Experimental assessment of gull predation on the Jonah crab Cancer borealis (Stimpson) in New England rocky intertidal and shallow subtidal zones

Predation by the Herring Gull Larus argentatus (Coues) and the Great Black-backed Gull Larus marinus (Linnaeus) on adult Jonah crabs Cancer borealis (Stimpson) was examined using exclusion cages at two rocky-shore sites on the coast of New Hampshire, USA. During the summer of 1990, crabs were tethered in exclusion and control replicates ,and survival monitored over two daytime low tides. Exclusion of gulls significantly enhanced crab survival, especially in lower tidal zones, and survival was higher overall during spring tides. The use of algal and crevice refuges significantly enhanced survival of crabs, especially at one site. Gulls may significantly depress densities of crabs in tidal zones accessible to them, limit the upper distribution of subtidal invertebrate species and influence the organization of intertidal communities.