Depth and timing of settlement of veligers from different populations of giant scallop, Placopecten magellanicus (Gmelin), in thermally stratified mesocosms

The present study was conducted to evaluate the relative roles that water column stratification intensity and possible inter-population behaviour differences play in determining depth of larval settlement of giant scallops, Placopecten magellanicus, in relation to thermoclines. Differences in timing of settlement of larvae from various populations were also examined. Two separate experiments were conducted in a 10.5-m-deep, 3.7-m-diameter, thermally stratified tank with larvae spawned from scallops collected from several adult beds located in areas with differing oceanographic regimes (Georges Bank, Mahone Bay, Passamaquoddy Bay). Previous mesocosm experiments had shown that veligers from these various populations differ in their vertical migration patterns. In the first experiment (December 1992-February 1993), larvae from all three populations were held in separate 9.5-m-deep tubes and exposed to a 1.5 °C temperature differential established over a depth interval of 1 m. The number of settled juveniles (spat) of each population collected at the end of the experiment increased with depth and showed no peak at or above the thermocline. This depth distribution of spat was most likely driven by preferential larval settlement. In the second experiment (February-May 1994), larvae spawned from Georges Bank and Passamaquoddy Bay stocks were held in 9.0-m-deep tubes and exposed to a 5 °C temperature differential established over a depth interval of 1 m. The number of settled spat of both populations was greater above this thermal boundary and increased with decreasing depth. This depth distribution was most likely driven by preferential settlement above the thermocline followed by upward post-settlement migration. The results from the two experiments indicate that larvae from various populations show similar trends in settlement patterns in response to similar thermal stratifications. Stratification intensity, however, does affect depth of larval settlement. In the second experiment, both populations of larvae settled throughout the time interval of collector deployment (larvae 32-82 days old). These results extend the range of planktonic developmental times generally reported in the literature and may be more indicative of natural planktonic development in the field. While Georges Bank larvae settled in consistent numbers through time, Passamaquoddy Bay larvae showed peaks in settlement at certain time periods, indicating that pulses of larval settlement may occur even from an individual spawning event.