Trophic effects of fishing southern rock lobster Jasus edwardsii shown by combined fatty acid and stable isotope analyses

Abstract

The southern rock lobster Jasus edwardsii is a commercial species that has benefited from the complete protection offered by no-take reserves, with higher abundances and larger ani- mals recorded in reserves than in adjacent fished areas. What remains unclear is whether there is any change in the diet of lobsters in reserves, for example, as a result of increased intraspecific competi- tion for food. We used combined chemical tracers to examine the diet of lobsters in fished and reserve areas in 2 bioregions in eastern Tasmania. δ15N values of lobsters were richer in fished than in reserve areas, indicating that lobsters eat a greater proportion of food items from higher trophic levels in fished areas. Mixing models suggest that ascidians, sea urchins and the turbinid gastropod were all important food sources for lobsters, but the importance of these food items differed between biore- gions. This spatial variability may suggest thatthe small size of the reserve in one bioregion is inad- equate at ensuring the diet of lobsters is protected from fishing pressure. Fatty acid profiles of lobsters supported the importance of these food sources to lobsters. Differences between bioregions, or inside and outside of reserves, were not apparent using fatty acids. The present study highlights that lobster fishing has the capacity to alter the trophic status of prey for generalist predators and suggests that fatty acid analyses may be limited in detecting changes in the dietary composition of such generalist feeders.