Western Gulls as a Possible Predator of California Sea Lion Pups

Abstract

The breeding period of Western Gulls (Larus occidentalis) extends from April to mid-August (Sowls et al. 1980), overlapping with that of California sea lions (Zalophus californianus) that breed from late May to the end of July (Peterson and Bartholomew 1967). Both species may occupy the same beaches, thus favoring the occurrence of a commensalist relationship in which gulls consume sea lion placentas (Hunt and Butler 1980). Sea lions on the other hand, may be alerted by gull squawks when the seabirds detect some possible danger (pers. observ.). Interactions among gulls and sea lions, however, may take another context as referred to below. During the beginning of the California sea lion breeding season of 1982, we visited the rookery located on Santa Margarita Island (24018' to 24032'N; 111042' to 11201 'W) and counted the first 20 pups born that year. Of these, three animals were dead and exhibited rounded holes on the belly and no eyes. At that time, about 200 Western Gulls were present on the beach, many of them walking around the sea lions. During our observations we made some noise that caused many mothers of the pups to go to the sea. Immediately after the mothers were gone, many gulls surrounded the pups and began to peck them. Based on these observations we supposed that the gulls could have caused the wounds on the dead pups. To test this hypothesis, we planned an experiment for the first week of June 1983. For that purpose we selected a place for observing a small section of the sea lion colony from which the disturbances were minimized. The numbers of females, pups, and gulls were recorded, as well as the interspecific attacks. Pecking on a pup body was considered to be a gull attack and a sea lion attack was any attempt to bite or pursue the gulls. The observations were registered under two conditions, undisturbed and disturbed colony. In the first case, the gulls walked around the sea lions sometimes attempting to peck the pups and doing so on a few ' Received 5 December 1986. Final acceptance 1 June 1987. SHORT COMMUNICATIONS 923