The ability of endangered blue whales to gather and breed may be being put at risk by equipment used to search for oil and gas lauded for its low environmental impact.

Research has discovered that whales forced to compete with the seismic testing work, which involves bouncing sound waves off the sea bed, markedly increase the number of times they repeat the same calls.

It is feared the repetition shows that they struggle to get their meanings across through the interference.

Little is known about the breeding habits of blue whales, which now number between 5,000-10,000. They are solitary most of the year, but when they come together in feeding areas like Canada’s St Lawrence Estuary, where the study took place, they may also be looking for a mate.

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Italian zoologist Lucia Di Lorio, who conducted the research, said: “They come to eat, but also to check out each other, maybe find a mate.

“We don’t yet know a lot about what these calls mean. They are only made when individuals are close together.

“These whales are endangered and the calls might be very important, but unfortunately they have a similar acoustic structure and are in the same frequency as seismic pulses.”

Blue whales make two types of sound. One takes the form of a long, low song that lasts hours, sometimes days, and consists of complex phrases that are repeated over and over again. The other is a short, higher-pitched call used only when the whales are closer together.

Light cannot penetrate more than a few metres in water, but sound travels 1,000 times faster than in air, so whales rely primarily on hearing for navigation, locating prey and tracking predators, as well as for communicating with each other.

While some of the short calls – which last less than four seconds – may be amorous, others are associated with aggression.

It is feared the distinctions may be lost when confronted with the noise of seismic exploration.

Ms Di Lorio, whose research is published in the Royal Society journal Biology Letters, was studying the social behaviour of the whales – the largest animals ever to have lived - when a seismic survey vessel began operations nearby.

Over the eleven days of the survey, Ms Di Lorio recorded the vocalisations of the whales and found there were significantly more when seismic pulses were being fired.

“The whales made more calls on days when the testing was happening. It seems they are having to repeat themselves in order to not lose information,” Ms Di Lorio said.

While military sonar and traditional seismic surveys using loud air guns are known to affect whale behaviour, the equipment being used near Ms Di Lorio’s whales was of a new variety developed for its low environmental impact, known as a ‘Sparker’.

“Unfortunately the company doing the testing only got in touch with us after they’d surveyed the area. If we’d known they were coming we could have got a lot more information.”

Previous research has shown blue whales stop singing their repetitive, long-distance songs when forced to compete with the noise of seismic tests, but the short, higher pitched calls appear to be a different matter, necessitating a repeat rather than risk the message not getting through.

“We need careful reconsideration of the potential behavioural impacts of even low source level seismic survey sounds on large whales. This is particularly relevant when the species is as at high risk of extinction as is the blue whale,” her study concludes.

Discovering exactly how much stress man-made noise causes a blue whale is difficult, but in future Ms Di Lorio hopes to be able to attach a heart monitor, or even analyze stress hormones collected from the cloud of vapour created when the animal exhales.

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