Alarm sirens intended to scare highly endangered whales away from ships that often kill them may actually be more likely to cause a collision, suggests new research.
North Atlantic right whales are on the brink of extinction, with only 300 left despite being protected from whaling since 1935. The biggest risk to them now is collisions with ships.
鈥淓ven if one individual is killed 鈥� it can be significant,鈥� says Douglas Nowacek who led the study at the Woods Hole Oceanographic Institution in Massachusetts. Understanding how whales respond to the sounds of ships is also important because many other whale species die in collisions.
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鈥淚t鈥檚 not all that uncommon to have a whale come in draped on the bow of a ship,鈥� he says.
Nowacek, now at Florida State University, and colleagues found the right whales ignored general ship sounds. However, when exposed to the sound of an alarm, they responded strongly. First, they rushed to the surface to breathe and then sink back to just under the water鈥檚 surface 鈥� making them more vulnerable to an oncoming vessel.
The results cast doubt over a leading idea to prevent whale 鈥渟hip strikes鈥�. 鈥淭here鈥檚 been a lot of discussion over several years about using a siren or signal to move whales out of the way,鈥� he told New 杏吧原创. But implementing strategies without understanding the effect they will have is risky, he warns.
Social noises
The team tested an alert signal on six whales foraging during summer in the Bay of Fundy in Canada. They attached an acoustic recording tag to each whale which monitored the whale鈥檚 received sound levels and its movements. They then played the whales recordings such as the social noises of other whales, general ship sounds and the alarm.
The whales responded mildly to the social sounds of other whales. But they showed no response to the ship sounds. 鈥淢any ships pass by these whales, so it鈥檚 probably not surprising that they are habituated to the sounds,鈥� says Nowacek.
But when the team played the alert sound, five of the six rushed up from their foraging depth of 150 metres down, caught a breath of air and then loitered about five metres below the surface leaving them 鈥渧ulnerable 鈥� but not visible鈥�.
Nowacek speculates that the whales behaviour may be an anti-predator response. Killer whales hunt right whales using acoustic and visual cues. Coming straight to the surface means the right whale will not suffocate while hiding, but remaining just below the water means it is not seen either.
The signal used was based on hospital type alarms -aiming to alert but not to scare. It is different, notes Nowacek, to the unpleasant sounds used by 鈥減ingers鈥� to repel dolphins from fishing boats.
Journal reference: Proceedings of the Royal Society of London B (DOI: 10.1098/rspb.2003.2570)