According to Riedman (Reference Riedman1990), the predation of pinnipeds by sharks in tropical or temperate regions is a significant cause of mortality of this group. Predator–prey interactions between these two groups have been observed in different regions through observations of living and wounded pinnipeds, the stomach contents of sharks and carcasses of animals killed with mutilations caused by bites (Le Bouef et al., Reference Le Bouef, McCosker and Hewitt1987; Rosas et al., Reference Rosas, Capistrano, Di Beneditto and Ramos1992; Klimley et al., Reference Klimley, Pyle, Anderson, Klimley and Ainley1996; Long et al., Reference Long, Hanni, Pyle, Roletto, Jones, Bandar, Klimley and Ainley1996). Despite these observations, little is known of these interactions off the Brazilian coast, mainly in the north-eastern region, where the occurrence of these species are sporadic. Most of the specimens that arrive are juveniles, mostly weak, debilitated and because of that they become vulnerable prey for sharks.

The objective of this study is to report the predator–prey interactions between sub-Antarctic fur seal (Arctocephalus tropicalis) and cookiecutter shark (Isistius brasiliensis) off the coast of Bahia.

The present study was based on numbers of pinnipeds stranded in the State of Bahia, north-eastern Brazil (13°33′S 038°54′W and 13°58′S 038°88′W). Two specimens of sub-Antarctic fur seal Arctocephalus tropicalis (Jefferson et al., Reference Jefferson, Leatherwood and Webber1993) had been registered, measuring 115.5 and 131 cm total length (TL) respectively. The diagnostic identification of shark predators was through analysis of the morphology and conditions of the wounds (Table 1), and compared with specialized literature (Le Boeuf et al., Reference Le Bouef, McCosker and Hewitt1987; Long et al., Reference Long, Hanni, Pyle, Roletto, Jones, Bandar, Klimley and Ainley1996; Cunha & Gonzalez, Reference Cunha and Gonzalez2000). Skin, organ and bone materials, as well as the photographs taken and moulds of the wounds were deposited in the scientific collection of Instituto Mamíferos Aquáticos (IMA), with catalogue numbers CCPM0168 and CCPM0202.

Table 1.

Details of pinnipeds stranded on the coast of Bahia with shark bites.

In August of 2003 and 2004, two juvenile male individuals of A. tropicalis had been found alive and had died shortly afterwards, due to the condition of the wounds and acute dehydration. In the first case (CCPM0168), the specimen showed a bite mark with a circular format in the distal part of the dorsal region of the left pectoral paddle (Table 1), caused by a cookiecutter shark. The second case, specimen CCPM0202, was found very weak and presented three distinct wounds, two of them caused by sharks and the other of unknown origin. The first wound, with a circular format, located in the superior face of the left previous paddle also was caused by Isistius spp. The second wound had a half-circular format and was situated in the posterior portion of the back. The wound presented eight perforations in approximately straight lines, each about 20 mm in length. Despite the format of the bite, few affirmations can be made about the predator species. The third wound, of unknown origin, was situated on the superior face of the right posterior paddle. After the analyses of the circular measures made here, the same ones are fit in category III, to whose height and width they are similar, suggesting to have been caused by I. plutodus (Cunha & Gonzalez, Reference Cunha and Gonzalez2000). These data corroborate the findings of Souto et al. (Reference Souto, Abrão-Oliveira, Nunes, Maia-Nogueira and Sampaio2007), where the authors had found in cetaceans a prevalence of 80% of the bites provoked by I. plutodus off the coast of Bahia. Individuals of this sort are small cosmopolitan sharks, of the family Dalatiidae, opportunists which were registered in the bati and epipelagic zones of tropical and subtropical waters, being able to occur in some regions of the continental and insular shelfs (Soto & Nisa-Castro-Neto, Reference Soto and Nisa-Castro-Neto1998; Gadig, Reference Gadig2001). Although this species is less known in the south-western Atlantic (Moore et al., Reference Moore, Steiner and Jann2003; Zidowitz et al., Reference Zidowitz, Fock, Ousch and von Westernhagen2004), this is the second observation for north-eastern Brazil, extending the data about its distribution. In this case, beyond the considered erratic occurrence of the Otariidae, one of the reasons for the stranded could be the injuries in pectoral paddles, difficulty of displacement. This study confirms the necessity to continue studies on the interactions between sharks and pinnipeds in order to understand better the predator–prey relations off the Brazilian coast, as well as to question the extent to which these interactions cause the strandings of this group.