THE GEOGRAPHY AND BODY-SIZE DEPENDENCE OF TOP-DOWN FORCING IN NEW ENGLAND'S LOBSTER-GROUNDFISH INTERACTION

We examined changes in the body-size dependence of the groundfish American lobster interaction across the steep environmental and biogeographic gradient from the Gulf of Maine to southern New England, USA. Lobster (Homarus americanus H. Milne-Edwards, 1837) abundance has increased dramatically in the Gulf of Maine since the 1980s, A parallel upsurge in southern New England ended in 1997 with the onset of a disease epizootic. To characterize the geography and body-size scaling of the predator-prey interaction, and to determine whether changes in the predator assemblage could explain the increases in lobster abundance, we drew on field predation experiments and surveys on nurseries where lobsters first settle, as well as trawl-survey time series of fish and lobster abundance on habitats exploited by older lobsters. Our results suggested that the fish predators frequenting lobster nurseries in the Gulf of Maine are less diverse or abundant than those in southern New England and that predation risk in the Gulf of Maine is correspondingly lower. Trawl-survey time series suggested that the expansion of the lobster population was more strongly correlated with declines in average body mass of fish predators in the Gulf of Maine than in southern New England. The resultant relaxation of predation pressure on the American lobster in the Gulf of Maine reflects a functional change in a key link of the food web induced by a change in predator body size. A closer examination of size-specific interactions of individual species and functional groups in the groundfish assemblage is therefore warranted.