Relative role of operational patterns and environmental conditions on cookie cutter shark damage in the Hawai‘i longline fishery Marine Ecology Progress Series

Cookie cutter sharks Isistius spp. (nahunaiki) are a small shark species that act as a micropredator for large pelagic organisms. Little is known about these sharks due to their cryptic behavior, resulting in most inferences about their distribution coming from patterns in the wounds they leave on prey. Here, we used documentation of these cookie cutter shark bites on fish caught in the Hawai‘i longline fishery, coupled with catch location and timing, to better understand how fisher behavior and environmental conditions are associated with cookie cutter shark bites. Specifically, we analyzed patterns in cookie cutter shark bites for the nighttime shallow-set sector targeting swordfish Xiphias gladius (a‘u kū) and the daytime deep-set sector targeting bigeye tuna Thunnus obesus (‘ahi po‘onui). Our results indicated that cookie cutter shark bites primarily occurred on hooked fish when operations extended longer into nightfall, during low lunar illumination, and along the edges of the North Pacific subtropical gyre. Bites were more common on average in the shallow-set fishery than the deep-set fishery, likely due to the nearly full nighttime operational window. The frequency of cookie cutter shark bites on fish caught in the deep-set sector has increased over time due to fishing extending further into nighttime hours. Our findings provide insight into the ecology of cookie cutter sharks, context for the increasing frequency of cookie cutter shark bites in the deep-set Hawai‘i longline fishery, and important information for fishers regarding the potential costs of increased effort leading to fish damaged by cookie cutter shark bites.