Specifically, each boat creates a "halo of influence" across an area measuring about 22 kilometers. That's 13.6 miles—a distance a little longer than a half marathon.
"While we knew that seabirds, including gannets, regularly followed fishing vessels, we were surprised at the distance at which the birds' behavior was affected, expecting it to be a more localised phenomenon," says Thomas Bodey of the University of Exeter.
Mr Bodey and his colleagues tracked the movements of individual gannets representing six colonies around the British Isles using high-resolution GPS data over a couple of months and compared that data to fisheries' GPS data. Models based on that data indicate that gannet behavior can be influenced up to 11 kilometers away from a ship (hence the 22 km halo).
The birds are also rather savvy in assessing vessel type and activity, the researchers report. Gannets were less likely to be attracted to boats that were simply drifting along than to boats with active fishing.
Mr Bodey says the broad influence of fishing boats on bird behavior has led researchers to wonder about the forces that bring birds to other resources, such as large groups of fish, and how information transfer may cascade across the seas to bring birds into an area. Understanding those behavioral phenomena may have implications for fisheries and conservation agencies alike.
"The impact of fisheries' discards on seabirds is complex, with both positive and negative effects depending on the species," Mr Bodey said. "However, being aware of the sphere of influence of vessels may help marine planning, highlighting areas where impacts may be greatest and enabling fisheries to strategically choose the least sensitive areas in which to discard."