A humpback whale breaching. Photo credit: Project BRAHSS (Behavioural Response of Australian Humpback whales to Seismic Surveys).
Little is known about the effects of loud sounds on migrating whales, but Rebecca Dunlop, from the University of Queensland, Australia, explains that there is a great deal of concern about the impact of noise as our oceans become rowdier. Explaining that one of the main sources of noise is oil and gas exploration, Dunlop describes how geologists fire off loud acoustic air guns to probe the structure of the ocean floor in search of fossil fuels. However, it was unclear how humpback whales might react to these sounds and whether the proximity of the disturbance is a concern. ‘If you heard the quiet noise of a burglar in your house at 3 a.m., I'm guessing you'd respond much more to that than to your loud toddler singing at the top of his or her voice – so context matters’, points out Dunlop. In addition, it wasn't clear whether the animals were disturbed by the gas guns themselves, or the noise of the towing boat's propeller. Wondering whether migrating humpback whales might be more likely to veer away from nearby soft sounds than remoter loud noises, Dunlop, Douglas Cato and an extensive team of collaborators investigated the animals’ reactions to strings of repetitively firing air guns as they were towed for an hour across the migrating whales’ path.
Locating one team of observers on a cliff above Peregian Beach and a second team 11 km south on ‘Mount Emu’, Dunlop and her colleagues were able to track the whales’ movements before the air guns began firing, while the guns were going off and for an hour after. ‘Michael Noad ran what probably ended up being one of the biggest studies of its kind with 100 plus people in the field’, says Dunlop. Then, having entered the animals locations as they surfaced into the VADAR tracking system, Eric Kniest, from the University of Newcastle, Australia, was able to reconstruct the whales’ courses and clearly saw them slowing and veering away from the airguns as they drew closer, suggesting that the whales were actively avoiding the air guns.
However, the team needed to understand exactly how loud the air gun discharges were as they reached each animal to disentangle the effects of the sound intensity and proximity on its behaviour. Rob McCauley, from Curtin University, Australia, painstakingly reconstructed maps of the intensity of each air gun blast as it reverberated through the water and reached the whales by taking account of the acoustic qualities of the sea floor. In addition, he calculated the propeller noise intensity as the vessels towed the silenced air guns.
Then, with all of the observations in hand, Lindsay Scott-Hayward, from the University of St Andrews, UK, created maps that related the air gun noise, and sound from the ship, to the vessel's distance from the whale groups – which ranged from 10 km to about 500 m – to show that both proximity and air gun noise intensity had a significant effect on the animals’ behaviour. ‘Humpback whales were more likely to avoid the air gun arrays, but not the boats alone, within 3 km of the source at levels over 140 re. 1 μPa2’, says Dunlop. Having identified the least invasive locations for prospecting acoustic air guns in relation to migrating humpback whales, Dunlop adds, ‘Developing mitigation measures based on more accurate dose–response models can only help… improve interactions between humans and cetaceans’.
