Basin-scale coherence of population dynamics of an exploited marine invertebrate, the bay scallop: implications of recruitment limitation

Direct effects of a red tide (Ptychodiscus brevis) outbreak on bay scallops Argopecten irradians concentricus in North Carolina, USA, from October 1987 to February 1988 were necessarily limited to the 2 generations then present (adults of the year-class fished in winter 1987-88 and their progeny harvested in 1988-89), yet the 3 subsequent year-classes of bay scallops (those fished in the 1989-90, 1990-91, and 1991-92 winter seasons) also have been extremely depressed. The fishery crashed to about 10 to 15 % of the recent (25 yr) historical average harvest: no recovery has yet begun. This inability to recover quickly along with evidence of a previous gradual (5 to 12 yr) though erratic recovery from low population size in the early 1970's implies that this scallop population may be recruitment-limited at low density. Sampling of adults in August just before spawning and of new recruits surviving until early December in both 1988 and 1989 revealed that recruitment was positively related to adult abundance. This relationship was assessed across 9 bay scallop grounds in North Carolina. Settlement onto spat collector bags deployed at these sites d u r ~n g early autumn also tended to follow the same patterns of adult and recruit abundance among sites in 1988 but not in 1989. Patterns of adult and recruit abundance were broadly coherent across water bodies, with densities in Core Sound high, Back Sound relatively low, and Bogue Sound extremely low. Compared to identical sampling conducted during 3 years just preceeding the red tide outbreak, average autumn recruitment rates of bay scallops for 1988 and 1989 were normal in Core Sound, about 29 % of normal in Back Sound, and about 5 U/o of normal in Bogue Sound. This pattern continues to conform to the documented pattern of immediate direct effects of the red tide. Changes in area of its seagrass habitat appear incapable of explaining the sound-specific pattern of bay scallop declines following the red tide. This coherence of population dynamics on a basin scale implies that the bay scallop may be recruitmentlimited at low density by mesoscale or sound-wide abundance of spawning adults, creating lasting effects of the red tide propagated through basin-scale population inertia. Recruitment limitation for an exploited species would have important consequences for sustainability of harvest, if based on an effective stock-recruitment relationship, and may imply need for management intervention to enhance spawning stock within depleted basins.