Patterns and drivers of resilience in over-fished populations

Globally, roughly two-thirds of exploited fish and invertebrate populations are thought to be below target limits, with a large number near critical limits. Deterministic population models for fished populations suggest that recovering populations should be as easy as scaling back fishing effort. Yet, some stocks seem to defy this logic, and a large number of theoretical and empirical studies have attempted to describe factors that may prevent populations from recovering. In this seminar, I will first present some basic ideas and theory about exploited populations and their recovery from near-critical levels. I will then present the framework and results from a global meta-analysis that attempted to establish how well populations conform to theories of recovery. For this analysis, we described over-exploited populations using a stochastic process framework, and treated them as ailing patients. Using survival analysis methods commonly employed in medical studies, we assessed the chance of recovery under a number of conditions. Although our results suggest that the predictability of recovery was strongly negatively affected by very intense fishing prior to recovery, we also showed that for moderately over-exploited stocks, recovery is often faster than expected. To put these results into context, I will discuss some theoretical arguments that suggest possible mechanisms for resilience in exploited populations - and mechanisms that may impair long term resilience in the face of continued high exploitation. The former need to be promoted in management systems to enhance the sustainability of fisheries in the long term.