It is evident that there are strong regional, environmental and ecological differences. In India, for example, there are three basic geographical zones which differ in their rainfall, temperature ranges and in the indigenous fish populations. The Gangetic river system, characterized by heavy seasonal rains and a high silt content, is dominated by major Indian carps, to the west in the more arid environments these are replaced by species of Barbus; in peninsular India the dominance of major carps gradually declines. Indian major carps stocked into reservoirs in this latter region grow well but do not breed. Thus, for maintenance of fisheries, stocking with artificially produced seed is necessary.

It has been recommended that technologies for fish seed production should be based on ecosystem approaches. Stocking of arid zone reservoirs requires their annual restocking as their water volume and surface area are usually drastically reduced, and salinity may increase. Some of such reservoirs may retain residual fish stock. Long-term stocking programmes are required for the Gangetic plain reservoirs where the effect of stocking is usually visible only after three-four years. For such reservoirs, long-term observations are necessary to appreciate the impact of stocking.

Tilapia is known to interact in reservoirs with major Indian carps, resulting in a slowdown of carp growth. A selective intensive removal of tilapia will return the situation to that prior to stocking. Tilapia become very common in reservoirs with a high carbon level as observed for example in West Bengal and Maharashtra. Other interactions mentioned included those observed between grass carp and common carp with the first species destroying common carp spawning areas (i.e., macrophytes), resulting in the decline in common carp production. Stocking silver carp into reservoirs with the existing Catla population leads to food competition, subsequently resulting in a decline in catches of Catla.

A high organic level can be utilized by some fish. It has been pointed out in the discussion that raw organic material is utilized, for example, by Prochilodus in South America and also by T. mossambica and that such material has been shown to contain important amino-acids and vitamins elsewhere in the world. Common carp has been successful in alleviating pollution and the grass carp macrophyte problems of a Kashmir lake. Also Labeo calbasu has been stocked into reservoirs dominated by Microcystis, on whose products of decay it feeds. Tilapias have been stocked into Hong Kong and Singapore water supply reservoirs to contain high phytoplankton densities, and this has been very successful. In Singapore, the overcrowding of tilapia has been successfully controlled by snakehead and the income from tilapia production (300 kg/week) is sufficient to pay for the upkeep of the Biological Unit working on the reservoir.

Better planning for maximizing fish production would be greatly assisted by establishing baseline environmental data. Continuous monitoring of such data is then recommended as a means of evaluating the success or failure of management strategies.

A number of aspects still require attention. For example, it is not clear which factors cause the excellent growth of tilapias in southern Indian reservoirs. Nor is it clear whether T. nilotica is preferable to T. mossambica for stocking such waters. The low cost of tilapia makes it accessible to the rural poor but the fish is not appreciated by urban population. In some countries, tilapia is not very popular, but is acceptable in others, indicating the uncertain status of fishes of this genus.

Stocking of cold waters

The cool waters of the highlands were identified as a special area requiring attention. Rivers of these areas have been largely managed by the introduction of trout. However, stocking with this species has been criticized because of the risk of introducing diseases into the countries such as India and Sri Lanka and also because such stockings lacked a clear purpose. The present approach in countries where trout is still being stocked is to use it for cage culture, although this does not prevent the escape of the fish into the river system. In Irian Jaya, the introduction of trout is being advocated for lack of other species suitable to occupy highland rivers. However, there are several native species within the IPFC region which are suited to cold water reservoir conditions. Schizothorax and Tor spp. have been suggested as suitable candidates for stocking but little data are available on their ecological requirements. Human populations in areas with cold water rivers, lakes and reservoirs are often in great need of animal protein, and the workshop recommends that much more effort be spent on investigating local species for their suitability for stocking.

Fish ladders

Fish ladders are highly uncommon in the region and they appear to be impractical for high dams. Their use in connexion with low height irrigation systems has been successful in Thailand. There is a general lack of information on migratory habits of indigenous fish species in the region and there is consequently a low demand for construction of fishways. However, the Workshop recommends that data be collected on the present and potential needs for fishways.

Use of alternative species

The present need for alternative fish species for stocking has been highlighted for the Philippines and Sri Lanka, where almost all economic species had to be introduced. A similar situation is evident for the small islands of the Pacific and for cold, high-altitude waters. The biology and ecology of only salmonids are sufficiently known at present to carry out reasonably controlled stocking. Australia has successfully bred artificially a number of indigenous species and stocked them into their tropical and sub-tropical reservoirs. Several species have established self-breeding populations. These species could be used especially for stocking reservoirs, particularly on Pacific islands to supplement some other better known species.

Economic aspects of stocking

The economics of stocking reservoirs are complex as a number of factors are usually involved. In many large reservoirs, the returns on investment may be small but these economic losses can be counterbalanced by social benefits; hence, a combined socio-economic evaluation of reservoir stocking should be taken into account. Good returns on investment of stocking have been recorded for a number of Indian reservoirs stocked with major Indian carps (see Table below). A similar evaluation should be made for southern reservoirs dominated by tilapias, to be able to assess this aspect more thoroughly.

Table 1 Profit to Government from some Indian reservoirs

Jatiluhur reservoir in Indonesia has shown that the production cost is highest for gourami (I.Rs. 2 700/kg) and lowest for Tilapia nilotica (I.Rs. 658/kg), with common carp in between (I.Rs. 1 200/kg). Tilapia is economically a loser if marketed dead as it fetches only I.Rs. 600/kg as compared with I.Rs. 1 800/kg for the fresh fish. Overall the Jatiluhur stocking programme is profitable. Introductions of tilapias for control of eutrophication of water supply reservoirs in Hong Kong and Singapore appear economically advantageous due to the savings in the cost of water treatment, decrease in water losses (in Hong Kong reservoirs) and profit from fish (Singapore). It has been stressed that in many situations economics must be considered together with the social impacts of stocking, and more attention should be paid to this aspect. Reservoir fisheries would greatly benefit from reinvestment of profits into reservoir management, such as is practised in Thailand.

Aquaculture practices in reservoirs and lakes

Aquaculture practices associated with reservoirs are now widespread in the region. Three main types of practice are commonly used: (i) open water culture; (ii) cage culture, and (iii) pen culture.

1. Open water culture: This is practised mainly in small dams and reservoirs and relies heavily upon stocking as well as supplementary feeding and fertilization of the water. The species utilized vary according to locality and local preference. Thus, Chinese carps may be preferred in cool waters, whereas tilapias are better adjusted to tropical waters. Stocking size varies greatly in the region but indications show that returns increase with the size of fingerlings stocked. Also, while information on stocking densities is scanty, stocking rates in China are based on such factors as fertility of the water and the size of the water body. Further development of open water culture is limited, however, by several factors, among which are the paucity of relevant data on lakes and reservoirs, insufficient understanding of indigenous fish and their interaction with the exotic species, and the unavailability of information on socio-economic impacts of this form of culture.

2. Cage culture: The length of experience with cage culture varies with the different countries in the region, some having had long years of experience in cage culture while others have adopted this culture technique more recently. As in open water culture, selection of fish species depends on local preference. Materials used for cages range from local bamboo to steel angle frames. Supplementary feeding is generally required although in some cases, culture depends on natural productivity. In such cases, limnological studies are needed to determine the basis for such free rearing. Cage culture is well developed in China where it is used for production of fingerlings and for table fish. The use of cages for production of fingerlings has proved to be very economical since much of the infrastructure and feed needed for land-based installations is avoided. Trials on cage culture are now being carried out in Sri Lanka under IDRC assistance, involving mainly the determination of appropriate stocking densities, feeding regimes, feed composition, etc.

3. Pen culture: This form of culture is adopted mainly in shallow lakes and has reached its highest expression in Laguna de Bay in the Philippines, where at present there are about 34 000 ha of pens for milkfish culture, contributing more than 20 percent of the fish supply of the Metropolitan Manila area. Fish pens in Laguna de Bay are constructed using bamboo and palm tree poles and nylon netting, the edges of which are submerged in about 0.5 m of mud. These are stocked with 20 000 to 30 000 milkfish fingerlings per hectare and no supplementary feeding is usually required. Pen culture, although proven successful in Laguna de Bay has not spread to the other countries in the region. This may be due to several factors including, but not limited to, seed availability, material and labour availability and depth of water. It was suggested that studies be made to determine the real causes/factors that constrain the expansion of fish pen culture in areas outside the Philippines. It was noted, however, that experimental trials are now being conducted in a number of other waters including lagoons and shallow reservoirs where fluctuation of water level is low (e.g., Sri Lanka) and oxbow lakes and oxygenated depleted swamps, as in India.

Structures analogous to the pens in Laguna de Bay are found elsewhere, principally in brackishwater coastal lagoons. Brush parks consisting of bamboo mattresses are used in India (“janos”) where they capture prawns and in Sri Lanka (“attakothu”) using mangrove twigs where they are mainly non-selective. It was pointed out, however, that these aggregating devices are different from fish pen culture as known and practised in the Philippines in that the pens are not collecting or catching traps and have to be stocked regularly with fingerlings of the species for culture, principally milkfish.

It was suggested that limnological studies be conducted prior to the development of fish pen culture. This is necessary to ensure that the carrying capacity of the water body is not exceeded, especially since the culture system, as done in the Philippines, relies heavily on natural productivity. The example of Laguna de Bay was cited where the normal culture period of six months had to be extended to almost a year as a result of the overcrowding of fish pens.