Page last updated at 08:35 GMT, Wednesday, 23 September 2009 09:35 UK

Seismic bangs 'block' whale calls

By Richard Black
Environment correspondent, BBC News website

Whalers standing on blue whale carcasse
Blue whales were hunted to the brink of extinction in the last century

Scientists have turned up new evidence showing that ocean noise can affect the communication of whales.

Studying blue whales off the eastern Canadian coast, they found the animals changed their vocalisations in response to an underwater seismic survey.

The survey was conducted using gear considered to have a low impact.

Writing in the journal Biology Letters, the researchers say this is the first evidence that whales will increase calls in response to underwater noise.

At this site, on a feeding ground, the whales make frequent calls of just a few seconds' duration, rather than the long "songs" that can be heard across vast tracts of ocean.

"The calls are used for short-range communication within a range of a few hundred metres," said Lucia Di Iorio, based at the University of Zurich in Switzerland.

We should definitely reconsider these things, because clearly it's not only the sound level that's important
Lucia Di Iorio

"And the frequency band they use is exactly where the main energy of those seismic pulses is located," she told BBC News.

Initially, Dr Di Iorio's group tried to persuade the Canadian university conducting the seismic survey to co-operate in the research, and to give details of where and when the underwater bangs were being produced.

That attempt failing, the scientists recorded the pulses with an array of detectors mounted on the sea bed in the St Lawrence Estuary.

The detectors also recorded the blue whales' calls, which are thought to be associated with feeding and socialising.

Information gap

On days with seismic surveys, the whales made two-and-half-times more calls than on days without.

The ratio was the same when the recordings were analysed in blocks of 10 minutes; survey noise induced more than a doubling of calls.

The researchers suggest the whales are having to "repeat information", as some of the calls are blocked or degraded by the seismic bangs.

Humpback whale breaching

"Our research doesn't say anything about whether this increase in call rate is negative for the animals, but of course it's not positive and it may be stressful," said Dr Di Iorio.

This survey was carried out using "sparkers", devices that generate a bang from an electrical discharge between two electrodes.

Sparkers produce sounds quieter than the ones generated by airguns, another technique engineers use for underwater surveys.

"It's used [here] because it's thought to have a lower impact on marine life," said Dr Di Iorio.

"But we should definitely reconsider these things, because clearly it's not only the sound level that's important; and one thing might be not to do the test when there are lots of whales around."

Gray area

A number of recent reports have highlighted the increase in ocean noise brought about by humanity's use of the oceans, in particular shipping.

One study indicated that the level of background noise from ships' propellers was doubling every decade in the Pacific Ocean.

Conservation groups are raising the issue because many marine animals, including whales and dolphins, use sound to communicate and to hunt.

The sharp sounds of seismic surveys are a particular concern. Engineers use very sharp, very loud bangs because these produce the clearest images of geological structures below the sea floor.

The surveys are typically used to map oil and gas deposits.

Earlier this year, companies involved in the Sakhalin Energy consortium agreed to suspend seismic work after seeing evidence that it was driving the critically endangered western gray whale, of which only about 130 remain, away from its summer feeding ground.

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