Bird dilemma: more seabirds killed when boats discard fewer fish.

Records stretching back 30 years indicate a tricky problem for marine conservation: When fishing vessels discard less fish-waste material, scavenging seabirds called great skuas attack and kill more neighboring seabirds.

The link shows up in an unusually long dietary study of great skuas nesting on the Scottish island of Foula in the western North Sea, says Stephen C. Votier of the University of Glasgow. Birds eating less of the waste fish pose a "potentially serious threat to some seabird communities," Votier and his colleagues report in the Feb. 19 Nature. Vulnerable species include black-legged kittiwakes, common guillemots, and Atlantic puffins.

The discovery that reducing discards from boats could ripple through the ecosystem and threaten seabirds strikes ecologist Julia Parrish of the University of Washington in Seattle as one of the more important lessons of the analysis. Ecosystems are complex, and "we'd better start looking at the big picture," she says.

Fishing fleets worldwide throw away more than 25 to 30 million tons a year of undersize fish and leftovers from on-board processing, says Votier. Great skuas (Stercorarius skua) are excellent at scrambling for the windfall, and the researchers suggest that abundant discards contributed to a jump in European skua populations during the past century. The other major boost for skua populations came from decreased human hunting of the birds.

Skuas are "halfway between a gull and bird of prey," says Votier. They feed on whatever's available and have always preyed on other birds. The large skuas take chicks of other species when available but can also kill adults of smaller species. Votier says he's seen a skua catch a bird and hold it underwater to drown it.

To determine the skuas' eating patterns, researchers relied on a project started by coauthor Robert Furness, also of the University of Glasgow. Since the mid-1980s, his team has been studying a great skua breeding colony, analyzing bones and feathers in pellets that skuas cough up after feeding.

Votier and his colleagues looked at the fluctuations in fish and bird contributions to the skuas' diet and compared them with variations in reported discards of whiting and haddock waste from fishing vessels in the North Sea. Those waste statistics have fluctuated with such factors as the size of fish populations, designs in fishing gear, and fishing intensity. The changes in discards matched changes in the skuas' diet, the researchers report.

The analysis also revealed a promising ecological link. When the population of sandeels, which are little schooling fish, is relatively high and boat waste is low, the hungry skuas don't prey as heavily on other birds. Conservation of sandeels, which some fishing fleets target, should become a greater priority, Votier and his colleagues argue. Otherwise, some of the bird species that the skuas attack might drop in numbers, the researchers warn.

For marine conservationists, says Parrish, it's a daunting challenge. Increasing discards doesn't make sense, sufficiently curbing sandeel fishing could be hard to justify, and killing skuas is drastic. "We will often have to make difficult choices," Parrish says.