Research Document - 2011/039

Changes in size-dependent mortality in the southern Gulf of St. Lawrence marine fish community

By H.P. Benoît and D.P. Swain

Abstract

In this document we review the evidence for size-dependent mortality in the marine fish community of the southern Gulf of St. Lawrence (sGSL), and consider the support for fishing and predation as likely causes for this mortality. This work was undertaken as a contribution to the October 2010 Zonal Advisory Process on the Impacts of Grey Seals on Fish Populations in Eastern Canada. The review is based on analyses of data from the annual bottom-trawl survey of the sGSL. Size-based indicators suggest that since the early 1990s, the survival rate of small fish in the ecosystem has improved considerably and the survival rate of the largest fish in the community has declined considerably. These size-related changes are largely consistent both within species and at the community level. The improved survival of small fish is consistent with release from predation by formerly abundant large groundfish. Declines in the abundance of larger fish appear to be related to fishing and predation by grey seals. The overall intensity of fishing on groundfish species has declined considerably since the early 1990s, and has been at a very low level for more than a decade. Decreases in the abundance of larger fish, coincident with a rapid increase in fishing intensity during the late 1980s and early 1990s, is consistent with a direct effect of fishing. Failure of those populations to recover despite the recent period of little fishing is not consistent with a direct fishing effect. Species that have failed to recover are all known grey seal prey. It is possible to calculate age or stage-dependent mortality rates for a small number of sGSL species. The rate of survival for juvenile skates, white hake and American plaice has improved, in some cases dramatically, over the past 20-30 years. In contrast, the rates of adult survival have declined over the same period. Because rates of fishing mortality also declined over this period, the worsening adult survival is concluded to be due to natural mortality.

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