CAN FISHERIES INFLUENCE INTERSPECIFIC COMPETITION IN SYMPATRIC POPULATIONS OF HERMIT-CRABS

Marine fisheries have resulted in large-scale manipulations of predato r populations. The practice of discarding by-catch, under-sized fishes and offal from trawlers has led to an increase in the populations of certain scavenging bird species. The greatest population increases hav e occurred in the most aggressively competitive species, while some su bordinate species have decreased in abundance. Several recent studies indicate that a similar situation may exist for epibenthic scavengers. We explore the possibility that enhanced food supply in the form of f isheries-generated carrion may affect the competitive balance between sympatric populations of hermit crabs, Pagurus bernhardus and P. pride aux. Under normal conditions, both species have overlapping diets and occur in equal abundance at the site studied. In laboratory investigat ions both species consumed the same types of carrion. However held obs ervations revealed that while P. bernhardus aggregate in areas disturb ed by trawls where they consume animals damaged in the trawl path, P. prideaux do not. Morphometric analyses revealed that P. prideaux have smaller chelae relative to P. bernhardus. Thus, P. prideaux may avoid dense aggregations of P. bernhardus which are more aggressive. These b ehavioural differences, coupled with higher fisheries mortality for P. prideaux, may provide a mechanism whereby P. bernhardus could outcomp ete P. prideaux in areas were fishing disturbance and discarding occur frequently.