Effect of fishing intensity and selectivity on trophic structure and fishery production

Fishing intensity and selectivity patterns affect ecosystem structure and fisheries yield, the 2 fundamental performance measures in the ecosystem approach to fisheries. We used a simple multispecies predation model to explore the effect of alternative fishing strategies on a 3-trophic-level food chain. Fishing strategies included highly selective fishing, nonselective fishing, and balanced harvesting that harvests all species at an instantaneous fishing mortality rate either proportional to intrinsic population growth rate or proportional to current population growth rate. The results showed that harvesting species at higher trophic levels has a low impact on total biomass but results in very low yields and severe impacts on trophic structure. Selectively harvesting species at the bottom of the food chain reduces the biomass of all fish, results in high yields, and is the only strategy that maintains unfished trophic structure. Non-selective fishing produces high total yield, but can cause extinction of fish at high trophic levels, and severely alters the trophic structure. Balanced harvest strategies produce higher total yield than harvesting species only at the bottom of the food chain, and have a smaller impact on trophic structure than selectively harvesting the top predator or nonselective fishing, but cannot fully maintain trophic structure. While these findings from a very simple model can provide insight into results from more complex models, analysis of sensitivity to structural assumptions in such simple models will be required to shed further light on the dynamic consequences of fishing across multiple trophic levels.