Timing, intensity and sources of autumn mortality of adult bay scallops Argopecten irradians concentricus Say

Abstract

Intensive sampling for the bay scallop Argopecten irradians concentricus Say at five sites in sounds around Cape Lookout, North Carolina, in 1983–1984 demonstrated little change in density of adult (>4 cm in shell height) scallops from August to October in either year but several significant declines from October to December 1983. Because the first sampling interval brackets the period of autumn migration through the North Carolina sounds of the molluscivorous cownose ray Rhinoptera bonasus (Mitchill), a reputed scallop predator, these sampling results imply that cownose rays have little effect on adult bay scallop densities during autumn in North Carolina. Furthermore, eight young-of-the-year cownose rays failed to consume any adult bay scallops when confined for 6 days with 104 tethered adult scallops in a 48-m 2 field enclosure. Mortality of tethered scallops within the enclosure, probably caused by predation from whelks Busycon spp., was significantly greater on unvegetated bottom than inside a seagrass meadow, yet did not vary with the presence or absence of epibiotic cover on the scallop's top valve. The October–December declines in density of adult scallops preceded the commercial harvest but coincided with the arrival of large numbers of overwintering herring gulls Larus argentatus and ring-billed gulls Larus delawarensis. Field experiments revealed extremely rapid predation by these gulls on adult scallops aerially exposed on intertidal flats and negligible losses for scallops covered by as little as 1–3 cm of water. Gull predation did not vary with epibiotic cover on the scallops. Because adult bay scallops can be shown to emigrate more rapidly from sandflats than from seagrass beds, which are deep enough to avoid aerial exposure on all but the most extreme low tides, it is unclear whether gull predation can explain the full magnitude of observed October–December declines in scallop density. Storms may be necessary to transport enough scallops onto intertidal flats where vulnerability to gulls is enhanced to enable gulls to exert substantial mortality.