The fast-growing activity of whale-watching can interfere with communications between cetaceans (whales), and this could have an impact on their survival, biologists fear.

If there are too many vessels there is too much background noise from engines — this requires the animals to "sing" longer in order to make themselves heard by other members of their group, the experts say in Thursday's issue of Nature, the British weekly science journal.

They compared recordings of calls made by killer whales living in the inshore waters of Washington state that had been made at three periods, from 1977-81, from 1989-92 and 2001-03.

The first two periods of recordings showed no significant difference in the primary calls — a tribal sound specific to each orca pod, or group, that comprises about half of the noise that the mammals make.

But in the third period, the call duration was about 15 percent longer, reflecting a fivefold increase over the previous decade in the number of whale-watching vessels.

The killer whales there, known as the southern resident community, comprise three pods whose population has been in decline since 1996.

Whale-watching off Washington is now a big business, the authors note. During daylight hours, a typical fleet of 72 commercial vessels and 22 private boats follow a pod.

"These whales adjust their behaviour to compensate for anthropogenic (man-made) noise once it reaches a threshold level," the study says. "Background noise can interfere with the detection and discrimination of crucial signals among members of a species."

The killer whale (Orcinus orca), the largest of the dolphin species, is a highly social animal, living in groups headed by females. The speculation is that their calls are probably used for mating and feeding as well as threat warning.

AFP