Seasonal prevalence of cookiecutter shark (Isistius brasiliensis) bites on short-finned pilot whales (Globicephala macrorhynchus) in Hawai‘i

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 are resident to the island of Hawai‘i and are often observed with fresh and healed bites from these sharks. As cookiecutters are pelagic, observing their behaviour directly can be problematic, but studying their bite patterns allows us to infer the dynamics of their predatory behaviour. We used a photo-identification catalogue of 405 resident whales (representing 5,871 identifications from 365 encounters from 2003-2012) to look at prevalence of fresh bites and examine possible seasonal variation in shark behaviour. For each bite we noted: status (fresh, healed or scared), location on body, and mean sea surface temperature. The mean proportion of the body visible for documenting bites was 22.2% (SD=10.0), thus many bites and scars were likely missed. We recorded 9,293 bites of all states, and bites were documented on 396 of 405 whales (97.8%). Fresh bites were recorded from 161 identifications, healing bites were recorded from 490 identifications, and scars were recorded from 2,779 identifications. Bites were most frequently documented on the head (31.7% of all bites), even though this and the dorsal fin had the smallest surface area, followed by the lateral sides (31.1%) and peduncle (26.6%), while the dorsal fin had the lowest percentage of bites (10.9%). The presence of fresh bites varied with day of the year and were negatively correlated with sea surface temperature. Peaks in fresh bite probability were at approximately day 115, 200 and 290 of the year.  The peaks in bite probability may be an indicator of fluctuations in other potential prey species or movements of the whales into areas where they may be more likely to be bitten.

Citation:

Walker-Milne, N., Y. Papastamatiou, R. Baird, S. Mahaffy. 2017. Seasonal prevalence of cookiecutter shark (Isistius brasiliensis) bites on short-finned pilot whales (Globicephala macrorhynchus) in Hawai‘i. Abstract (Proceedings) 22nd Biennial on the Biology of Marine Mammals, Halifax, Nova Scotia, October 22-27, 2017.