WHELK (BUCCINUM-UNDATUM) MOVEMENT AND ITS IMPLICATIONS FOR THE USE OF TAG-RECAPTURE METHODS FOR THE DETERMINATION OF BAITED TRAP FISHING PARAMETERS

The tag-recapture method has been used to measure the field of attraction and the effective fishing area of a baited trap. The possibly conflicting experimental conditions required for the determination of these two parameters are, respectively, that tagged animals do not move and that they behave as undisturbed animals, prior to exposure to bait odour. Field observations and in situ experiments were conducted on the north shore of the Gulf of Saint Lawrence in 1988 and 1989 to compare behaviour of undisturbed and tagged whelks (Buccinum undatum). In the daytime, approximately 75 % of undisturbed whelks were quiescent, being either buried and oriented randomly or static and oriented downstream, while the remainder moved across stream at 2-5 cm.min-1. In contrast, > 97 % of tagged whelks oriented in nonrandom directions and moved at equal or greater speeds, on average, 2.2-9.2 m from release points in less than 6 h. Because neither of the two basic conditions was met, tag-recapture experiments would produce biased estimates of baited trap fishing parameters.