Trophic interactions between key predatory fishes and crustaceans: comparison of two Northwest Atlantic systems during a period of ecosystem change

Abstract

This study shows that a sharp ecosystem transition in dominant communities during the early 1990s, from finfish to crustaceans, was common to the 2 northernmost Northwest Atlantic ecosystems, the NewfoundlandLabrador shelf (NL) and the northern Gulf of St. Lawrence (nGSL). Fishery and survey data show that populations of Atlantic cod Gadus morhua, typical of most finfish species, collapsed during the late 1980s to early 1990s in both systems, while Green- land halibut Reinhardtius hippoglossoides populations changed little. Biomass of northern shrimp Pandalus borealis increased following the collapse of cod in both systems, likely due, at least in part, to release of predation pressure. Predation appeared to have relatively little effect on bio- mass of snow crab Chionoecetes opilio. Shrimp replaced capelin as the principal prey in the diet of NL cod and nGSL Greenland halibut in the mid-1990s. The contribution of shrimp to the pred- ator diets was generally highest when neither capelin nor other suitable prey (fish or squid) were available. We conclude that the NL and nGSL ecosystems are similar in form and function, differ- ing from Canadian Atlantic ecosystems further south. The implications of a change in predomi- nant forage species from capelin to shrimp in these systems are unknown, but could conveivably include changes in the pathway and efficiency of energy flow.