Management strategy evaluation applied to the conservation of an endangered population subject to incidental take

Abstract

Management strategy evaluation (MSE) is a simulation method commonly used to evaluate the expected performance of harvest strategies with respect to target populations. However, MSE is also useful for evaluating strategies designed to manage incidental take from endangered populations. We adapted MSE to the case of Sacramento River winter Chinook salmon, an endangered population that is subject to incidental take by mixed-stock ocean salmon fisheries that target more abundant stocks. Pursuant to the US Endangered Species Act, the US National Marine Fisheries Service called for a new fishery management strategy that would link the allowable fishing mortality to the population’s risk of extinction. Our objective was to evaluate extinction risk under different harvest strategies. We simulated the dynamics of the salmon population and the management process simultaneously. The management process included harvest strategies that set allowable fishing mortality rates as a function of population size. Strategies that reduced fishing mortality rates in response to low estimates of population size resulted in lower extinction risk than strategies that set fishing mortality rates at current or historical levels. The number of years of data taken into account by a strategy and the extent of reductions in fishing mortality resulted in different frequencies and durations of reduced fishing opportunity. By quantifying the trade-off between the risk of extinction of an endangered population and the opportunity for fisheries to harvest target stocks, our study illustrated the utility of MSE for applications to endangered populations.