Squaring the circle: reconciling fishing and conservation of aquatic ecosystems

Size-at-entry regulations in fisheries cause major disruption to aquatic ecosystems, including truncation of age- and size-structures, destabilization of fish stocks, directional selection on phenotypic traits and a by-catch of unwanted species and sizes. Here, we use simple dynamic models of size-spectra to examine an alternative, so-called balanced harvesting. Balanced harvesting helps in retaining the approximate power-law size-structure of natural ecosystems, whereas size-at-entry regulations do not. Balanced harvesting is less likely to destabilize steady states than size-at-entry regulations set close to the size at maturation. Surprisingly, our numerical results suggest that steady-state biomass yield can be substantially increased by switching from size-at-entry to balanced harvesting. On the basis of these results, we argue that the goals of conservation and of greater yields seem less difficult to reconcile than have previously been thought. However, to work towards these goals require a change in our approach to fishing.