Covariation between grey seal (Halichoerus grypus) abundance and natural mortality of cod (Gadus morhua) in the southern Gulf of St. Lawrence

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Abstract
More than 10 years after the collapse of Atlantic cod (Gadus morhua) fisheries in Canada, the role of increased seal populations in the decline and lack of recovery of the stocks continues to be discussed. Using removals and abundance indices from synthetic populations, we found that sequential population analysis can uncover trends in natural mortality. We used this approach to examine variation in natural mortality (M) of southern Gulf of St. Lawrence cod. M increased from the late 1970s to the mid-1990s but declined slightly recently. Results were consistent with previous work indicating that M increased in the 1980s. Changes in estimated M for this cod stock matched fluctuations in grey seal (Halichoerus grypus) abundance. The increase in grey seal abundance from the late 1970s to the mid-1990s corresponded with the increase in estimated M of cod over this time period. The correspondence between seal abundance and M of cod supports the hypothesis that seal predation may be a cause of increased M. However, the diet information available suggests that seals consume mainly juvenile cod, whereas our evidence for an increase in M is for larger cod (ages 3 years and older).

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