Mitigation of predation by the European green crab Carcinus maenas on publicly cultured hard shell clams Mercenaria mercenaria

The European green crab Carcinus maenas appears to pose a substantial threat to coastal fisheries wherever it has invaded, prompting efforts to mitigate its effects on invaded communities.; To establish the potential threat of the green crab to clam fisheries, I experimentally measured the effect of this crab upon the venerid Katelysia scalarina in Tasmania, Australia. Juvenile clam abundance tended to be lower at sites with C. maenas relative to those without, and relative predation intensity on these juveniles was significantly higher at invaded areas. Using manipulative experiments, I found that green crabs had much higher predation rates than any native predator tested and, unlike native crabs, increased their per capita predation with increasing prey density.; I next examined the effect of the green crab on current efforts to restore hard shell clams (Mercenaria mercenaria Linne) in Massachusetts, USA. I undertook three components. First, I surveyed regional managers to identify the extent of current programs and perceived obstacles. Although restoration success varied, all identified predation as a major source of loss. Among predators, green crabs were cited most frequently and considered a significant source of mortality.; Second, I examined the relative abundance of green crabs in marine soft substrate habitats at multiple scales: along the northeastern US coast, within embayments of Martha's Vineyard, Massachusetts, and within Sengekontacket Pond, Martha's Vineyard. In trap surveys, green crabs were common and the most abundant decapod. Snorkel surveys generally confirmed this prevalence.; Third, I used a combination of quantitative and experimental approaches on Martha's Vineyard to: (1) quantify patterns of predation upon stocked juvenile clams, (2) characterize size- and density-specific predation patterns and (3) explore the effectiveness of selective harvest of green crabs as a means of protecting clams. Public stocking underwent rapid (∼1 wk) declines, and there green crab abundance was negatively correlated with clam survival. A series of field and laboratory experiments demonstrated the importance of green crab predation upon clam juveniles. Finally, selective harvest of green crabs (over limited spatial and temporal scales) was found to be effective at protecting clams, though it did not significantly affect green crab abundance.