Impacts of Seasonal Stock Mixing on the Assessment of Atlantic Cod in the Gulf of Maine

Atlantic cod (Gadus morhua) in the Northwest Atlantic off New England and southern Atlantic Canada exhibit a complex population structure. This region has three independently assessed stocks [Georges Bank, Gulf of Maine (GOM), and the 4X stock south of Nova Scotia], all of which are known to mix with one another. Population assessments of these stocks, however, assume no interpopulation mixing. Using simulations, we evaluated impacts of ignoring mixing resulting from seasonal migrations on the GOM stock assessment. The dynamics of the three stocks were simulated according to different scenarios of interstock mixing, and a statistical catch-at-age stock assessment model was fitted to the simulated GOM data with and without mixing. The results suggest that, while mixing causes measurable bias in the stock assessment model, under the conditions we tested in this study, this model still performed well. The relative timing of seasonal migration of the three stocks plays a critical role in determining the magnitude of bias. The scale and trends among years in the bias of parameter estimates were driven by how representative the catch and survey data were for the GOM stock; this representation changed depending on the mixing rates.