A Moveable Feast

To a hungry seabird, a commercial fishing boat is like a floating, all-you-can-eat buffet. Large quantities of unwanted catch often get tossed back in the water, and enterprising seabirds happily feast on these discards.

But new research out of the University of Exeter suggests that the boats impact seabird feeding behavior over a much greater distance than previously thought. The study is the first to estimate the ecological influence of individual fishing vessels.

The scientists tracked 74 Northern Gannets from six different colonies around Ireland over the course of the breeding season. The choice to study Northern Gannets in particular was a logical one: GPS devices weigh about 40 grams, making them too bulky for most of the UK's seabirds. But it's "like having some change in your pocket for the gannet," which tips the scales at up to eight pounds, said Thomas Bodey, an ecologist at the University of Exeter and the study's lead author. All large fishing boats in the EU are required to use a vessel monitoring system, which allows fisheries authorities to monitor their location and speed. The scientists simply matched the gannet and boat data together.

The results, published in Current Biology, show that fishing vessels have an enormous ecological footprint of 22 kilometers in diameter; gannets are influenced by fishing boats up to 11 kilometers away.

The gannets are discerning—rather than gravitate towards all vessels, they differentiate between types of fishing boats and their activities and adjust their response accordingly. For example, gannets forage more often near trawlers that are fishing than those that are drifting, but the opposite is true of non-trawlers—reflecting the times when discards are most readily available.

Bodey and his team hope to learn how the gannets are homing in on the boats. Because Northern Gannets plunge headfirst into the sea when they hunt for fish, they have no external nostrils, making it unlikely that they locate the vessels by smell. "We don't know what it is that they're picking up on from that distance away," Bodey explained.

Although the study only focused on one species, the knowledge that individual vessels can affect wildlife behavior at a large scale is valuable information for marine planning and fisheries management—especially in the face of an EU proposal to ban wasteful discards.

It's a fine kettle of fish indeed.