There have been huge fights in the past decade over Naval sub-hunting sonar and its effects on certain species of whales. In several cases, mass strandings of marine mammals have occurred shortly after naval exercises where mid-frequency active (MFA) sonar was used. There is particularly strong evidence that several strandings of beaked whales—a family of small whales that look a bit like oversized dolphins—were associated with naval exercises involving mid-frequency sonar.
One odd feature of these strandings is that the sonar, while loud, is highly unlikely to be loud enough to physically harm the whales where they encounter it. As the sound spreads outward from its source, its intensity drops off very rapidly as the finite energy in the signal is spread over more and more area. Unless the whales just happened to be within, say, 100 yards of the ship when it started transmitting, the sound would be too faint to do damage (at least as we currently understand their physiology). What’s even odder is that the whales shouldn’t even be able to hear the sonar that well—their hearing and vocalizations are centered around 40 kHz, while the sonar is around 3-8 kHz.
Since it appears unlikely that the strandings are caused by direct physical trauma, scientists began considering other explanations. One hypothesis emerged from the fact that the calls of hunting killer whales also fall in this 3-8 kHz frequency range. Could the beaked whales be mistaking the sonar for the sounds of a predator, and altering their behavior in a way that might ultimately led to their deaths?
To investigate, a team of scientists from several research institutions, NOAA, and the U.S. Navy, undertook the first real experimental study of how beaked whales respond to MFA sonar. The location was a Navy test range near the Bahamas, featuring an array of hydrophones spread over its 1124 km2 (434 mi2) area. Though intended for listening to submarines, they also pick up the foraging clicks of Blainville’s beaked whales, Mesoplodon densirostris. This set up the first part of the study. During a normal Navy sonar exercise, the researchers monitored the hydrophone array for beaked whale foraging noises. Before, during, and after the exercise, they noted which hydrophones heard whales and which didn’t.
Once the noise starts, no whale calls are heard near the center of the range. At the same time, more whale clicks are heard at the edges, suggesting the whales have moved away. Over the next couple of days, they slowly return. Before a later sonar exercise, the researchers managed to stick a satellite tracking tag on one whale, who promptly hightailed it out of the range once the exercise began.
The scientists then moved on to phase two. They tagged two whales with tags that not only measured their depth and location, but recorded the ambient noise. Once each whale started diving again as normal, they played one of three recordings: mid-frequency sonar, an orca call, or a random signal. Gradually, they turned up the volume to see how the whales would react. Once the volume of each playback reached a certain level, the whales’ behavior changed abruptly. They immediately went silent, halting their echolocation clicks, and began swimming towards the surface—but at a much slower rate than normal. It looked for all the world as if they were trying to be inconspicuous under the perceived threat of a predator. The threshold for these changes was about 120 decibels (dB) for the sonar and random signal, but only 100 dB for the orca calls.
What does all this mean? Well…nothing for sure, since the sample sizes in question are so small. Tagging beaked whales is hard to do, given their elusiveness and deep-diving habits. A grand total of three tagged whales and a small handful of experimental sound playbacks just isn’t enough to reach firm conclusions. But what was observed is suggestive. The beaked whales appear to actively avoid the area where the sonar is being used—there’s something about it they don’t like. And it seems that something may be related to the rough similarity between mid-frequency sonar and their predator’s hunting calls.
The sounds are not actually that similar. But since they’re right on the lower edge of the beaked whales’ hearing, maybe they have a hard time telling the difference. Are they getting freaked out and changing their dive behavior in a dangerous way? Maybe their apparent reluctance to surface has something to do with the gas bubble lesions seen in some stranded whales—lesions very similar to those that occur in human scuba divers when they get the bends. (This is odd, though, since the whales ascended more slowly in the presence of noise, and decompression bubbles would be expected from ascending too fast.) At this point, we don’t know for sure. But this study, despite the difficulties imposed by confidential military technology and highly elusive cetacean subjects, appears to have already shed a great deal of light on the problem.
Tyack PL, Zimmer WM, Moretti D, Southall BL, Claridge DE, Durban JW, Clark CW, D’Amico A, Dimarzio N, Jarvis S, McCarthy E, Morrissey R, Ward J, & Boyd IL (2011). Beaked whales respond to simulated and actual navy sonar. PloS one, 6 (3) PMID: 21423729
T.M. Cox, T.J. Ragen, A.J. REad, E. Vos, R.W. Baird, K. Balcomb, et al. (2006). Understanding the impacts of anthropogenic sound on beaked whales Journal of Cetacean Research and Management, 7 (3), 177-187
Jepson, P., Arbelo, M., Deaville, R., Patterson, I., Castro, P., Baker, J., Degollada, E., Ross, H., Herráez, P., Pocknell, A., Rodríguez, F., Howie, F., Espinosa, A., Reid, R., Jaber, J., Martin, V., Cunningham, A., & Fernández, A. (2003). Gas-bubble lesions in stranded cetaceans Nature, 425 (6958), 575-576 DOI: 10.1038/425575a