Research Document - 2012/034

Revised population model for Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence

By D.P. Swain

Abstract

The status of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence (4T cod) has been assessed using sequential population analysis (SPA) implemented using the adapt software. In order to accommodate large changes in the instantaneous rate of natural mortality (M) experienced by 4T cod, recent assessments have fixed age-aggregated (2+) M at 0.2 in 1971-1979 and at 0.4 in 1994-1997 and estimated it in 5-7 year blocks in other years. The February 2009 assessment of this stock identified discrepancies between the observed abundance indices and those predicted by the assessment model. A revised population model has been developed to address this problem. The revised model, implemented using the AD Model Builder software, estimates separate trends in M for ages 2-4 and 5+ years, starting from fixed values in 1971-1976. M is estimated either in blocks of years or using a random walk. Both approaches to estimating M produce similar results. Estimated M of 5+ cod increased sharply in the late 1980s and early 1990s, and remained at a high level throughout the 1990s and 2000s. In contrast, estimated M of cod aged 2-4 years decreased in the early 1990s and remained at a relatively low level in the following years. The revised model provided a much better fit to the abundance indices than did models that used age-aggregated (2+) M. Retrospective patterns were negligible for the revised model. Like earlier models, the revised model indicated that the abundance and biomass of 4T cod are at record low levels and are declining.

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