VERTICAL MIGRATION FOR HORIZONTAL TRANSPORT WHILE AVOIDING PREDATORS - II - EVIDENCE FOR THE TIDAL DIEL MODEL FROM 2 POPULATIONS OF SCALLOP(PLACOPECTEN-MAGELLANICUS) VELIGERS/

We examined the vertical migration behavior of scallop (Placopecten ma gellanicus) veligers in mesocosms and in previously reported field stu dies. Evidence suggests that these bivalve veligers migrate in respons e to both tidal and diurnal stimuli in a manner similar to a proposed tidal/diel model. Both populations have a diurnal response to solar cu es. The response to tidal cues differs between the Georges Bank and Pa ssamaquoddy Bay populations. Georges Bank veligers appear to utilize t he differences in tidal phase that occur with depth to transport them in a northeasterly direction, thus maintaining the population on the b ank. Passamaquoddy Bay veligers respond by swimming up at slack water (high and low tides) and down when currents are strongest. Such behavi or would minimize dispersal on the strong tidal currents in the Bay of Fundy and thus also tend to maintain a population within an area. Hor izontal transport resulting from vertical migration is the most likely selective pressure to create and maintain these different behaviors a gainst the homogenizing effects of migration between the two populatio ns. The implications of inherited differences in behavior probably req uire consideration in the management of both wild and cultured populat ions. Common sampling practices that obscure the tidal part of tidal/d iel migration, including averaging the results from several days of sa mpling, sampling too infrequently to perceive a tidal periodicity, and assuming that only behavior that changes at high and low tides will a ffect horizontal transport, need to be avoided in studies of vertical migration.