Comparison Between Maximum Sustained Yield Proxies and Maximum Sustained Yield

Attaining maximum sustained yield (MSY) is a central goal in U.S. fisheries management. To attain MSY, fishing mortality is maintained at FMSY and biomass at BMSY. Replacing FMSY and BMSY by “proxies” for FMSY and BMSY is commonplace. However, these proxies are not equivalent to FMSY and BMSY. The lack of equivalency is an important issue with regard to whether MSY is attained or whether biomass production is wasted. In this paper we study the magnitude of the equivalency. We compare FMSY/BMSY (calculated using the ASPIC toolbox) with the proxy estimates, F40%/B40%, published in GARM III. Our calculations confirm that in general the FMSY/BMSY calculations differ from the GARM III proxy estimates. The proxy estimates generally indicate that the stocks are overfished and are at relatively low biomasses, while the ASPIC estimates generally reflect the opposite: the stocks are not overfished and are at relatively high levels of abundance. In comparing the two approaches, the ASPIC estimates appeared favorable over the proxy estimates because 1) the ASPIC estimates involve only a few parameters in contrast to the many parameters estimated in the proxy approach, 2) “real variance” estimates for the proxy are not available so that it is difficult to evaluate the statistical adequacy of the proxy approach relative to the ASPIC approach, and 3) the proxy approach is based on many components (e.g., growth, stock and recruitment, etc.) that are subject to considerable uncertainty.