Rapid withdrawal from a sound source by open-ocean sharks

Abstract

Studies undertaken in the Straits of Florida and the Tongue of the Ocean (TOTO), Bahamas, have established that silky sharks, Carcharhinus falciformis, while approaching a source of underwater sound, will withdraw rapidly from its vicinity if specified changes occur in the nature of the transmitted sound. These changes include: 1) an increase in level of the sound being transmitted (approximately 20 dB) and 2) an abruptness by which that level is achieved. Augmenting factors may include sudden changes in the spectral or temporal qualities of the transmitted sound. Although a biological sound (killer whale scream) could elicit clear withdrawal, it possessed no unique quality; rather, more simply constructed sounds, possessing only a restricted band of frequencies and lacking frequency-modulation, were equally effective. Habituation of the response was apparent during successive tests. All results closely followed those obtained on lemon sharks, Negaprion brevirostris, during a concurrent study (to be reported elsewhere). In the presence of added stimulants (chopped, fresh fish) withdrawal could be elicited in small silky sharks for only a short time. Limited testing of oceanic whitetip sharks,Carcharhinus longimanus, showed that extremely limited withdrawal could also be elicited in that species.