To humans, the world beneath the waves may seem a quiet place. To the animals that live there, it is anything but. Numerous species, from whales to shrimps, use sound to communicate, navigate and threaten across distances from centimeters to hundreds of kilometers; and, increasingly, human activities – from pile driving to sonar to ship propellers to seismic testing – compete with and drown out the natural cacophony.
According to Cornell University’s Christopher Clark, this growing noise pollution is “doubling every decade in an urbanized marine environment,” as a consequence of which, for example, a blue whale that was born in 1940 would have seen its ‘acoustic bubble’ – the distance over which its vocalizations can travel and the vocalizations of others can be heard – shrink from 1,000 miles to 100 miles.
Blue whales create the loudest noises of any living thing in order to communicate across vast distances, and most previous studies of the impacts of noise pollution on their behavior have focused on anthropogenic sounds generated within their vocalization range. However, new research just published in the online open-access journal PLoS One has found that blue whales appear to be affected even by sounds outside of that range.
In the study, Mariana Melcon and colleagues from the Scripps Institution of Oceanography observed the response of blue whales in the Southern California Bight to mid-frequency sounds created by military sonar, which occur between 1000Hz to 8000Hz, much higher than blue whale calls, which are 100Hz and lower.
They collected thousands of hours of recordings over two summers, and found that, when mid-frequency active (MFA) sonar (which is used to find submarines) was active in the region, blue whales’ low-frequency vocalizations, known as “D-calls”, dropped by almost half: a sign, the authors posit, that despite being well outside the whales’ vocalization range, such relatively high-frequency sounds are within their hearing range. One possible explanation for being able to hear sounds of such high frequency, Melcon and colleagues suggest, “is that it may be advantageous, for instance, to hear their predators, i.e. killer whales, which vocalize in the same frequency range as MFA sonar.”
Conversely, when confronted with the noise of shipping, which does largely fall within their vocalization range, the whales increased their D-calls – which, the authors write, “may be the vocal response of the animals to overcome the noise.”
In other words, one set of human sounds apparently cause whales to keep quiet so they can listen out for predators; and another set forces them to speak up so they can hear each other over the din. Far from being world of peace and quiet, the underwater realm is, at least for the largest animal on Earth, becoming an ever noisier and ever more difficult place to live.
Image: Artistic image of a blue whale. (Denis Scott/Corbis)