A Simulation Study of the Effects of Spatially Complex Population Structure for Gulf of Maine Atlantic Cod

Abstract

There is growing evidence that numerous fish stocks consist of several smaller, reproductively isolated units that vary in their ability to produce new recruits each year because of a mismatch with environmental conditions. It has been suggested that spawning aggregations exist for the Gulf of Maine stock of Atlantic cod Gadus morhua and that approximately one-half of the historic aggregations are locally depleted. We developed a spatially explicit, age-based projection model to investigate the effects of complex population structure and spatially variable recruitment on Gulf of Maine Atlantic cod. The results of this study demonstrate that there are significant disparities in the estimates of spawning stock biomass, annual yield, and annual recruitment when a stock with a complex population structure is treated as a single stock. In addition, treating a stock with a complex population structure as a single stock can mask a steady decline in the values of these variables as the spawning aggregations gradually become depleted. To achieve rebuilding of the simulated Gulf of Maine Atlantic cod stock within 10 years, we determined that F = 0 would be required. Because continued bycatch mortality in other fisheries would prevent this even with the closure of the Gulf of Maine cod fishery, it may not be possible to rebuild this particular stock in a short time frame. When fishing mortality is removed, the stock only rebuilds to one-half of the initial carrying capacity because about 50% of the reproductive capacity of the stock has been lost. Without recolonization, the simulated stock has lost its biological capacity to return to the original unfished condition. Based on the simulations in this study, the behavior of the Gulf of Maine Atlantic cod stock is more consistent with that of a stock with a complex population structure than with that of a truly single stock. In light of this, the management measures for this stock and other stocks with the potential for complex population structure should be carefully evaluated. Managing such stocks as single units may be inappropriate and lead to both inadequate protection of the resource and reduced fishery yields.