On balanced exploitation of marine ecosystems: results from dynamic size spectra

Fisheries are often managed to protect small young fish and to harvest big old fish. This can be wasteful, leading to large parts of catches being discarded. A recent suggestion is that it could be better to distribute fishing more widely across species and body sizes, balancing it more closely to the natural productivity of different organisms. Here, we test effects of such fishing against more traditional methods using a model of a single fish species with a dynamic size spectrum together with a fixed spectrum of plankton. This has the feature that productivity is determined by the bookkeeping of biomass in the model, and decreases as fish grow larger. The results show that harvesting smaller fish (which have higher productivity) allows a greater sustainable biomass yield than harvest-ing larger fish (which have lower productivity); the greater spawning-stock biomass that comes from protecting large fish contributes to this. Balanced exploitation brings fishing mortality more in line with this natural variation in productivity. In addition, the resilience of the ecosystem to perturbations can be improved, and disruption to the size distribution of organisms in the ecosystem reduced. We argue that there are potentially real benefits to be gained by moving towards more balanced exploitation of marine ecosystems, unconventional though this is