How sound from human activities affects marine mammals
WHOI MS #50
Woods Hole MA 02543
Popular version of Talk 1aID1
We can usually see things that are miles away, but if you have ever snorkeled, you know that vision is limited to a few tens of meters underwater. Vision is the best way to sense distant objects in air, but sound is the best way to sense objects that are far away under the sea. Low frequency sounds can travel hundreds of miles in the right conditions. When mammals entered the ocean tens of millions of years ago, they evolved mechanisms to sense objects by listening for echoes from their own sounds, and to use sound to communicate over long distances.
Most baleen whales migrate thousands of kilometers every year and often swim a hundred miles in a day. They produce low frequency sounds that a century ago could be heard hundreds of miles away. Modern ships generate enough noise from their engines and propellers to have reduced the range over which whales can communicate. The low frequency noise from ships travels so well in the ocean that it has raised the noise levels ten to one hundred times compared to a century ago. This means that a whale call that could have been heard hundreds of miles away now can only be heard tens of miles away. We do not know how far away whales need to communicate, but this reduction in range of communication could affect the ability of whales to find a mate or their young.
Recent research has shown that whales have different strategies to compensate for noise. As noise levels increase, some whales increase the loudness or duration of their calls. Marine mammals have also been shown to increase the redundancy of their calls in noise, like repeating “Mayday, mayday, mayday” in a noise radio channel. If the noise is limited in frequency, whales may shift the frequency of their calls out of the noise band.
There has been growing concern that the noise humans have introduced into the sea might disrupt the behavior of marine mammals. Studies have demonstrated that marine mammals may avoid some sound sources at ranges of kilometers. Some whales even appear to have abandoned habitat following coastal development. Even activities usually considered benign, such as whale watching, can reduce use of habitat and impact reproduction in coastal populations of dolphins.
During the 1990s regulators recognized the need to establish what levels of sound might be harmful to marine mammals. A series of experiments were conducted based on the theory that hearing is the organ system most sensitive to noise. These experiments measured how much sound exposure was required to reduce the sensitivity of hearing in dolphins or seals. It turned out that the shorter the exposure, the higher the level of sound required to reduce hearing sensitivity. This relationship suggests that a specific dose of sound energy fatigues the ears and reduces hearing.
The only cases with strong evidence that anthropogenic sound has killed marine mammals involve atypical mass strandings of deep-diving toothed whales, called beaked whales, that coincided with naval sonar exercises. There are few mass strandings reported for these whales until the 1960s when a new kind of powerful naval sonar was developed. Since that time, there have been a series of 10-20 unusual mass strandings of beaked whales. Normal mass strandings of species such as pilot whales typically involve a group of animals coming ashore together. By contrast, the atypical mass strandings of beaked whales involve multiple individuals or groups stranding over many kilometers of coastline within a few hours. This scale is consistent with an effect of a sound intense enough to carry over this distance in such a short time. There is a consistent pattern of these strandings coinciding with naval sonar exercises.
For a sonar system to produce enough exposure to affect hearing, it would need to pass within about a hundred meters of whales for a reasonable time interval. It seems unlikely that a ship would pass so closely to enough groups to cause the number of animals stranded in some of the atypical mass strandings. While a number of hypotheses have been suggested for the cause of these strandings, not enough is known about where whales were before they stranded to inform us about their exposure or the cause of stranding.
Some hypotheses about the cause of sonar-related strandings suggest the sonar may elicit a behavioral response that leads to stranding. A study was started last summer to test behavioral responses of tagged beaked whales to sounds of sonar and natural sounds. Five Blainville’s beaked whales were tagged with a tag that can record sound and behavior, and one of these whales was exposed to sounds of sonar during one deep foraging dive, and sounds of killer whales in the next foraging dive. The whale exposed to these sounds stopped foraging with echolocation clicks earlier than in any of the other foraging dives from unexposed whales. The whale then did a longer and slower ascent than usual. After exposure to the killer whale sounds, the whale moved steadily away from the sound source and out of the area for ten hours until the tag came off.
When the tagged beaked whale was exposed to killer whale sounds, she stopped clicking after just five minutes at an exposure level that was not much above the levels of ambient noise, well below levels that cause direct effects on hearing. It is remarkable that such a short duration and low exposure level would stimulate such a prolonged response, but it makes sense that a whale might show a prolonged avoidance response to a predator. The whale took longer (10 minutes) to stop clicking when exposed to sonar sounds, and it took a higher sound level to elicit the response to sonar, but this level was still well below levels expected to reduce the sensitivity of hearing or cause any direct physical effects. The experiment demonstrated that the whale showed a similar but weaker response to the sonar compared to the killer whale sounds, and it took a longer and louder exposure to sonar to trigger the response to sonar than to killer whale sounds. These observations support the idea that the sonar might trigger an anti-predator response. While the response observed in this experiment did not seem to pose any risk to the whale, it seems possible that longer and much louder exposures in an actual sonar exercise might trigger panicked flight, perhaps leading to stranding in some settings.
The numbers of beaked whales known to have stranded during sonar exercises is quite small compared to the sizes of their populations. While these strandings are dramatic, causing the death of individual whales, the cumulative effects of milder problems such as masking or stress may have more serious impacts on marine mammal populations. However, we do not know if more beaked whales were injured than those observed to strand. Protecting these whales from effects of sonar will require both understanding the cause of strandings along with better methods to monitor for these elusive species. Even more important is the hypothesis that a behavioral response to low level sounds can lead to injury or death. If this is correct, then current policies that assume that sounds that do not damage hearing are safe, may not adequately protect marine mammals. One of the most important areas for future work to focus involves understanding when cumulative subtle behavioral and physiological responses to noise start to affect individuals’ abilities to grow, survive, and reproduce enough to impact the health of marine mammal populations.