Maximizing Fisheries Yields While Maintaining Ecosystem Structure

Under the Ecosystem Approach to Fisheries an optimum fishing pattern is one that gives the highest yield while causing the least structural impact on the community. Unregulated, overall non-selective open access African inland fisheries have been observed to sustain high sustainable catches by harvesting a broad spectrum of species and sizes, often in open conflict with current management regulations in terms of mesh and gear regulations. We use a size based trophic model to theoretically explore the optimal fishing pattern in terms of the effort on a given size range. The results are validated and compared with long term multispecies data from man-made Lake Kariba under fished and unfished conditions. Both model and observations show that the highest yields with the least structural impact on the ecosystem is obtained by predominantly targeting the smallest components of the community. These results call for a re-evaluation of the size based management regulations that are ubiquitous in most fisheries.