Where there are fishery boats docking after a day's catch, there are usually seabirds hovering in hopes of lifting scraps. Discards from fisheries supplement the diets of these flying scavengers, but it is less clear how commercial fishing affects the foraging behavior of birds when they are out at sea and out of sight.

Using satellite transmitters to track bird movement, a study published in the January 28 issue of Current Biology found that the foraging range of two types of seabirds off the eastern coast of Spain—Balearic shearwaters and Cory's shearwaters—shrinks on days when fishery boats are operating. Instead of covering hundreds of kilometers, these birds stay in smaller areas that coincide with the routes of commercial fisheries. The authors suggest that fishery activity, which has previously been shown to be related to higher shearwater reproductive rates, could be reducing birds' foraging time and giving them more time to breed.

"The birds are not spread all over the foraging area," says Frederic Bartumeus, who is a theoretical ecologist at the University of Barcelona's Climate Science Catalan Institute and lead author of the study. In areas where there are fisheries, "it's like they are trapped by an attractive force," he says—the attractive force being demersal, deepwater fish that have been swept into nets and thrown overboard because they are too small to sell. Without these handouts, seabirds dive for smaller pelagic (open-sea) fish that live closer to the surface.

Bartumeus and his colleagues studied the effect of fishery boats on a total of 10 Balearic shearwater and 18 Cory's shearwater birds between 1999 and 2005. To track the birds, the researchers placed a lightweight satellite transmitter on the birds' backs while they were in breeding colonies on the Balearic Islands, which are in the Mediterranean between Spain and the west coast of northern Italy. For about four days after gearing up the birds with the transmitters, the team recorded their positions within a two- to three-square-kilometer area, every 3.5 hours on average. This was accomplished via radio signals the transmitters sent to the Argos data collection system comprising about 2,000 thousand platforms on Earth that relay information to a tracking instrument on each of the National Oceanic and Atmospheric Administration's Polar Operational Environmental satellites. Researchers have been using this satellite system, run by the French-based company CLS, to track animals since the 1980s.

To get a sense of the effect of fisheries on foraging, the researchers planned their tracking to cover both weekdays, when the fisheries run 12 hours a day, along with weekends, holidays and government-enforced moratorium days, when the fisheries do not operate. The researchers found that the average distance that seabirds covered between recordings decreased from about 25 to 22 kilometers when the fishing fleet was at sea. In addition, the team observed that seabirds would fly as far as 300 kilometers when fisheries were not operating, compared with a maximum distance of 165 kilometers on fishing days. "These large traveling distances are rare but are much more probable when fisheries are not present," Bartumeus says. He points out that the boats travel up to 100 kilometers between fishing areas, and seabirds could be following them on these trips.

Although the researchers did not track the fishing boats in their study, Bartumeus says that the team compared the foraging to a map of the average boat positions in the region. "The [fishing] areas are more or less the same and also the timing is more or less the same," he notes. In future studies, he plans to track the fisheries as well as seabirds using the global positioning system, which will give the researchers location updates every 15 minutes or so.

"The novelty of this paper is that it's not just what the birds eat, but it's the way in which they use the environment in a spatial sense," says Stephen Votier, an ecologist at the University of Plymouth School of Marine Science and Engineering in England. Votier, who was not involved in the current work, found that the diet of seabirds in fishery areas in the northeastern Atlantic consists of the deepwater fish that fisheries catch.

Dieting on fisheries' scraps has so far been beneficial for the seabirds off the coast of the Iberian Peninsula. After Spain scheduled fishery moratoria in the 1990s shearwater reproduction rates plummeted, according to work by Daniel Oro, a population ecologist at the Mediterranean Institute for Advanced Studies in Majorca, Spain, and one of the authors on the current study.

One explanation for the improved reproduction is that the birds spend more time and calories hunting when they have to do it the natural way, Bartumeus says.