By Paul Rincon
BBC News Online science staff
Killer whales living off the west coast of the US are extending the length of their calls to each other to be heard above the din of heavy boat traffic.
When noise reaches a threshold, orcas make longer calls
The findings come from an analysis of killer whale, or orca, calls by British and US researchers which has been published in the journal Nature.
The orcas make longer calls when boats are present in an apparent attempt to be heard above the engine noise.
But the orcas only take this action when noise reaches a critical level.
The killer whales observed in the study came from a population that lives close to the shore in waters off Washington state.
There has been a sharp increase in the number of boats in the area over the past decade. A major commercial shipping lane cuts through the waters, while tourism and whale-watching have become increasingly popular.
Numbers of killer whales have been dropping here since 1996.
Researchers from the University of Durham, UK, and the Whale Museum in Friday Harbor, US, compared recordings of calls made by orcas over the periods 1977-81, 1989-92 and 2001-03 in waters made in the absence and presence of boats.
Although no significant difference was found in the length of calls over the 1977-81 and 1989-92 period, the team found a 10-15% increase in the duration of calls made by the orcas during the 2001-03 period.
This would appear to suggest that the whales are altering the length of their calls to be heard above the din of background noise from boats.
"The whale-watching vessels quite often act as a beacon attracting the tourist boats," co-author Andrew Foote of the University of Durham told BBC News Online.
"This increases the amount of traffic around the whales even more. While the whale-watching vessels behave responsibly - try not to start their engines up when they're on top of the whales and so on - the tourists aren't always aware of quite how to behave with the whales."
Orcas are declining in the coastal waters of Washington state (Image: Fred Felleman)
If the growth in boat traffic continues apace, it could start interfering with the orcas' ability to find food, says Mr Foote. The animals partly make calls to keep in touch, but also to co-ordinate foraging.
However, the researchers suggest that because the number of boats increased about fivefold between 1990 and 2000, the orcas only start making longer calls once boat noise reaches a threshold.
Numbers of boats following the killer whales, including registered whale-watching boats and private tourist boats, increased roughly fivefold from 1990 to 2000.