Acoustic and foraging behavior of tagged sperm whales under natural and depredation foraging conditions in the Gulf of Alaska

Sperm whales have been depredating black cod (Anoplopoma fimbria) from demersal longlines in the Gulf of Alaska for decades, but the behavior has now become pervasive enough that it may be affecting government estimates of the sustainable catch, motivating further studies of this behavior. Over a three-year period, 11 B-Probe bioacoustic tags have been attached to seven adult sperm whales off Southeast Alaska, permitting observations of the animals’ dive profiles and acoustic behavior during natural and depredation foraging conditions. Two rough categories of depredation were identified: “deep” and “shallow.” “Deep depredating” whales consistently surface within 500 m of a hauling fishing vessel, have maximum dive depths greater than 200m, and display significantly different acoustic behavior than naturally foraging whales, with shorter inter-click intervals, occasional bouts of high “creak” rates, and fewer dives without creaks. “Shallow depredating” whales conduct dives that are much shorter, shallower, and more acoustically active than both the natural and deep depredating behaviors, with median creak rates three times that of natural levels. Occurrence of slow clicks and the behavioral context in which these vocalizations are produced were also investigated. These results provide insight into the energetic benefits of depredation behavior to sperm whales. [Work conducted under the SEASWAP program, supported by the North Pacific Research Board and the National Geographic Society.]