Impacts of environmental change and direct and indirect harvesting effects on the dynamics of a marine fish community

We describe dramatic shifts in the species composition of the marine fish community of the southern Gulf of St. Lawrence using a 35-year time series of catch rates in an annual bottom-trawl survey. We attempt to understand the causes of these changes using a traits-based approach that relates the similarity among species in their abundance trends to similarities in their ecological traits. We selected traits based on a priori beliefs of how each should reflect susceptibility to changes in a different external factor potentially affecting the community. We found evidence for an effect of ocean climate and top-down effects of fishing and seal predation, but not for bottom-up effects of prey availability on adult fishes. Mean body length in the community decreased dramatically in the 1990s. This reflected the removal of large-bodied fishes by fishing and sharp increases in the abundance of small fishes. The biomass of small fish was inversely correlated with an index of predation on those fish by larger fish, suggesting strong predator control of the abundance of small-bodied fishes. Our results suggest that changes in ocean climate combined with direct and indirect effects of harvesting can dramatically and rapidly alter the composition of marine fish communities.