The three case studies of small river basins in humid tropical Asia have shown that in most situations inland fisheries can successfully develop under the multiple-use conditions of river basin land and water resources. Of the three basins discussed above, the Mahaweli in Sri Lanka has had engineering structures such as dams and canals in existence for a long time. Until recently the major constraint to development of fisheries in the Mahaweli was not manipulation of the water resource, but religious constraints which reduced the use of fish to a bare minimum and thus did not provide stimulus to any improvements. This has changed, however, in the second half of this century. In the other basins (Agno and Nam Pong), prior to river modifications by dams, the inland fisheries was limited to rivers and had a subsistence character.
The rapid environmental changes in river basins in the last few years, with inevitable damming of rivers, establishment of new reservoirs and irrigation systems, together with the change of people's thinking, have provided a challenge to fishery planners and managers.
The best preparatory work for rational management of new reservoirs was undertaken in Thailand where fisheries statistics have been collected since the very beginning of the Nam Pong reservoir's existence. Monitoring of the basic fisheries parameters there has helped in planning for an increase of fish yield through regular fish stocking.
In the Mahaweli Basin in Sri Lanka, exotic tilapias have proved to be the best adapted for reservoir environment, and the current fishery is largely based on them. The Accelerated Mahaweli Development Programme with its formation of several large reservoirs gives a major impetus to further development of inland fisheries, and in parallel with the large reservoirs the intensification of fisheries in small tanks and floodplain lakes is also being considered. With careful planning, followed by efficient management, the currently completed or nearly completed reservoirs, as well as the other island waters, should serve as a major example for complex river system reservoir fishery development under multiple-use conditions.
The best monitoring and statistical records available for the Nam Pong have greatly assisted in the preparation of a fishery model which alongside several non-fishery models provide further guidance for the rationalization of the Nam Pong river basin resources, including fish. Such a modelling has clearly shown that in the Ubolratana reservoir regulatory fishery mechanisms must be introduced to optimize fish yield.
With the exception of the Agno River Basin, reservoir fisheries are becoming an important source of fish protein. While the pre-reservoir period was characterized by a prevalence of subsistence fishery of a small number of fishermen, the presence of reservoirs has led to establishment of commercial fishery and to the full involvement of fisheries authorities in the reservoir fish stock improvement. A plan for the Mahaweli Basin fishery development indicates that reservoirs should contribute about 60% to the total fish production, the rest coming from tanks, floodplain lakes and aquaculture. In the Nam Pong Basin, aquaculture in ponds, paddy fields and in other waterbodies also significantly contributes to the fish protein supply. This shows that while the reservoir fishery may not be a panacea for solving fish protein requirements, it greatly contributes towards this goal.
The cost-benefit evaluation for the Nam Pong Basin, based on economic parameters only, shows only marginal benefits for investors. Social benefits, not included in the evaluation, would probably increase considerably the total benefits if a socio-economic approach were applied.
In focusing on the reservoir fishery, the well documented analysis of Nam Pong reservoir fisheries shows constraints to the fish stocks being imposed there by the fishery itself. Lack of fishery regulations have led to changes in the character of fish stocks and to their overexploitation, i.e. a tremendous increase in fishing pressure, which has led lately to a declining fish yield. Thus, it is not an impact of other users of the resource which limits the fisheries of these reservoirs. In the Agno situation where reservoirs are under the impact of mining waste, with the resulting high sediment input, it has been estimated that the reservoir fishery provides enough fish to satisfy approximately 50% of the protein demand for the subsistence fishermen community of some 500 people. The lowest (third) reservoir, nearing completion, is considered to have the best fishery development opportunities of all the reservoirs, and the fisheries authorities are now concentrating their efforts predominantly there.
The coordination of resource management is still weak in some river basins. The lack of a coordinating river basin authority for the Nam Pong Basin has been recognized as a major obstacle to the realization of the potential benefits. A similar problem is evident also for the Agno River Basin where, although collaboration between some authorities and government departments is excellent, the low level of inter-provincial contacts prevents an effective introduction of ameliorative measures.
The Mahaweli Authority, a government corporation set up in 1979, is given the responsibility for implementation of the Mahaweli Ganga Development Programme, i.e. for promoting and arranging the cooperation of government departments, state institutions, local authorities, public corporations and other persons in the planning and implementation of the scheme and in the development of any special area, e.g. fisheries.
Some of the lessons learned from the three case studies are summarized below. They show especially the need for improvement in approaches, management and coordination. The analysis of the three case studies indicates that in spite of the complexity and many interactions between various factors involved in multipurpose river basin development projects, remedial measures for optimizing fisheries under constraints are available for all situations, and it is only lack of their implementation which prevents achieving better results in river basin planning and management.
There is a need for better coordination between individual governmental and other bodies during the planning stage of major developments of land and water resources. The existence of a central coordinating authority has been shown to be a definitive advantage for the coordination of activities and for the implementation of the programme of development. Although government fisheries authorities have been active in all three river basins in varying degrees of efficiency, they have been weakly integrated into the overall basin development programmes. Fisheries experience gained from one river basin could be of benefit to another river basin of a similar character within the same region, and there is scope for transfer of such knowledge and experience through TCDC.
In the three case studies the major negative impact from users other than fisheries is from the discharge of pollutants, especially mining waste. In the Agno Basin the lack of a central authority for the basin makes enforcement of any preventive measures difficult. In such cases the government should request the enforcement of necessary remedial measures.
In all three basins non-reservoir associated inland fishery represents an important component of the total freshwater fish yield and highly contributes to the total fish production. This fishery utilizes floodplain lakes and tanks (Mahaweli), brackishwater ponds (Agno), and ponds and irrigation systems (Nam Pong). Fisheries development planners consider fish production from these waterbodies equally important as that from the reservoirs.
In all three basins a sound managerial approach to inland fisheries, both capture and culture, requires a good understanding of the current environmental situation, which should be based on regular data collection. Regular monitoring of selected limnological parameters, fish stocks and fishing effort should be introduced where still lacking, and improved where effort on such data collection is not consistent. This would enable one to more carefully estimate the impact on fishery by other uses of the same resource than is possible at present.
In two river basins, i.e. Nam Pong and Mahaweli, excessive fishery pressure on reservoir fish stocks has resulted in declining yield. A model constructed for the Nam Pong (Ubolratana) reservoir fisheries in Thailand shows that the major constraints imposed on the fisheries in the multiple-use context are those originating from the fisheries itself. Similar models for the other two basins would greatly assist in clarifying the level of impacts and would help to identify remedial measures for fisheries improvement. As such models have to be based on sound data, the establishment of careful data collection is a prerequisite for the modelling process.
Amongst factors most closely related to the reservoir fish yield are (i) fishing pressure (Nam Pong and Mahaweli): the fish stock composition has been shown to change unfavourably as a result of heavy fishing pressure under the situation of non-regulated fishery (Nam Pong), while a better regulation is wanting in the Mahaweli; (ii) engineering manipulation of water level (Mahaweli); (iii) the presence or absence of exotic fish (Mahaweli); (iv) stocking (Nam Pong, Mahaweli).
The selection of the most suitable fish species for reservoir stocking is still limited by our poor knowledge of species interactions. While exotic species introductions into some reservoirs may result in a long-term success in the form of their dominating fish landings, e.g. tilapias in the Mahaweli, elsewhere the success may be only temporary, e.g. Trichogaster pectoralis in Nam Pong. Long-term observations in the Nam Pong reservoir have shown that indigenous fish figure more prominently in fish landings than exotic fish.
There is a need for guidelines for technical advice on optimizing inland fisheries in multiple-use context. The use of such guidelines during the preparatory stages, i.e. during the planning and design process when negotiations between the future users take place, would assist both the fisheries and non-fisheries planners in a better understanding of specific requirements of fisheries in regulated waters and in otherwise modified conditions. In turn, this should promote the appreciation of fisheries needs and interests under new situations resulting from other impacts.
The case studies have shown the need for a manual on suitable fish species for stocking Indo-Pacific inland waters, especially reservoirs and lakes. Such a manual should summarize the current experience with each species under various reservoir and aquaculture conditions, and to show its suitability for multispecies situations.
The only available evaluation of cost/benefit ratios for the Nam Pong river basin development is based on economic parameters only. Although it is widely recognized that fisheries have contributed considerably towards the favourable opinion about the success of the reservoir component of the scheme, in economic terms the total Nam Pong river basin development has been considered only marginally successful. It is recommended that social aspects be included in future cost/benefit evaluations of multiple-use projects.
