Varying components of productivity and their impact on fishing mortality reference points for Grand Bank Atlantic cod and American plaice

Population productivity is determined in large part by growth, maturity and recruitment. This determines the level of fishing that the population can sustain without declining. We examined how these components of productivity have varied during warm and cold periods for two Grand Bank fish populations, and how this variation affects fishing mortality reference points. Productivity of both Div. 3NO Atlantic cod and Div. 3LNO American plaice has varied considerably over time. Projections of population size under different conditions showed that the level of recruits per spawner played a major role in determining the level of fishing mortality that did not result in population decline. For plaice there was also a substantial impact of varying proportion mature at age. The impact of factors other than temperature on recruits per spawner and maturity meant that the association of productivity with temperature was not consistent. When productivity was at its lowest, the level of fishing mortality that could be sustained without causing rapid population decline was very low. The results of this study clearly demonstrate that the impacts of changing productivity can be rapid and very large and if fishing mortality reference points are not adjusted accordingly the results can be catastrophic. It may be helpful to develop fishing mortality reference points that incorporate all components of productivity and are updated using only recent data. Such reference points could be used in combination with buffers around a biomass limit reference point (Blim) and/or low risk tolerances for declining below Blim to account for the uncertainties that would remain even in frequently updated reference points.