CURRENTS, PREDATORS, AND THE AGGREGATION OF THE GASTROPOD BUCCINUM-UNDATUM AROUND BAIT

Aggregation of the gastropod Buccinum undatum Linnaeus around bait was studied at 2 subtidal sites, with different current regimes, in the northern Gulf of Saint Lawrence, Canada. Time-lapse photography, SCUBA and current recordings were used to measure and interpret movement and behavior of whelks up to 25 m away from the bait. Objectives were to test the hypothesis of upstream movement to bait, and to determine relations between whelk arrival rate at the bait and currents and predators. In the absence of bait, 75.8 % of whelks were static or buried. In the presence of bait, whelks in a 20 to 60-degrees downstream sector to d maximum distance of 20 m moved towards the bait. Whelks apparently oriented to the bait by chemotaxis. Average crawling speed towards the bait varied from 7.3 to 15.1 cm min-1 and was independent of current speed. More whelks arrived at bait per unit time when current direction changed gradually than when current was directionally stable; this was due to exploitation of new grounds and to depletion of the downstream population respectively. A model and indirect field evidence indicate that whelk arrival rate is depressed by sudden and marked changes in current direction, because whelks are unable to track the shifting odor plume. Arrival rate was positively correlated with current speed with time lags of up to 3 h, but tended to be negatively correlated with longer time lags. Arrival rate was negatively correlated with the number of predators (Cancer irroratus and Hyas araneus) at bait with time lags of up to 1 h, but positively correlated with numbers of H araneus at bait with longer time lags. These correlations are explained by changes in the characteristcs of the odor plume and by agonistic interactions.