Site-specific and density-dependent extinction of prey by schooling rays: generation of a population sink in top-quality habitat for bay scallops

Bay scallops (Argopecten irradians concentricus) are patchily distributed o n two dominant spatial scales: (1) geographically restricted to highly sali ne marine lagoons, and (2) locally abundant within such lagoons only in rel atively discrete beds of seagrass habitat. In the Cape Lookout lagoonal sys tem of North Carolina, adult bay scallop abundance in the most densely occu pied, seagrass bed (Oscar Shoal) exhibits repeatable declines from up to 70 m(-2) to near zero in a 2- to 4-week period, during late summer. This cras h is completed before fall spawning can be initiated, thereby creating a po pulation sink in what is the singly most productive patch of habitat. Field experiments conducted in the summers of 1996 and 1998 demonstrated that th e seasonal extinction of bay scallops on Oscar Shoal can be prevented by th e erection Of 1-m(2) stockades, made of 50-cm-high vertical poles, spaced e very 25 cm, which inhibit access by cownose rays. Because these stockades w ere porous to emigration and physical transport, and open to access by all other predators of adult scallops, predation by migrating cownose rays is t he only viable explanation for the crash. Consequently, the natural predati on process in this system achieves the reproductive extinction of prey in t he habitat patch of highest productivity. Over 7 years of observation, the mortality rate in this patch increased with summer density, reaching the as ymptote of 100% at 10 m(-2). The site-specific habitat selection by schools of rays may be based on prey density, which could render this example repr esentative of a widespread generator of population sinks in habitat patches of high quality. The virtual extinction of scallops within Oscar Shoal des pite nearby patches with relatively high density may be related to the high ly efficient feeding behavior of schools and the high vulnerability of bay scallops in a context of multiple alternative prey types.