Dynamics of foraging interactions between cookiecutter sharks (Isistius spp.) and short-finned pilot whales (Globicephala macrorhynchus) in Hawaiʻi

Abstract Cookiecutter sharks ( Isistius spp. ) are small pelagic squaloid sharks found throughout tropical and sub-tropical waters that are known to feed opportunistically on a range of prey, including animals much larger than themselves. Short-finned pilot whales ( Globicephala macrorhynchus ) are resident to Hawaiʻi Island and are often observed with fresh and healed cookiecutter shark bites. In this study, cookiecutter bites were used to infer the spatiotemporal patterns of the foraging behaviour of sharks on pilot whales off the Hawaiian Islands (21°N, 158°W to 18.5°N, 154.5°W). A photo-identification catalogue of 399 resident short-finned pilot whales (representing 5,859 identifications of known individuals from 365 encounters from 2003 to 2012), were used to infer the prevalence and seasonal variation in shark presence. The mean proportion of the pilot whale’s body visible for documenting shark bites was 22.2% (SD ± 10.0). A total of 9,281fresh, healed, and scarred bite marks were documented on 396 of 399 whales (99.2%). Bites were most frequently documented on the head (32.9% of all bites), followed by the lateral sides (29.8%) and peduncle (26.1%), while the dorsal fin had the lowest percentage of bites (11.2%). The presence of fresh bites varied with ordinal date, with peaks in April, July and mid-October and were also negatively correlated with sea surface temperature. There was also a peak in fresh bites in the transition between crescent and quarter lunar phases. These results provide further evidence that cookiecutter sharks in Hawaiʻi may perform seasonal migrations or dietary shifts.