The effects of fishing on demersal fish communities of the Scotian Shelf

Abstract

Since the 1970s, average individual weight of commercially targeted demersal fish on the Scotian Shelf (Canada) decreased by 51% on the eastern shelf and by 41% on the western shelf. For both systems, the integrated community size spectrum showed long-term declines in proportions of large fish and survey cpue of most targeted species. In the east, these changes were coincident with a doubling of fishing effort and a decline in bottom temperatures to the lowest value observed in 50 years. In the west, fishing effort more than doubled while bottom temperatures reached the highest value in 50 years. In both areas, declines in survey cpue and average weight were more prevalent for target species than for non-target species. Since introduction of the closure of the cod fishery on the eastern shelf in 1993 and the restrictions on landings on the western shelf, average weights and the community size structure have stabilized. In the east, this stability is associated with increasing bottom temperatures; in the west it is concurrent with high bottom temperatures. I conclude that both fishing and changes in bottom temperature have influenced demersal fish size but that the relative effects cannot be determined from current observations.