Research Document - 2011/036
Alternative hypotheses for causes of the elevated natural mortality of cod (Gadus morhua) in the southern Gulf of St. Lawrence: the weight of evidence
By D. P. Swain, H. P. Benoît, M. O. Hammill, G. McClelland and É. Aubry
Abstract
Natural mortality (M) of older (ages 5+ yr) cod in the southern Gulf of St. Lawrence has been unusually high throughout the 1990s and 2000s. We compared the weight of evidence for a suite of alternate hypotheses for the causes of this elevated M. A significant portion of the losses attributed to M in the late 1980s and early 1990s may instead be due to unreported catch, but the contribution of unreported catch to estimated M from the mid 1990s to the present can only be negligible. The hypothesis that the losses represent emigration rather than mortality can be rejected. While data are limited, there is no evidence to support the hypothesis that disease is a major contributor to the elevated M. Likewise, the hypothesis that contaminant-induced mortality is a significant component of the elevated M is not supported by the evidence. Life-history change (early maturation) in combination with poor fish condition may have contributed to moderate increases in M (by 0.1-0.2) in the early to mid 1980s, but M due to these causes would have declined when fish condition subsequently improved. Neither life history change (early maturation, early scenescence) nor poor fish condition are supported as important factors in the current high level of M in the 2000s. Parasite-induced mortality related to direct damage to organs and tissues or depletion of energy reserves is small in this population. However, it is possible that parasite infection may contribute the elevated M by increasing the susceptibility of heavily infected fish to predators. The sharp increase in M of 5+ cod as their abundance collapsed in the late 1980s and early 1990s is consistent with the predator-pit hypothesis for the cause of this high M. Given the diets, distributions and abundances of potential predators of large cod, grey seals are most likely to be the predominant predator producing this pit. The available diet information indicates that grey seals consume large cod (>40 cm in length), that they appear to show positive selection for large cod over small cod, and that when foraging in the vicinity of cod aggregations large cod can be a major component of the diet. Due to data gaps, the quantity of large cod consumed by grey seals is uncertain. However, some assumptions for filling data gaps lead to consumption estimates that account for a high proportion of the M of 5+ cod. There is also indirect (correlative) evidence that grey seal predation plays a role in the elevated M of adult cod and other large demersal fish. The hypothesis most strongly supported by the weight of evidence is that a major component of the current high M of 5+ southern Gulf cod is due to predation by grey seals.
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