Foraging behavior of fish-eating sperm whales in the Gulf of Alaska in the presence and absence of fishing vessels

Abstract

Historical whaling records indicate that sperm whales off southeast Alaska incorporate fish into their diets, particularly black cod (Anoploploma fimbria). Since 1995 this fact has become relevant to fisheries’ concerns in the form of increased depredation encounters between longline fishermen and over 40 sperm whales. Since 2002 the SE Alaska Sperm Whale Avoidance Project (SEASWAP) has been studying this phenomenon using fishermen reports, photo-ID, biopsy, and (since 2004) passive acoustics using both towed arrays and autonomous recorders placed on longline deployments. By using acoustic multipath the range and depths of foraging whales can be determined. Findings to date indicate that, under natural conditions, sperm whales are foraging at mid-depth in the water column (e.g., 250 m in 500-m-deep water), and that their dive cycle durations are similar to those reported in other oceans. This information is being compared with depth measurements of black cod at various stages of their life cycle. There is increasing evidence that distinctive acoustic cues made by longline vessels lead to changes in diving and acoustic behavior by the animals, when the animals are less than 10 nautical miles away. [Work supported by the North Pacific Research Board.]