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Multi-year Sperm Whale Study in GOM Shows Lack of Avoidance at Long-Range, Possible Foraging Changes

Science, Seismic Surveys Add comments


Jochens, A., D. Biggs, K. Benoit-Bird, D. Engelhaupt, J. Gordon, C. Hu, N. Jaquet, M. Johnson, R. Leben, B. Mate, P. Miller, J. Ortega-Ortiz, A. Thode, P. Tyack, and B. Würsig. 2008. Sperm whale seismic study in the Gulf of Mexico: Synthesis report. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS Study MMS 2008-006. 341 pp. [DOWNLOAD REPORT]
For four summers, from 2002-2005, a diverse team of researchers studied the sperm whale populations of the northern Gulf of Mexico; this final report presents the results of three distinct lines of research: to learn more about the population sizes, social patterns, and group and individual behavior of this population of sperm whales, to characterize habitat use in this area, and to examine possible changes in behavior in response to the noise of seismic survey airguns. By all accounts, the study was very successful on the first two counts, dramatically increasing our understanding of the overall populations and habitat use, especially in the key areas of the Gulf where the oil and gas industry is moving into deeper waters. It is the third topic, effects of noise, that especially interest us here at AEI, and on this count, the results were not as clear-cut. Over the course of two field seasons in which researchers attached acoustic D-tags to sperm whales, only a total of eight whales were tagged and subsequently exposed via controlled exposure to air guns towed by ships participating in the study. In each case, the whale was tagged while the seismic survey vessel was 7-13km away; after tagging, the researchers collected pre-exposure data, then the ship fired up its airguns and slowly moved closer, aiming to keep the sound going through two complete hour-long dive cycles, so as to monitor changes in foraging clicks (seeking prey) and buzzes (zeroing in and attempting to capture prey). A final post-exposure period followed, while tags were still attached. Exposures occurred at levels from 111 to 147 dB re 1uPa (rms) , alternatively measured as 131-164dB peak-to-peak, at distances of 1.4-12.6km.

While it appeared that there was no horizontal avoidance (i.e., the whales did not move away from active airguns or change direction of movement when guns were turned on and ramped up, or when they were at full power), the study notes that “the sample sizes were insufficient to be completely confident that there were no horizontal or vertical responses,” while also stressing that this population may have been habituated to such noise over many decades of oil and gas exploration and development, so that the results can not be easily applied to populations in more pristine locations. Further, the study found that “it is more likely than not that some decrease of foraging effort may occur” when airguns are active, at least in some individuals. Using complex statistical analysis to try to tease patterns from this limited data set, the researchers conclude that a decrease of 20% in foraging activity is likely. Strikingly, one whale apparently ceased foraging—and this was the whale that had the highest received sound levels; it rested at the surface for over four hours during airgun activity (much longer than normal), and dived to feed immediately after the airguns were shut off. Nevertheless, the study did conclude that the population of sperm whales appears to be healthy and thriving, and that oil and gas exploration activity does not drive the animals away. Supporting the controlled exposure data was a larger number of animals that came within earshot of industrial surveys taking place in the Gulf. As part of the long-term population studies, S-tags (satellite tags, noting horizontal location, but not dive patterns) were attached to 53 whales. Of these, 34 came within 100km of active survey vessels (sometimes within 100km or two or three vessels), while 12 whales came within 25km of survey vessels. Whale locations appeared random; that is, statistical analysis suggests there was no avoidance of the active airguns. It is noted that no whales came within 5km of an active survey vessel, though the researchers suggest that a total sample size of 75 would be necessary for this to be considered a statistically significant indication of close-range avoidance.

A final important finding was that the actual received levels of sound did not match those expected based on the models generally used to predict sound exposures and safety radii. Several factors were noted, especially that any given airgun blast results in a series of noise arrivals at a distance, as the sound travels different direct and reflected pathways, and that surface ducts can allow sounds to travel further than expected. At times, received levels reached over 160dB (peak to peak) at ranges over 10km.There was significant energy at higher frequencies than typically considered in airgun effects modeling (up to 3kHz, rather than the expected frequency range of below 500Hz).

(Ed. note: It seems incongruous to me that press coverage of this report, and indeed the content of key academic press releases, focused on the apparent lack of avoidance behavior. Since the data set is so small, the report itself cautions against extrapolating these results elsewhere, and repeatedly calls for further study. As well, the very limited data collected for whales at closer range and higher received levels are generally disconcerting, even in this likely habituated population: notably the cessation of foraging in the one whale who received sound levels at 164dB, peak-to-peak (which can occur up to 10km away in some conditions), and the apparent lack of whales within 5km of active industrial survey vessels. While granting that we cannot make any generalizations from these truly tiny data sets, the widespread “take-away message” that the study proved that whales did not react to seismic surveys seems far out of line with the subtleties of the actual reported results. The report’s results do not offer any clear rationale for the currently established shut-down distance of just 500m, which is designed to avoid physical injury, but is unlikely to prevent the behavioral effects hinted at in this study.)


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