The state of Canada’s iconic Northern cod: A second opinion

Abstract

Canada’s Northern cod stock (NAFO Divisions 2J3KL), once one of the world’s most important stocks, declined rapidly in the early 1990s and remains at historically low biomass despite a recent comeback. In contrast to many studies, recent stock assessments based on a new state-space model (hereafter Canada Department of Fisheries and Oceans, or NCAM model) concluded that natural (M) and not fishing mortality (F) was the main cause of stock depletion and has kept it at low levels since. Here we offer a second opinion on those conclusions, using the same catch and survey data, but based on a VPA model that allows for variability in M and unreported catch. This model posits that the biomass, both total and spawning from the NCAM model, is overestimated (35%), that fishing mortality was a major contributor to the early 1990s decline and slow rebuilding, and that the decline began in the late 1980s and was not as knife-edged as suggested by the NCAM. In agreement with the NCAM results, M has been variable and episodically elevated, but much less than suggested by the NCAM. VPA using a non-variable M did not fit well. In addition, the stock recruitment relationship suggests an asymptote at near 106 t, similar but slightly higher than the current limit reference point for spawning biomass. Of importance, the present analyses suggest that fishing has impacted rebuilding and will slow achievement of a spawning biomass that avoids recruitment overfishing.