A humpback whale (Megaptera novaeangliae), one of the many cetaceans relying on sound across vast distances.
Credit: Louis Herman/NOAA
Others disagree, arguing the human addition to natural ocean noise is vestigial, being localised to certain busy spots, and falling away rapidly only metres from the source as the water blankets it. While it is possible to deafen fish in a tank, it is very difficult to prove that changes in the health or behaviour of wild whales, seals or fish are due to sound alone. Indeed, the fine details of their behaviour patterns are extraordinarily difficult to monitor and interpret, so who really knows? In short, they claim, ocean noise is just another overheated issue generated by environmentalists and fed by a media frenzy based on too much emotion and too little data.
We know sound can travel both fast and far in the ocean - but for many reasons it does not travel evenly. "In the sea, noise drops off very quickly with distance from the source, then it drops off much more slowly," said Rob McCauley, a marine biologist at Perth's Curtin University of Technology who has studied ocean bioacoustics for the past 15 years.
"It's a logarithmic decay. High-frequency sound attenuates very rapidly because the water literally absorbs it. But low-frequency sound in the range 10 to 200 Hertz can travel a long, long way," he said.
To make the picture more complicated, shallow waters absorb sound both at the sea-bottom and at the surface - so they are, potentially, less 'noisy' than deep waters.
The oceans also contain a structure that allows low-frequency noise to propagate along a 'sound channel', about 1,000 metres down, through which it can travel for thousands of kilometres, all over the globe. It is this sound channel, says McCauley, that has given rise to a misconception that all noise in the ocean travels all round the world.
"That's not true," said McCauley. "There is certainly a lot of manmade noise in the busier parts of the Atlantic, but it rapidly decays to background levels and we are not seeing its effects in waters around Australia. There's a lot of noise here but it is mostly biological, made by whales and so on."
However, he adds, there are still local problem areas for noise - including busy ports and Australia's northwestern coast where ship-borne air-guns blast away in the seismic quest for oil and gas.
And some species are more susceptible than others: toothed whales and dolphins, for example, are sensitive to high-frequency sound, whereas baleen whales operate at lower frequencies. Seismic survey ships "act like a giant plough", McCauley says, scaring away sea life with each blast. "But they tend to come back afterwards. Permanent displacement of sea life due to noise has been claimed, but there's not much evidence for it."
However McCauley has firsthand proof that seismic blasts do affect snapper. In a behavioural experiment in Western Australia's Cockburn Sound, he exposed a cage of pink snapper to shots from a seismic air-gun over a distance of 500 metres and less. At each shot, the snapper bunched together, circled wildly and dived for cover at the bottom of the cage. But when McCauley repeated the experiment 58 days later, he was surprised to see the fish didn't react at all.
"When we came to look at their ears, the sensory part was a mess," he said. "We concluded they were stone deaf, and that any fish in the wild that suffered such an injury would be dead, eaten by sharks or whatever."
Michael Smith, a biologist at the University of Maryland, has likewise demonstrated noise-induced stress and deafness in goldfish, a species that tends to rely on its hearing. And Sonja Amoser, a research associate at the University of Vienna, has shown that catfish, which communicate vocally and rely on sound even more than goldfish, suffered longer hearing loss.
