Everyone knows that seabirds cluster around fishing boats, hoping for a feast from discards thrown back into the ocean. But only now have researchers investigated the size of fishing vessels’ ecological footprint – and it turns out to be much larger than marine biologists had expected. A boat that is actively fishing can influence birds as far as 11km away.
Scientists at Exeter University and University College Cork monitored 74 gannets from six colonies around the Irish coast; each bird was fitted with its own miniature GPS tracker. The birds’ movements over two months were correlated with those of fishing boats, which also carry GPS equipment.
“While we knew that seabirds, including gannets, regularly followed fishing vessels, we were surprised at the distance at which the birds’ behaviour was affected, expecting it to be a more localised phenomenon,” says Thomas Bodey of Exeter. “Our work suggests each fishing vessel has a substantial footprint, with the behaviour of seabirds affected within a 22km diameter circle surrounding it.”
Studying the interaction between boats and birds, the researchers found that individual gannets adjusted their foraging, depending on whether the vessel was actively fishing. Boats that were just drifting or steaming to a new location attracted many fewer birds. The data, reported in the journal Current Biology, also show that gannets can distinguish between trawlers and other types of fishing boat.
“The fact that birds responded differently to boats depending on whether they were fishing or not, and the type of gear they were carrying, indicates just how finely attuned these animals are to the opportunities humans can provide,” says co-author Mark Jessopp of Cork.
Gannets fish almost entirely during the day, using their fine sense of sight. The study does not indicate how they spot the attractive feeding opportunities presented by human fishing but the distant view of other birds, already wheeling around the boat, must play a role. At shorter range the birds may recognise visual differences between various types of fishing activity and boats.
Although gannets and gulls are attracted to fishing boats because discards offer an easier meal than diving for live fish, it is also possible that the birds have learnt that human activity indicates the presence of a rich fish stock.
The researchers hope their findings and follow-up studies will help define the spatial influence of fisheries – key knowledge for marine policy makers. It will be interesting to see, for example, how the impending EU fish discards ban, which will be phased in from next year, affects seabirds.