FUNCTIONAL-RESPONSE OF THE PREDATORS AMERICAN LOBSTER HOMARUS-AMERICANUS (MILNE-EDWARDS) AND ATLANTIC WOLFFISH ANARHICHAS-LUPUS (L) TO INCREASING NUMBERS OF THE GREEN SEA-URCHIN STRONGYLOCENTROTUS-DROEBACHIENSIS (MULLER)

The predatory impact of the American lobster Homarus americanus and the Atlantic wolffish Anarhichas lupus on the green sea urchin Strongylocentrotus droebachiensis was investigated in a multifactorial experiment. Both predators exhibited either Type 2 or Type 3 functional responses to increasing prey density, but the magnitude of the response differed for the two predators, and for different prey sizes. Predation on large (> 20 mm) sea urchins increased approximately three-fold, i.e., from 1.12 to 3.54 urchins.day-1 for wolffish, and from 0.35 to 0.97 urchins.day-1 for lobster, when prey density was increased sixfold, i.e., from 5 to 30 urchins.tank-1. Although more urchins were killed at the highest density, the proportion of available urchins that were killed dropped by almost-equal-to 50%. Predation on small (less-than-or-equal-to 20 mm) sea urchins was similar for both predators, increasing from almost-equal-to 0.5 to almost-equal-to 1.9 urchins-day-1 with increased prey density. In the lobster treatment, a rock crab, Cancer irroratus Say, was included as alternative prey. The lobsters killed 5.78% of the total number of available rock crabs, and 5.01% of the total number of available urchins, but lobster predation on urchins was nearly halved in replicates where the crab was killed. Experimental factors other than prey density and prey size, i.e., season; availability of food and physical refuges for the prey, and whether the prey had been fed or starved prior to the experiment, had no significant effects on the predation rates of either predator. These results are consistent with the hypothesis that green sea urchin outbreaks may be triggered by reductions in predation pressure. There was no evidence of significantly increased predation on morbid sea urchins although almost-equal-to 1% of the experimental sea urchin population exhibited symptoms of amoeboid, Paramoeba invadens (Jones), disease.