Thursday, 8 March 2012

Do blue whales hear mid-frequency anthropogenic noise?

Although a number of marine acoustic studies have investigated how whales are impacted by masking anthropogenic noise--or sounds that overlap with the whales' own vocalizations--less research has focused on noise that does not seem to directly threaten the animals' abilities to communicate with each other. However, many whales can detect a wide variety of acoustic stimuli that occur outside the frequency range utilized when vocalizing with conspecifics. For instance, foraging blue whales (Balaenoptera musculus) produce low-pitched (<100 Hz) sounds, called D calls, that appear to help them coordinate group feeding behaviors; however, the animals are probably capable of hearing much higher frequencies, such as those characterizing their own echolocation clicks (reportedly reaching frequencies as high as 31 kHz). Thus, it stands to reason that higher-pitched human noises might also elicit behavioral responses in this species.

(Blue whale, Balenoptera musculus)

A team of researchers from the Scripps Institution of Oceanography recently explored this possibility using an acoustic recording device deployed in the Southern California Bight, an important summer feeding ground for the endangered blue whale. The recorder collected continuous acoustic data during four separate 2- to 4-month periods over 2 different summers. From these data, the researchers could isolate occurrences of anthropogenic noises (sonar, ship noise, and explosions--all occurring within the 1-8 kHz frequency band), "natural" noise events (wind, rain, earthquakes, other animals), and blue whale D calling. The researchers calculated the probability of D calling during each of the anthropogenic noise sources, as well as during all "natural" acoustic events. The latter was used as a baseline indicating how the whales should respond to anthropogenic noise events if they are seen as comparable to "natural" sources of noise. Thus, ratios of anthropogenic to "natural" noise should be high for human sounds that don't disturb the whales, but low for human sounds that do.

Ratios were fairly high (0.83) for ship noise, indicating that the whales are not particularly troubled by this sound. However, calling dropped dramatically during explosions (ratio = 0.63) and, especially, sonar (ratio = 0.54). To explore this pattern further, the researchers then investigated D calling behavior with respect to the maximum sound pressure level (or volume) of "natural" and anthropogenic noises throughout the study period. Although whales were equally likely to produce D calls during "natural" noises of all volumes, there was a higher probability of vocalizations with increasingly loud ship noise, and a lower probability with increasingly loud sonar events.

(Transmission patterns for side-scan and multibeam sonar)

The researchers hypothesize that ship noise is associated with increased calling activity because this behavioral shift may enable whales to overcome potential masking--a tactic appropriate for this type of noise pollution because it is a broadband signal that has acoustic energy in the lower pitches at which blue whales communicate. On the other hand, sonar seems to act as a call deterrent. Interestingly, although blue whales in this population typically D call more often during sunset and just before sunrise, they did not appear to be more impacted by anthropogenic noises during these times of day.

Overall, the blue whale behaviors documented here suggest that these animals can hear mid-range (1-8 kHz) anthropogenic noises. Although this seems like an obvious conclusion, it actually isn't; nobody has been able to directly measure the hearing sensitivity of this species, and examinations of their vocalizations have led to hypotheses that the whales hear best around 20 Hz, and decently up to 200 Hz. There have been some doubts as to whether decades-old recordings of high-pitched (4-10 and 21-31 kHz) echolocation clicks are accurate, so researchers have been unsure whether blue whales could detect, and potentially be bothered by higher-pitched sounds such as anthropogenic noises. The current results indicate that the answer to this question is "yes."

In fact, the whales altered their behaviors even after stimuli that were "relatively low intensity." Thus, even isolated noise events that happen several kilometers away could be problematic for these animals. Although the current study found no relationship between time of day and strength of whale response to noise, the researchers still suspect that whales are likely to be most susceptible to noise during their peak vocalization times (at sunset and just prior to sunrise); additional studies that focus on other whale behaviors and obtain an even bigger bigger sample size may be able to detect such patterns.

For supplementary images associated with this post, visit the Anthrophysis pin board at Pinterest.

Melcon, M.L., Cummins, A.J., Kerosky, S.M., Roche, L.K., Wiggins, S.M., and Hildebrand, J.A. 2012. Blue whales respond to anthropogenic noise. PLoS ONE 7(2):e32681.

Thanks to the following websites for providing the images used in this post:

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