Quantifying factors influencing American lobster (Homarus americanus) predation on the invasive green crab (Carcinus maenas) in Newfoundland, Canada

The European green crab (Carcinus maenas) is a newly invasive species in Newfoundland, where it has likely been present for <= 15 years. The green crab has been found in stomach contents of American lobster (Homarus americanus) in New England and Nova Scotia, Canada, but predation on this species has not yet been quantified in Newfoundland. We conducted feeding experiments to determine whether lobsters from Newfoundland were as likely as those from Nova Scotia (which have coexisted with green crabs for >60 years) to recognize and prey upon this new species. We also performed experiments to determine whether green crabs reach a size refuge from predation and whether factors including starvation, availability of alternate food sources, or habitat complexity would influence the probability of lobster attacking or feeding on green crabs. In our trials, lobster origin had no significant effect on crab predation; lobsters, irrespective of origin, were more likely to consume small (CW]) and medium (40-65 mm CW) crabs than larger (>65 mm CW) ones. Nevertheless, even small lobsters (73-76 mm carapace length, 300 g) were able to kill and consume the largest green crabs (78 mm CW, 100 g). Green crabs were less likely to be attacked or eaten when an alternative food source was present, suggesting that the lobsters were preying on the crabs, rather than simply killing them in a dispute over territory. The addition of a shelter provided a refuge for the green crabs; however, the crabs were only able to avoid being injured or eaten if this shelter was structurally complex. The green crab is slowly spreading westward around the island of Newfoundland, and so its long-term effects, interactions with other organisms, and contribution to the diet of Newfoundland lobsters remain to be seen.