Cookiecutter Shark Bite Wounds on Cetaceans of the Gulf of Mexico

During line-transect surveys, marine mammal observers may detect the presence of fresh wounds and scars produced by cookiecutter sharks (Isistius spp.) bites on cetaceans. Of the 19 different species evaluated 12 showed evidence of predation by cookie-cutter sharks. The vast majority of sightings with positive records included dolphin species like the common bottlenose, Atlantic spotted, pantropical spotted and spinner dolphins. Several biases may affect the detection of wounds and scars on cetaceans. One of the most important ones is approachability of small cetaceans to the ship. Dolphins have the tendency to bowride, therefore allowing for a close inspection of their bodies by marine mammal observers. Large cetaceans like sperm whales spend a considerable amount of time under water, therefore making detection of wounds and scars more problematic, however they do happen. Despite all limitations, the positive detections of wounds and scars on multiple species of cetaceans in the Gulf, does indicate the widespread cookiecutter shark predation on cetaceans. In addition to observations from line-transect surveys, cetaceans that strand may have wounds and scars and marine mammal stranding network scientists who respond to these strandings may note on the Level A data form the presence of these. Eleven species of stranded cetaceans were documented with bites and similar to the occurrences recorded during the line-transect surveys, indicate predation of these sharks upon various species of cetaceans. In addition to dolphins, the stranding data revealed predation on the pygmy or dwarf sperm whales and melon-headed whales. Fin whales were the only stranded baleen whale species with bites detected. Assessing predator-prey associations can be further investigated by comparing stable isotopes ratios. Stable isotope ratios for cookiecutter sharks place them in a similar trophic level to sperm whales, indicating that these sharks are in a higher trophic level than sperm whale prey for example. Studying the feeding ecology of these small sharks on relatively large prey, like cetaceans, allowed us to infer distributional overlap between cetaceans and cookie-cutter sharks. In addition, detection of bites on generally coastal species, like the common bottlenose dolphin may indicate that cookiecutter sharks can have a broader, shallower water depth than indicated by their previous capture locations. 

Mark A. Grace, Laura Aichinger Dias, Katherine Maze-Foley, Carrie Sinclair, Keith D. Mullin, Lance Garrison, and Lauren Noble. Aquatic Mammals 2018, 44(5), 491-499.  https://doi.org/10.1578/AM.44.5.2018.491