Slowly raising the level of sonar noise – known as “ramp-up” – is also used by geophysical explorers who use airguns to search for oil and gas reserves as a way of reducing the harmful impact of very loud sounds produced by humans in the sea.
It was hoped the technique would encourage the whales to move away calmly before the sound reaches full volume and damages their hearing.
However, it was not known how effective these warning strategies were, until an international team of researchers led by Dr Paul Wensveen and Professor Patrick Miller of the university’s Sea Mammal Research Unit recorded the responses of 13 humpback whales to sonar in the Barents Sea off Norway.
After attaching data logging tags, recording sound and movement, to individual whales via suction cups, the team then reproduced the riskiest situation for whales at sea: when a navy vessel sails towards the mammals towing a transmitting sonar source. The team analysed the response of whales as they gradually increased the sonar to full intensity.
The research, published in the Journal of Experimental Biology, found while evasive action did reduce the chances of whales encountering damaging levels of sound, only five of the 11 animals veered away from the ship on the first occasion “ramp-up” sounds were transmitted, so overall sound levels received by humpbacks were only reduced by a few decibels.
However, the study indicates that the hearing of those most responsive animals would be more effectively protected by the “ramp-up” procedure.
Animals responding when the sonar first sailed towards them were more reluctant to veer away from the gradually increasing sound on the second occasion.
Professor Miller said whales reacted in different ways to sonar noise.
“Our research suggests humpback whales may be reluctant to avoid sonar if they have heard it before, or are distracted by food.
“Unfortunately, seemingly common-sense measures like ‘ramp-up’ may not be as effective as we hoped for protecting hearing of whales from loud sounds like sonar. However, there was no indication in our study that ‘ramp-up’ increases risk to hearing, and that ‘ramp-up’ may be effective with those species or individuals more sensitive to sound disruption and therefore move away more strongly.”