Potential Yield of Small Reservoir Fisheries in South Asia

Background

Despite long traditions in reservoir construction in Asia, fisheries within them have only recently been developed. Natural fish production from Asian reservoirs is generally low, as indigenous lacustrine fish species able to colonize reservoir habitats are rare. Since the 1950s, fish production has been increased in two ways. In Sri Lanka, lacustrine African tilapia has been introduced, establishing self-perpetuating populations, while in China, India and Thailand, extensive aquaculture systems (culture-based) have been developed in reservoirs, regularly stocked with farmed seed fish. Self-perpetuating populations can be assessed using conventional stock assessment tools. However, culture-based fisheries require specific methods to assess both harvesting and stocking regimes including their interrelation. Culture-based fisheries management has remained largely haphazard, thus development of specific assessment methods has been urgently required.

Approach

A literature review identified a lack of appropriate assessment methodologies for culture-based reservoir fisheries as an important constraint to their development. Thus the project concentrated at first on population dynamics modelling of culture-based fisheries, and later on developing practical methods for biological and socio-economic assessments.

Mathematical models were developed for the two key population processes in culture-based fisheries, density-dependent growth and size-dependent mortality, and tested on data sets from literature. The growth and mortality models were combined in a length-structured population model, which was subsequently used to explore the effects of various management strategies on the yield of both seasonal and perennial reservoir fisheries.

Field case studies were carried out in China and Thailand to obtain an overview of Asian reservoir fisheries and their problems, to assess the environmental and socio-economic constraints and opportunities linked to culture-based reservoir fisheries, to test the newly developed assessment methodology in practice and to assess needs for further research.

Findings

The project has substantially improved the conceptual understanding of culture-based fisheries and provided qualitative guidelines for the management of such fisheries, and tools for their quantitative assessment.

A population dynamics model for culture-based fisheries has been developed which incorporates simple sub-models for the two key population processes of density-dependent growth (using von Bertalanffy theory) and size-dependent mortality. This general model can be used to simulate the effects of any possible stocking and harvesting pattern on the production from a culture-based fishery.

Stocking and harvesting regimes for perennial and seasonal culture-based reservoir fisheries have been evaluated quantitatively using the population model and a quantitative assessment method developed on the basis of modelling results has been tested on stocking and catch data from a Chinese reservoir. Results indicate that optimal stocking and harvesting in a perennial reservoir fishery are interrelated and must be considered together in management.

The project has identified major constraints to fish production from small and medium size reservoirs, namely: inappropriate stocking and harvesting regimes adopted largely due to lack of assessment methodology, conflicting uses of the reservoir, and unsatisfactory distribution of benefits from communal culture fisheries. The modelling study has also made important contributions to the development of adaptive management strategies for culture-based fisheries.