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Research Document - 2011/038

Recent trends in abundance of larval anisakine parasites in southern Gulf of St. Lawrence cod (Gadus morhua), and possible effects of the parasites on cod condition and mortality

By G. McClelland, D.P. Swain, and É. Aubry

Abstract

Larvae of the marine mammal parasites, Pseudoterranova decipiens, Anisakis simplex and Contracaecum osculatum, were surveyed in cod (Gadus morhua) from the southern Gulf of St. Lawrence between September, 2007 and May, 2009. In 2008-09, abundances of all three nematode species surged to levels unprecedented for Northwest Atlantic cod, an event that may be related to a warming trend in sea temperatures. A weak positive relationship between condition and intensity of infection was observed in both small and more heavily infected large southern Gulf cod. This relationship is likely indirect, resulting from variation in feeding intensity or foraging success among cod. Cod which consume more food are in better condition but acquire more parasites. There was no tendency for the relationship between condition and parasite abundance to become less positive over the winter period when little feeding occurs. These results do not preclude a direct negative effect of parasite infection on cod condition, but they do indicate that any such effect is weak relative to other factors affecting condition. Because sub-lethal effects should become apparent before lethal effects, these results also suggest that parasite-induced mortality related to direct damage to organs and tissues or depletion of energy reserves is small in this population. Analyses of changes in frequency distribution of parasite abundance with cod size or age do not appear to indicate mortality of heavily infected southern Gulf cod, but losses resulting from parasitism may be offset by increases in incidence of infection as cod grow and exploit more heavily infected prey. Analyses of worm count frequency distributions of larval anisakine nematodes have, however, provided evidence of parasite-induced mortality in cod from the Cape Breton and Central Scotian shelves, and in American plaice throughout the southern Gulf and Scotia-Fundy regions. Parasite infection may contribute to the elevated natural mortality of southern Gulf cod by increasing the susceptibility of heavily infected fish to predators.

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