Response of Bay Scallops to Spawner Transplants: A Test of Recruitment Limitation

Adult bay scallops were transplanted prior to spawning in summers of 1992 (135,000) and 1993 (100,000) from a donor site in Back Sound where scallops were abundant to receiver sites in western Bogue Sound, an estuarine water basin where scallops had not initiated recovery since their virtual elimination by a red tide outbreak in 1987-88. These transplants were intended as a test of the hypothesis that bay scallop populations are recruitment-limited on a basin scale within sounds, consistent with the limited physical transport of their short-lived pelagic larvae. This intervention also represents an empirical test of a process-based restoration option (spawner transplantation) with broad significance to managers of shellfisheries resources. Initial tests of alternative methods for transporting adult scallops during summer heat revealed that transport out of seawater but under moist conditions inside insulated coolers induced only 1.5 % mortality, independent of holding time ranging from 1.75 to 4 hr. In contrast, scallops maintained for 6 hr in flow-through seawater tanks exhibited between 10 and 50% mortality, with losses increasing with decreasing flow rates and oxygen concentrations. Although fewer than 2.5 % of available adult bay scallops were removed from the donor site of Oscar Shoal, the numbers of remaining adult scallops fell to near zero by early December each year. In contrast, mortality and emigration were both negligible from August to December for scallops at each of the 4 receiver sites in western Bogue Sound. Consequently, transplantation had the effect of protecting those adult scallops from high autumn rates of natural mortality at Oscar Shoal and allowing them to survive until winter, when the fishing season begins. On average, recruitment of scallops at two study sites in western Bogue Sound following the transplants in 1992 and 1993 was 654 % greater than in 1988 and 1989 when no transplantation had occurred. At two control sites in Back and Core Sounds, the average increase in recruitment was only 54% from 1988 and 1989 to 1992 and 1993. This temporal increase in recruitment of bay scallops to natural seagrass beds was significantly larger in western Bogue Sound than in the control sounds, demonstrating a positive effect of the transplant on bay scallop restoration. Larval settlement onto spat collectors at three of those same study sites did not correlate well with the recruitment data and failed to reveal enhancement in western Bogue Sound following the transplants. Thus, our data cannot confirm that the transplant succeeded through the mechanism of enhancing larval abundances. Nevertheless, settlement of scallop spat onto settlement bags deployed along a transect in the channel revealed a pattern of decreasing settlement with distance from Bogue Inlet, consistent with the hypothesis that scallop larvae become depleted with distance from their source and thus limit population size in this system. Furthermore, larval settlement onto collector bags and recruitment to natural seagrass beds was negligible during both years at a site in central Bogue Sound. Physical current-meter data on water transport show that this site lies outside the influence of tidal forcing from Bogue Inlet and thus is disconnected hydrographically from the source of competent larvae near Bogue Inlet in western Bogue Sound. Thus, transplant of pre-spawning adult bay scallops proved to be a successful restoration action in this system, probably because it enhanced otherwise limited abundances of larvae, although spat collector results do not unequivocally support the inference that larval enhancement was the mechanism by which increased recruitment was achieved.