Sunday, 29 January 2012

Distant noise pollution can have local consequences

Underwater noise pollution does not usually receive as much attention as its terrestrial counterpart, despite the fact that many aquatic animals may be negatively impacted by loud noises. Because the speed of sound is 5 times faster in water than it is in air, acoustic pollution can maintain its intensity over much greater areas and affect a wide variety of animals located some distance from the noise source. Depending on the strength of a particular disturbance event's sound waves, exposed animals may suffer injuries (some lethal), hearing damage, and/or disruptions to their behavioral time budgets. Although a number of species are thought to face such dangers, researchers are particularly concerned about the impacts of noise on whales, many of which rely heavily on acoustic communication to facilitate social interactions.

(Humpback whale, Megaptera novaeangliae)

Previous work on endangered right whales (Eubalaena glacialis) has shown that noise-exposed individuals alter the acoustic characteristics of their calls when their habitats are polluted by ship traffic noise. A recent study conducted in the Stellwagen Bank National Marine Sanctuary (SBNMS) has revealed that humpback whales (Megaptera novaeangliae) also make adjustments in response to anthropogenic noise--a result that is, perhaps, unsurprising given the previous findings. What is sure to raise some eyebrows is the fact that the whales in the current study were responding to noise pollution generated approximately 200 km away from the study site.

The humpback study was initiated after collaborators from Integrated Statistics, the Northeast Fisheries Science Center, and Marine Acoustics, Inc. found some intriguing patterns while reviewing data collected by an underwater array of autonomous recording units in the SBNMS. By chance, the units were running during the 2006 Ocean Acoustic Waveguide Remote Sensing (OAWRS) experiment, which used low-frequency noise pulses to create images of fish shoals over a 100-km area. The recorders picked up the noise pulses despite the fact that they were generated in a study site some 200 km off; they also collected recordings of vocalizations delivered by local male humpback whales during the OAWRS study period.

(Map showing the Massachusetts coastline and the Stellwagen Bank National Marine Sanctuary)

It seemed to the researchers that there were markedly fewer recordings of whale calls during the noise pollution period, but they couldn't be certain whether this stemmed from variations in the acoustic environment or from normal seasonal differences in communication patterns. In order to explore further, the scientists collected additional acoustic data at the same portion of the year (late September-early October) in both 2008 and 2009. This window of time was broken into 3 phases: before, during, and after the calendar period when the noise pulses were delivered in 2006.

During the focal periods of 2008 and 2009, environmental noise regimes were similar to each other and also to that recorded during the non-OAWRS period of 2006; in other words, they appear to reflect a "normal" ambient noise environment for the whales. In the years with no ambient noise pollution, whale songs were increasingly common throughout the focal period--there were more in the "during" phase than "before," and more "after" than "during." In 2006, however, there were significantly fewer songs recorded in the "during" phase of the focal period than during the "after" phase--a reverse of the pattern observed in normal years.

(Sonogram showing a humpback whale song; the Y-axis shows frequency and the X-axis shows time)

Massachusetts humpbacks migrate south in the winter in order to take advantage of warmer tropical waters. Thus, over the course of the focal period examined here, the study animals were getting ever closer to their departure date. The corresponding increase in singing behavior during the "normal" years suggests that the whales' song may be used to coordinate some aspect of migration, though currently scientists have little understanding of the role of song in humpbacks' life histories. If singing does play a role during this important transition period, then anthropogenic noise pollution during this time might have a serious impact on the animals' efforts to gather into appropriate social groups for their move south.

Remarkably, the whales appeared capable of hearing sonic pulses over 200 km from the noise source. The researchers calculated that the sounds were only 5-22 dB above local ambient noise levels by the time they reached the whales. Although these are not uncomfortably loud amplitudes, they appear to have been sufficient to disrupt the whales' normal behavior patterns. Unfortunately, the dataset collected here is not sufficient to indicate why less whale song was recorded during the noise pollution period: Did the animals respond to the noise by singing less, or by leaving the area altogether? Visual data collected at the same time for another study suggests that the whales were present, but chose not to vocalize; however, further studies would be needed to definitively characterize typical humpback responses to noise. Additional research might also investigate whether the songs that were produced during the noisy period differed from those delivered in normal conditions--for instance, by being louder or at a different frequency.

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

Risch, D., Corkeron, P.J., Ellison, W.T., and Van Parijs, S.M. 2012. Changes in humpback whale song occurrence in response to an acoustic source 200 km away. PLoS ONE 7(1):e29741.

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

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