Even low levels of sonar can drive whales to their deaths
Naval sonar exercises are much more distressing to beaked whales than previously believed, and are driving them to beach themselves, a new study has found.
Scientists have long been aware of a link between sonar exercises and mass strandings of beaked whales - indeed, environmental groups took the US navy all the way to the Supreme Court over the issue back in 2008.
The court ruled that the Navy could continue with its exercises, but said it was essential for the Navy to develop better methods to protect the whales. This was all very well, but with no scientific evidence as to what levels of sonar were acceptable, the ruling did little to help.
Now, Peter Tyack, a senior scientist at Woods Hole Oceanographic Institution (WHOI), has used two methods to investigate behavioral responses of beaked whales to sonar.
One was to monitor whale responses to naval exercises involving tactical mid-frequency sonars over several days, and the other was to play simulated sonar and control sounds to whales tagged with a device that records sound, movement, and orientation.
"These experiments were very difficult to develop, and it was a major breakthrough simply to be able to develop a study that could safely study these responses," says Tyack.
"All three times that tagged beaked whales were exposed experimentally to playback of sounds when they were foraging at depth, they stopped foraging prematurely and made unusually long and slow ascents to the surface, moving away from the sound."
During real naval exercises, the team found that whales moved away from the sonar as quickly as possible, in a response that could lead to the mass beachings. And they showed this behaviour at sonal levels well below those previously considered to be a problem.
"This suggests that beaked whales are particularly sensitive to sound. Their behavior tended to be disrupted at exposure levels around 140 decibels, so they may require a lower threshold than many current regulations that anticipate disruption of behavior around 160 dB," says Tyack.