Predation regulation of sedimentary faunal structure:: potential effects of a fishery-induced switch in predators in a Newfoundland sub-Arctic fjord

The collapse of the cod fishery in Newfoundland has coincided with marked increases in abundances of snow crab, pandalid shrimp, and other crustaceans that prey on sedimentary infauna. A 3-year sampling program in Bonne Bay, Newfoundland indicates differences in composition and number of these predators in the two main arms of the fjord that coincide with strong differences in benthic community structure. To test whether predation pressure contributes to the observed patterns in sedimentary fauna, exclusion field experiments with full and partial cages were deployed in both arms at 30-m depth and sampled along with ambient sediments at 0-, 4-, and 8-week periods. Predation significantly influenced species composition, abundance and, in some cases, diversity. The most striking changes included increases in the polychaetes Pholoe tecta and Ophelina cylindricaudata in exclusions relative to controls, and concurrent declines in the polychaete Paradoneis lyra and the cumacean Lamphros fuscata. In laboratory experiments, fresh non-disturbed sediment cores from each experimental area were either protected or exposed to snow crab, the most abundant predator in the bay. A snow crab inclusion experiment was also carried out in the field, using cages similar to those used for exclusions. Despite differences in sedimentary faunas in the two arms, both types of experiments detected a predator effect that was very similar to that documented in exclusion experiments. Thus, despite differences in the scales associated with each type of manipulation, our results suggest that crab predation is a significant structuring force in Newfoundland sedimentary communities. Given the historical changes that have occurred in predator composition as a result of cod over-fishing, we hypothesize that broad-scale community changes may be taking place in North Atlantic benthic ecosystems.