EFFECTS OF PREDATOR FORAGING BEHAVIOR ON PATTERNS OF PREY MORTALITY IN MARINE SOFT BOTTOMS

This study links spatial and seasonal patterns of mortality of the har d clam, Mercenaria mercenaria (L.), in marine soft bottoms with the pr edation rates and habitat use of its main predator, the blue crab, Cal linectes sapidus Rathbun. Patterns of predation on tethered juvenile c lams exposed to the natural assemblage of predators were compared amon g different habitat types in fall and summer. Between-habitat patterns of predation on clams varied with season. In fall, predation on tethe red clams was greater in subtidal sand bottoms and just inside the edg e of intertidal salt marshes than in intertidal sand hats. In summer, predation on clams was similar in all habitats. Experiments conducted in held enclosures showed that: (a) individual crabs spent more time i n the salt marsh habitat than in intertidal sand flats; (b) crab indiv iduals placed in a sand bottom habitat had greater predation rates in high-density prey patches than in low-density patches; (c) individuals had greater predation rates in prey patches located just inside the e dge of salt marshes than in intertidal sand flats, when prey density w as held constant between the two habitats; (d) at intermediate and hig h crab densities predation mortality of clams was similar between vege tated and unvegetated habitats; (e) both individual crabs and groups o f crabs consumed similar numbers of clams in the two habitat types whe n large predatory birds (mainly various species of terns, Sterna spp., herring gulls, Larus argentatus Coues, and ring-billed gulls, L. dela warensis Ord) were excluded from enclosures, but the crabs consumed mo re clams in the salt marsh than in the sand flat habitat in control en closures where birds were not excluded. In the fall, when Herring and Ring-billed Gulls were abundant in the study area, preference by blue crabs for safer and more profitable habitats may explain the greater p redation on clams in salt marshes than in intertidal sand flats. In th e summer, when Herring and Ring-billed Gulls were rare and crab densit ies are 1.5-3 times greater than in the fall, competition with conspec ifics may have caused crabs to disperse and feed in intertidal flats a nd may explain the general lack of differences in predation intensity among habitat types observed in the summer. Thus, patterns of predatio n and habitat use by blue crabs appeared to explain between-habitat an d seasonal differences in predation mortality of clams. Focusing on th e variation in the feeding rates of individual predators in response t o external conditions can produce the mechanistic understanding of spa tial and seasonal patterns of predation needed to understand and bette r predict the processes that structure benthic marine communities.