Whalewatching allows humans a glimpse of magnificent creatures in their natural habitat. But as the pastime becomes more popular, a new study suggests, noise from the boat traffic may be drowning out the animals ability to hear one anothers calls.

A. Rus Hoelzel of the University of Durham in England and his colleagues examined the vocalizations made by killer whales from the coastal waters of Washington State, analyzing calls uttered in the presence and absence of boat engine noise from recordings made during three time periods over the past 25 years. They observed no difference in whale calls recorded about 25 years ago and 15 years ago. But the most recent set of recordings, made between 2001 and 2003, revealed that whales changed the characteristics of their calls when boats were around, making them 15 percent longer compared to the calls they made without such background noise. Boat traffic in the region increased fivefold during the 1990s, the team notes, with groups of killer whales now followed by as many as 22 tourist vessels a day. The results of the study, published today in the journal Nature, suggest that there exists a critical level of noise beyond which the animals performance becomes impaired, forcing them to adapt their behavior in order to be heard above the din.

Killer whales are highly social animals and are thought to call to one another while jointly foraging for food. Although there is no direct evidence that the increased level of engine noise interferes with communication, the researchers note that the killer whale population in the area they study has been declining since 1996. "Whale watching is a really constructive thing," Hoelzel says. "It educates the public and its a good conservation tool. But we need to do the research to see the kind of impact were having and to regulate it effectively."