North Pacific Research Board Research Articles

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

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.]

The use of acoustic multipath for localization of sperm whales

Sperm whales produce impulsive sounds, or ‘‘clicks,’’ that are ideally suited for acoustic tracking. If a widely distributed passive acoustic array is available, then the relative time delays from a click’s direct path can be used to derive an animal’s location. There are many interesting circumstances, however, where deploying a wide-baseline 3D array geometry is either unavailable or impractical. In such situations acoustic reflections of clicks from the ocean surface and bottom can be used to effectively convert a few hydrophones into a wide-aperture array that permits localization in range, depth, and azimuth. In addition acoustic multipath, particularly surface reflections, can also simplify automated detection and tracking procedures, and permit a single hydrophone to be used to distinguish between multiple whales calling simultaneously. Reviewed here is recent research on how multipath effects detected on one or two hydrophones have been used to recover dive profiles of sperm whales in the Gulf of Alaska and the Gulf of Mexico. This information, in turn, has been used to explore whether the animals exploit multipath for their own purposes, to estimate signal directionality, and to observe how sperm whales remove fish from longlines. [Work sponsored by ONR, the Minerals Management Service, and the North Pacific Research Board.]

Acoustic observations of longline depredation by sperm whales

Sperm whales have learned to take sablefish off longline gear in the Gulf of Alaska. Over the past two years the Southeast Alaska Sperm Whale Avoidance Project (SEASWAP) has collected biopsy and photo-ID data concerning longline depredation, via collaboration with local fishermen in Sitka, AK. In 2004, in collaboration with SEASWAP, sets of autonomous acoustic recorders were attached to the anchor lines of several longline deployments, effectively converting the fishing gear into a vertical acoustic array deployed 200 m beneath the surface. The fishing vessels then left the area, leaving the acoustic instruments behind to monitor the region. Several hours later, typically the next morning, each vessel would return and begin hauling in the gear, a procedure that was also recorded by the instruments. In May 2004 an interaction of two sperm whales with the F/V Cobra was recorded. It was found that the animals started producing sounds within 20 min of the longline recovery and continued being extremely vocally active throughout the recovery. Acoustic multipath was exploited to estimate the range and depth of the whales. [Work sponsored by the North Pacific Research Board.]