Thode, A., Mathias, D., Straley, J., Calambokidis, J., Schorr, G. et al.
This report reviews the behavioral reactions of depredating sperm whales (Physeter macrocephalus) to a variety of acoustic playbacks generated at relatively low source levels, as measured by instrumented bio-acoustic tags. The goal of the study was to determine whether these signals might elicit a “mild alerting response,” such as avoidance and surfacing behaviors, for potential incorporation into mitigation efforts during seismic surveys. The tests were conducted in 2009 off a fishing vessel near Sitka, AK, in conjunction with a study, funded by the North Pacific Research Board (NPRB), to learn whether sound can act as an acoustic deterrent to sperm whales depredating longline gear in the area (Thode et al., 2007a; Thode et al., 2007b; Thode et al., 2007c). The region provided a convenient testing ground for sperm whales; the close shelf break off Sitka provided accessibility to the animals, and a history of collaboration exists between the local fishing industry and marine mammal researchers. Four distinct trips with a total of 11 longline hauls were conducted between June 4 and July 4, 2009, off Sitka, AK; playbacks were conducted during ten of these hauls. The trips were punctuated by shore stops due to weather, the need to recover tags still attached to whales, and the need to refuel and offload fish. A total of 12 bioacoustic “Bprobe” and DST (Starr-Oddi Data Storage) tags were deployed during the month, which recorded a total of 229 hours of animal depth, pitch and roll data at 5 second sampling intervals, as well as 79 hours of B-Probe acoustic data recorded on the animals themselves. The B-probe and DST tags were often deployed simultaneously on the same animal. Nine distinct animals were successfully tagged and identified, and two animals were tagged twice, with one tagged two weeks apart. At least one tagged animal was within a couple of kilometers of a playback during at least seven of the playback sessions, and acoustic tag data was obtained for four playback sessions. Every hauling site was encircled by at least four autonomous acoustic recorders sampling at 50 kHz. Satellite location tags and GPS-based tags were deployed as well. Roughly speaking, a playback commenced midway through a three-hour haul, in order to provide a baseline for visual, acoustic, and tag observations. During the first two trips, five different types of signals were played: FM sweeps, continuous white noise, white noise bursts, transient orca calls, and sperm whale creaks. The third trip played FM sweeps only, and the final trip played transient orca sounds only. The final two trips also altered the durations and intervals between playbacks. The tagging data were processed to distill parameters about dive, acoustic, and orientation behavior during fishing hauls with and without acoustic playback. A two-sided Kolmogorov-Smirnov test found statistically significant differences between haul-only and haul-playback situations, in terms of the acoustic and rotational behavior of the animals. Specifically, during playbacks animals clicked and “creaked” less, and the relative decrease in “pauses” following creak events suggest that the animals were not as successful in capturing prey. No significant changes in dive depths or durations were found, however. The sample size for playbacks was not large enough to determine which particular acoustic signal type was responsible for the observed differences, and the results may be confounded by differences in the behavior of animals between the start and end of a fishing haul. No HSE issues were encountered during the work. Lessons learned and suggested changes in fieldwork procedure are discussed.
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