Assemblages and Distributions of Groundfish Species off the West Coast of Vancouver Island

Abstract

Fluctuations in environmental conditions have been shown to alter the productivity of the marine ecosystem off the west coast of Vancouver Island, but it is unclear how individual groundfish species respond to changing seas, in part because the distributions of these species along environmental gradients are unknown. The recent global declines in stock biomass, biodiversity, and mean trophic level of landings have led to proposals for ecosystem-based fisheries management. Before such a system could be implemented, it would be necessary to identify the species assemblages that would define management areas. By analysing catch data from biennial research trawls conducted 2004-2008, with oceanographic data from the same region we attempted to determine the distributions of the 38 species that comprise the top 95% of the biomass and to identify any natural assemblages that may be present. Neither an Average Bray-Curtis dendrogram nor a Canonical Analysis produced any realistic assemblages, though a Canonical Correspondence Analysis identified the two major factors in determining species identity as surface chlorophyll concentration and depth. We examined species richness, finding that it also increased with depth, though this may be an artefact of the sampling gear used, which targets larger bodied species. We conclude by questioning the validity and value of assigning species to discrete assemblages when species identity appears to vary continuously along environmental gradients and by discussing potential problems with assemblage-based fisheries management.