Report: Locating Sperm Whales Using a Towed Array Hydrophone

A new report, Real Time Localization of Sperm Whales Using  Towed Array Hydrophone, has been released about fishermen's participation in conducting research with the Southeast Alaska Sperm Whale Avoidance Project (SEASWAP). Fishermen helped to test a system that can be used to find and record sperm whales while fishing or transiting. Read the full report here. 

Background and report abstract

Whale depredation on commercial sablefish and halibut longline gear increases harvesting costs and presents the risk of marine mammal entanglement issues.  In 2016, with funding from the Central Bering Sea Fishermen’s Assn, The Southeast Alaska Sperm Whale Avoidance Program (SEASWAP) tested a towed array hydrophone system that can be deployed and retrieved from fishing vessels while underway at normal transit speeds. The towed array equipment consisted of a pair of matched hydrophones within a streamlined housing that was towed on a strengthened cable 100 to 200 meters in length. The array was connected to a power supply, sound filtering equipment, and a computer running PAMGUARD software to enable sperm whale detection and localization.  The main objectives of this work were to: 1)  test the performance of this type of equipment on vessels typical of those used in Alaska’s longline fisheries; 2)  explore with fishermen how the equipment could be integrated onboard without impacting fishing activities; 3)  assess the efficacy of using towed array hydrophones to support whale avoidance strategies. 

 

The custom built, towed array hydrophone system was tested for a total of 20 days at sea during two cruises offshore of Southeast Alaska and one cruise in the deep, inside waters of Chatham Strait. Hand deployment and retrieval of a towed array from the deck of a long line vessel while underway proved to be relatively simple and straight forward. Waterproof connectors allowed the coiled cable to be disconnected when not in use and stored in locations that did not interfere with fishing operations. The major installation challenge encountered was to eliminate or minimize electrical noise unique to each vessel’s electrical system and computer.  Once electrical noise was eliminated, the vessel machinery proved quiet enough to allow detections of whales at ranges of several miles.

A total of 25 sperm whales were detected with the towed array system during a three day offshore survey which indicated a higher density of whales along the 200fm depth contour (a preferred fishing ground) than the 1000fm contour.  Detection ranges for whales varied from two to eight miles.  General localization of whale position ahead of, or behind the vessel, could be achieved by ear using headphones.  More accurate localization was possible using PAMGUARD software to calculate and display bearings on a computer screen, however the current PAMGUARD click detector and user interface was not sufficiently “fishermen friendly” to allow consistent use by non-trained individuals.  Optimum cable length to balance noise reduction and deck handling logistics was approximately 150 meters.  Detections were possible at normal transit speeds (6-8 knots), however, noise decreased and detection range improved with slower vessel speed. 

 

These initial sea trials demonstrate that the towed array equipment can be deployed and retrieved from longline vessels without the need to shift the vessel in and out of gear thus minimizing acoustic cues that attract sperm whales.  The ability to detect sperm whales at a distance of four to eight miles provides the vessel operator with a powerful new tool to identify “whale free” areas with a lower chance of depredation prior to setting the fishing gear. Placing towed array systems on vessels transiting to home or distant fishing grounds could provide information to map sperm whale distribution over a large geographic area, supporting a real-time reporting network to identify areas of high whale activity for avoidance.  Costs could be reduced by sharing towed array systems, and by placing equipment on strategically selected vessels engaged in cooperative reporting networks. 

Tracks and vessel locations when sperm whales were detected acoustically during SEASWAP’s towed array cruise on June 17th-19th offshore of Sitka, AK. The cruise approximately followed the 200fm contour and the 1000fm contour. The erratic lines in the south eastern section of the track indicates an interruption of the survey when sperm whales were encountered near a fishing vessel and a RHIB was deployed to collect tissue samples and photographic identifications.  There were 16 separate encounters with an estimated 20 individuals detected on the 200fm track and 5 encounters all with single animals on the ~1000fm track.

Tracks and vessel locations when sperm whales were detected acoustically during SEASWAP’s towed array cruise on June 17th-19th offshore of Sitka, AK. The cruise approximately followed the 200fm contour and the 1000fm contour. The erratic lines in the south eastern section of the track indicates an interruption of the survey when sperm whales were encountered near a fishing vessel and a RHIB was deployed to collect tissue samples and photographic identifications.  There were 16 separate encounters with an estimated 20 individuals detected on the 200fm track and 5 encounters all with single animals on the ~1000fm track.