Effects of predation by sea ducks on clam abundance in soft-bottom intertidal habitats

Recent studies have documented strong, top-down predation effects of sea ducks on mussel populations in rocky intertidal communities. However, the impact of these gregarious predators in soft-bottom communities has been largely unexplored. We evaluated effects of predation by wintering surf scotersMelanitta perspicillataand white-winged scotersM. fuscaon clam populations in soft-bottom intertidal habitats of the Strait of Georgia, British Columbia. Specifically, we documented spatial and temporal variation in clam density (clams m–2), scoter diet composition, and the consequences of scoter predation on clam abundance. Of the 3 most numerous clams, Manila clamsVenerupis philippinarumand varnish clamsNuttallia obscuratawere the primary prey items of both scoter species, while clams of the genusMacomawere rarely consumed by scoters. Between scoter arrival in the fall and departure in the spring, Manila clams decreased in density at most sample sites, while varnish clam densities did not change or declined slightly. Our estimates of numbers of clams consumed by scoters accounted for most of the observed declines in combined abundance of Manila and varnish clams, despite the presence of numerous other vertebrate and invertebrate species known to consume clams. ForMacomaspp., we detected an over-winter increase in density, presumably due to growth of clams too small to be retained by our sieve (