Endogenous Fishing Mortality in Life History Models: Relaxing Some Implicit Assumptions

Life history models can include a wide range of biological and ecological features that affect exploited fish populations. However, they typically treat fishing mortality as an exogenous parameter. Implicitly, this approach assumes that the supply of fishing effort is perfectly inelastic. That is, the supply curve of effort is vertical. Fishery modelers often run simulations for different values of fishing mortality, but his exercise also assumes vertical supply and simply explores a series of these curves as different scenarios. The seemingly innocuous assumption of vertical supply conflicts with a large body of empirical work on behavior of fishermen and fishing fleets. Economists and fisheries scientists consistently find that fishing behavior is responsive to economic opportunities over time and space as well as across target species. Accounting for this phenomenon requires that fishing mortality be made endogenous. This paper demonstrates an approach to endogenizing fishing mortality in life history models by allowing the fish stock in the previous period and other behavioral drivers to enter into the equation that predicts fishing effort in the next period. The paper discusses conditions under which the standard approach is approximately accurate and when endogenous fishing mortality dramatically alters model predictions. An empirical application to the Gulf of Mexico gag, a species of grouper, illustrates the importance of endogenizing fishing mortality. Accounting for fishing behavior ultimately will improve predictions from management models and avoid fisheries management failures.