Food webs and the sustainability of indiscriminate fisheries (original) (raw)
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Garcia, S.M. (Ed.), Kolding, J., Rice, J., Rochet, M.-J., Zhou, S., Arimoto, T., Beyer, J., Borges, L., Bundy, A., Dunn, D., Graham, N., Hall, M., Heino, M., Law, R., Makino, M., Rijnsdorp, A.D., Simard, F., Smith, A.D.M. and Symons, D. (2011). Selective Fishing and Balanced Harvest in Relation to Fisheries and Ecosystem Sustainability. Report of a scientific workshop organized by the IUCN-CEM Fisheries Expert Group (FEG) and the European Bureau for Conservation and Development (EBCD) in Nagoya (Japan), 14–16 October 2010. Gland, Switzerland and Brussels, Belgium: IUCN and EBCD. iv + 33pp The conventional selectivity paradigm is briefly reviewed and its performance examined from an ecosystem perspective. It is stressed that the overall (cumulative) selectivity of the harvest process in an ecosystem is the result of nested selection by fishers and fisheries of: (i) habitats; (ii) species assemblages; (iii) populations; and (iv) individuals. A range of ecosystem models predict the strong impact of concentrated fishing (selective fishing) on ecosystem structure stability, resilience and productivity. There seem to be advantages (in both yield and maintenance of ecosystem structure and functioning) to distributing fishing pressure broadly across available species and ecosystem compartments. Balanced harvesting was therefore defined by the workshop as a strategy that distributes fishing pressure across the widest possible range of trophic levels, sizes and species, in proportion to their natural productivity, reducing fishing pressure where it is excessive. The few attempts to verify the impacts predicted by models in real ecosystems with empirical data had limited success, indicating that such a demonstration might be a significant challenge. Data from African small-scale fisheries were presented as a possible example of the capacity of multiple fisheries targeting an extremely broad range of species and sizes to extract high yield with limited impact on ecosystem structure. There are also a number of examples of surprising consequences of selectivity regulations resulting in either operational changes in the fishery or to unexpected shifts in the ecosystem. Emerging research priorities and management implications are reviewed.
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Catching fish in proportion to their productivity, termed balanced harvesting, has been suggested as a basis for the ecosystem approach to fishing. Balanced harvesting has been criticized as uneconomical and unachievable because of the level of micromanagement it would require. Here, we investigate the consequences of allowing a fixed number of fishers in a small-scale fishery to choose what size fish to attempt to catch. We examine this from a game-theoretic perspective and test our predictions using an agent-based model for fishers’ decisions coupled with a size-spectrum model for the dynamics of a single fish species. We show that smallscale gillnet fishers, operating without size-based regulations, would end up catching small and large fish in proportion to their productivity, in other words balanced harvesting. This is significant because it shows that, far from being unachievable, balanced harvesting can emerge without external intervention under some circumstances. Controls are needed to prevent overfishing, but minimum size regulations alone are not sufficient to achieve this, and actually reduce the sustainable yield by confining fishing to a relatively unproductive part of the size-spectrum. Our findings are particularly relevant for small-scale fisheries in areas where there is poverty and malnutrition because here provision of biomass for food is more important than the market value of the catch