Atlantic Rock Crab, unlike American Lobster, Is Important to Ecosystem Functioning in Northumberland Strait

To compare the roles of American lobster Homarus americanus and Atlantic rock crab Cancer irroratus in the food web processes of a coastal ecosystem, distribution, abundance, stomach contents, diet overlap, and occurrence in stomachs of predators were described for the two species collected during July-August trawl surveys in Northumberland Strait, southern Gulf of St. Lawrence. Atlantic rock crab was more widely distributed, was more numerous on mud and muddy sand substrates, and occurred in deeper water than American lobster. Atlantic rock crab was the principal prey of American lobster while American lobster was almost never eaten by Atlantic rock crab. American lobster (benthic and pelagic stages) was a trace item in all fish and decapod stomachs examined except in those of the Shorthorn Sculpin Myoxocephalus scorpius and other American lobsters. In contrast, Atlantic rock crab larvae commonly occurred in stomachs of many pelagic fishes while small (<40 mm carapace width) benthic individuals were important prey of demersal fish species such as Winter Skate Leucoraja ocellata, Cunner Tautogolabrus adspersus, Longhorn Sculpin M. octodecemspinosus, and Shorthorn Sculpin. Atlantic rock crab had a broad diet consisting of fish, sevenspine bay shrimp Crangon septemspinosa, Atlantic rock crab (mostly cannibalism), bivalves, and polychaetes. American lobster had a narrow diet consisting mainly of Atlantic rock crab with lesser amounts of American lobster (old carapaces and cannibalism) and sea stars. Diet overlap between the two species was moderate and mainly due to joint consumption of Atlantic rock crab. Thus, Atlantic rock crab play a prominent role in the southern Gulf of St. Lawrence food web (and therefore energy cycling) while American lobster serves mostly as an energy sink whose loss from the ecosystem would mainly affect people dependent upon the fishery.