Can You Hear Me Now?: Ocean Noise And Minke Whales
Imagine you’re having dinner in a restaurant. As the number of patrons increases, so does the noise level. To converse with your companions, you raise your voice. This response is known as the “Lombard effect” or the “Lombard reflex.” It describes the involuntary tendency of a speaker to increase vocal effort in response to noise in their environment. We know that the oceans are getting noisier with increases in shipping, fishing, military traffic, cruise ships, and so forth. So, are marine animals also having to raise their voices to be heard?
Knowing how marine animals respond to noise is important. Industrial activities and military exercises may require counts and locations of species in the area before they can proceed. Estimates of how many animals are in an area are derived from the sounds they make. This process is known as acoustic density estimation, and its accuracy may be suspect in noisy waters. If animals are emitting louder sounds to compensate for localized noise, measurements of their numbers and locations will likely be wrong.
Researchers in this study looked at whether and how Minke whales would respond to increased background noise in their environment. Minke whales are relatively small, solitary baleen whales. There are two species, the common, or northern Minke and the Antarctic, or southern Minke. Much remains unknown about these whales, including their exact migration ranges and auditory system. Indeed, it wasn’t until 2005 the sounds made by Minke whales were definitively linked with them.
The U.S. Navy maintains a set of hydrophones on its missile range in Hawaii. For the period of August 2012 through July 2017, these hydrophones remained fixed in type and location. Recordings from these devices provided researchers the opportunity to learn if Minke whales display the Lombard effect. To measure the whales’ response, they detected, located, and tracked a total of 42,159 calls, or “boings” across a 20 by 60-kilometer area. The “boing” is a series of low-frequency thumps or pulses. Duration and pulse rates seem to vary by population, and even by whether another Minke is in the area.
Changes in animal source noise levels (SLs) were correlated with changes in background noise. The results of the analysis confirmed that Minke whales do display the Lombard effect in response to background noise. Not only did they increase the intensity of their calls, but they reduced the variance of the boing. Yet they didn’t fully compensate for the increased noise levels, either because they were unable or unwilling to do so. When background noise rose above 85db, it appears that the whales stopped calling altogether. This could impair the whales’ ability to communicate, and with it the health of the Minke whale populations.
The dangers of our increasingly noisy oceans are becoming more apparent. This study suggested that an increase in background noise of 20db would reduce the range of a Minke whale call from 114 kilometers to just 19. That’s a dramatic difference. Animal advocates can use this study as evidence that whales not only try to “shout louder” to be heard, but that their efforts may not be enough. Sound, no matter the volume, is invisible. For those who rely on their ears and a quiet environment to communicate, it’s a matter of survival.