Abstract
Under the ecosystem approach to fisheries, an optimal fishing pattern is one that gives the highest possible yield while having the least structural impact on the community. Unregulated, open-access African inland fisheries have been observed to sustain high catches by harvesting a broad spectrum of species and sizes, often in conflict with current management regulations in terms of mesh and gear regulations. Using a size- and trait-based model, we explore whether such exploitation patterns are commensurable with the ecosystem approach to fisheries by comparing the impacts on size spectrum slope and yield with the different size limit regimes employed in the Zambian and Zimbabwean sides of man-made Lake Kariba. Long-term multispecies data under fished and unfished conditions are used to compare and validate the model results. Both model and observations show that the highest yields and low structural impact on the ecosystem are obtained by targeting small individuals in the community. These results call for a re-evaluation of the size-based management regulations that are ubiquitous in most fisheries.
Highlights
Size selectivity is deeply rooted in fisheries theory and it has been a long-standing policy in fisheries management to protect the juveniles and target the adults (Caddy 1999; Kolding and van Zwieten 2011)
These regulations are justified by the results of the traditional yield-per-recruit models (Beverton and Holt 1957) that are widely used for simulations to optimize relative yields as a function of entry age and fishing pressure in single-species assessments
The rationale is that ecosystem function and resilience depends on a dynamic relationship within species, between species and their abiotic environment, so that the conservation of these interactions and processes is of greater significance for the long-term maintenance of biological diversity than simple protection of species (UNEP Conference of the Parties to the Convention on Biological Diversity (UNEP/CBD/COP) (1998)
Summary
Size selectivity is deeply rooted in fisheries theory and it has been a long-standing policy in fisheries management to protect the juveniles and target the adults (Caddy 1999; Kolding and van Zwieten 2011). These regulations are justified by the results of the traditional yield-per-recruit models (Beverton and Holt 1957) that are widely used for simulations to optimize relative yields as a function of entry age and fishing pressure in single-species assessments. The rationale is that ecosystem function and resilience depends on a dynamic relationship within species, between species and their abiotic environment, so that the conservation of these interactions and processes is of greater significance for the long-term maintenance of biological diversity than simple protection of species (UNEP Conference of the Parties to the Convention on Biological Diversity (UNEP/CBD/COP) (1998)
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