Automatic real-time GPS tracking of tagged cetaceans to conduct controlled exposure experimental protocols at sea

In addition to recording whale behaviour, animal-attached tags enable real-time tracking of a focal experimental subject—required for positioning a sound source to achieve a desired exposure design. Tracking has typically been accomplished with visual observers spotting the tagged whale, aided by radio signals from the tag, and recording its range and bearing. Here we describe an automatic system for near real-time GPS tracking of multiple tagged whales, developed to enable execution of long-duration exposure sessions in a behavioural response study. Real-time GPS tracking was automatically accomplished by intercepting satellite transmissions from a Lotek F6G 149 series GPS-Argos unit attached to a whale within a suction-cup multisensor tag, and decoding the data packets on the source vessel. Testing and refinement using a custom high-performance antenna demonstrated tracking of killer whales (Orcinus orca) and humpback whales (Megaptera novaeangliae) to the horizon and confirmed the benefit of specific tag placement flat on the body between the blowhole and dorsal fin. The system was successfully deployed in a behavioural response study testing how killer and humpback whales respond to continuous and pulsed sonar exposures over durations used by Navies. With receiving antennas placed 23 m above sea level, positions were reliably received < 10 min after they were recorded on the whale out to distances up to 17 km—the distance to the horizon, and at a somewhat slower rate at even greater distances. A MATLAB program was developed to plot automatic GPS positions and plan vessel movements in the sonar controlled exposure experiments. Using this system, we were able to carry out a challenging experimental protocol which required two approaches to within 1 km of each tagged whale subject over an 8-h period, while maintaining a distance of ~ 10 km between approaches. A publicly available PAMGuard module to plot GPS locations is also available to support smaller scale experimental efforts. Automatic GPS tracking represents a fundamental advance that enables at-sea experiments with marine mammals to be conducted without continuous visual tracking, substantially lowering cost and expanding contexts in which controlled exposure experiments can be conducted, including during darkness or rough weather.