Response of bay scallops to spawner transplants:a test of recruitment limitation

Adult bay scallops Argopecten irradians concentncus Say were transplanted prior to spawning in summer 1992 (135000). 1993 (100000), and 1994 (150000) from a donor site where scallops were abundant to receiver sites in western Bogue Sound, North Carolina, USA, a n estuarine water basin where scallops had not initiated recovery since their virtual elimination by a red tide outbreak in 1987-1988. Transplantation enhanced local adult density in receiver sites from < l in 1992 and 1993 and 3 in 1994 to 15 m-' These transplants were intended as a test of the hypothesis that bay scallop popu-lat~ons are recruitment-l~mited on a basin scale withln sounds, urhich is consistent with the Limited physical transport of their shoi-t-11ved pelagic larvae. Thls intervention also represents an empincal test of a process-based restoration optlon (spawner transplantat~on) w ~t h broad significance to managers of shellf~sh resources. Both mortahty and emigration were negligible from August to December for transplanted scallops at each of the 4 receiver sites in western Boguts Sound. On average, 1-ecru~tnlent of scilllops at 2 study sites in western Bogue Sound following the transplants in 1992, 1993, and 1994 was 568"; greater than in 1988 and 1989 when no transplantation had occurred. At 2 control sites in Back and Core Sounds. North Carolina, USA, the average change in recruitment was a non-significant 34'% increase over this same period. Adult density in Bogue Sound increased by 258% following spawner transplantation ds compared to a non-significant change of 8 % in control sounds. The absolute magnitude of the 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 transplants on bay scallop restoration. Larval settlement onto spat collectors at 3 of those same study sites d ~d not correlate well with recru~tment data and failed to reveal enhancement in western Bogue Sound follo transplantation. Thus, spat collector data cannot conflrm that the transplants succeeded through the nlechanis~n of enhancing larval abundances. Nevertheless, settlement of scallop spat onto collectors deployed along a transect in the channel rtvealed a pattern of decreasing settlement with distance from Bogue Inlet, which is consistent with the hypothesis that scallop larvae become depleted with distance from their sourcc and thus limit population size in this system. Furthermore, larval settlement onto collectors and recruitment to natural seagrass beds were negligible at a site in central Bogue Sound that 11es o u t s ~d e the influence of tidal forcing from Bogue Inlet and is disconnected hydrographically from the source of competent larvae in western Bogue Sound. Thus, recruitment appears to limlt population size of bay scallops in this system, imp11 ing that larval subsidy froin transplantation is the likely although unconfirnled mechaixsm of successful enhancement of recruitment following spawner transplantation