Evaluating Alternative Harvest Strategies for Alaska Groundfish Fisheries Based on Estimates of the Probability of Overfishing

Increasingly, management systems require that catch limits account for scientific uncertainty by specifying a buffer whereby the catch limit is lower than that determined based on an assumption of perfect information. A “P* approach” is a method whereby catch limits are specified such that the probability that a future fishing mortality (F) exceeds the F corresponding to maximum sustainable yield (F_MSY) remains at or below a pre-specified value (P*). Some sources of scientific uncertainty are typically ignored when calculating catch limits, leading to underestimates of uncertainty about population biomass. A management strategy evaluation was developed to examine the performance of a “P* approach” that included accounting for typically ignored uncertainties. The P* approach was considered because it seems mandated by U.S. law; however, our results demonstrate that the approach has concerning properties. First, the amount of uncertainty that is taken into account is arbitrary and influences resulting catch limits. Secondly, small changes in the acceptable probability of overfishing can result in large changes to resulting catch limits. Third, the estimated and true probability of overfishing diverge as additional uncertainty is taken into account. Simulated cases will be shown to demonstrate each of these properties. Alternative harvest policies will be discussed.