Evidence of shark attacks on Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas

Abstract

The rate of shark predation on small cetaceans is poorly understood, but information is increasing as more long-term cetacean and shark studies are conducted. Resident Atlantic spotted dolphins (Stenella frontalis), off the coast of Bimini, The Bahamas, have been the subject of ongoing photo-ID and behavioral studies since 2001. Sharks in this area have also been systematically documented for decades. Despite these efforts, an actual shark attack on a dolphin has not been observed in Bimini and such an observation is rare anywhere in the world. Because of this, photo-ID records were analyzed for evidence of shark-induced injuries, often in the form of a crescent-shaped injury or scar. Fifteen percent of cataloged individuals within the Bimini dolphin population exhibit evidence of a shark attack. Patterns in the location on the body of the attack and trends related to age and sex were calculated. There were few injuries to the ventral portion of the body, where most attacks would likely be fatal. Although the level of healing prevented most scars from being linked to a specific shark species, we suspect bull (Carcharhinus leucas), tiger (Galeocerdo cuvieri), shortfin mako (Isurus oxyrinchus) and dusky sharks (Carcharhinus obscures) to be possible predators of dolphins near Bimini. This study does not estimate the frequency of attacks on dolphins; here, we estimate the occurrence of unsuccessful attacks. Introduction: The amount of information on shark predation on small cetaceans is increasing as longitudinal cetacean projects and anecdotal information are compiled. Stomach content studies have shown delphinid species consumed by the following shark species: Zambesi/bull (Carcharhinus leucas), tiger (Galeocerdo cuvieri), great white (Carcharodon carcharias), and dusky (Carcharhinus obscures)(Cockcroft et al., 1989). First hand observations are rare (Mann & Barnett, 1999; Maldini, 2003) so alternative methods for assessing shark predation must be developed. One method is by assessing failed attempts; that is, documenting fresh injuries and scars on known dolphins that can be attributed to sharks, as was done in this study. Here, the percentage of Atlantic spotted dolphins, from a known population, with evidence of past shark attacks was determined and compared to other dolphin populations. Patterns in the body location of these scars as well as differences between ages and sexes were considered. Potential shark species responsible for the attacks are discussed.