Predation of juvenile sea scallops (Placopecten magellanicus) by crabs (Cancer irroratus and Hyas sp.) and Starfish (Asterias vulgaris, Leptasterias polaris, and Crossaster papposus)

A scallop fishermen's association has seeded sea scallops, Placopecten magellanicus, off Iles-de-la-Madeleine (Quebec, Canada) since 1993. When the scallops reach the bottom, they fall prey to starfish and crabs. Two size classes are available during the Iles-de-la-Madeleine seeding period: small scallops from collectors (15 to 25 mm shell height) and large scallops grown-out in pearl nets (35 to 45 mm). To evaluate predation on both classes shortly after seeding, experiments were performed under controlled laboratory conditions. Testing involved three species of starfish, Asterias vulgaris (120 to 200 mm diameter), Leptasterias polaris (120 to 200 mm diameter), and Crossaster papposus (110 to 130 mm diameter), and two species of crab, Cancer irroratus (85 to 120 mm carapace width) and Hyas sp. (90 to 110 mm carapace length). Scallops from both size classes were presented separately (nonchoice treatment) and together (choice treatment) to each predator species. Starfish and Hyas sp. consumed less than one scallop per predator per day as compared to C. irroratus, which consumed as many as 12 scallops per predator per day. Starfish and crabs did not show a clear prey size preference in both treatments. However, large scallops tend to be consumed faster by both crab species in choice treatments. These results indicate that bottoms with high densities of starfish or crab should be avoided in seeding. In addition, bottom seeding should be done if possible with scallops greater than the size classes used in this study (over 50 mm), according on these results and those of other studies.