Fishing can have devastating effects on seabirds. Not only do they get snagged on hooks and tangled in nets, but chronic overfishing can deprive the birds of their prey—the same small fish that boats are catching. Now a study has identified what appears to be a universal threshold for danger: when the biomass of so-called forage fish drops below one-third of its maximum, seabirds of many species start to have fewer chicks.
"This is one of the most important seabird papers to be published in some time," comments conservation biologist Dee Boersma of the University of Washington, Seattle, who was not involved in the research. The findings demonstrate the widespread reliance of seabirds on small forage fish, she says. To protect the birds, the papers' authors call for lower harvest levels of forage fish. "The problem remains that most fisheries are not properly managed and controlled," says co-author Philippe Cury of the Institute of Research for Development in Sète, France. Worldwide, about 25% of forage fish stocks have collapsed, he adds.
A classic example of seabirds that suffered from overfishing is the Atlantic puffin in Norway. When herring stocks in the Norwegian Sea crashed in the 1960s, the puffins had trouble reproducing and the population plummeted by 64%. But how typical was that response, and at what point does trouble begin for seabirds? To look for a general relationship between the abundance of forage fish and birds' breeding success, Cury and an international team of researchers examined data on fish and 14 species of seabirds from seven ecosystems around the world. The records ranged from 15 to 47 years in length.
All bird species showed the same response, the team reports in the 23 December issue of Science: the number of fledglings per breeding pair started to decline when the abundance of forage fish dropped below one-third  of the maximum observed amount. "The biggest surprise was the consistency of the relationship," says co-author Ian Boyd, a mammalogist at the University of St. Andrews in the United Kingdom. "It bowled us all over."
Boyd says that once prey become too scarce, hunting probably becomes inefficient and the birds lack enough food to successfully raise as many young. Although seabirds are adapted for the vicissitudes of life—forage fish numbers have large natural fluctuations—seabirds populations may decline when fishing depresses levels for many years in a row. "It's a corrosive effect," Boyd says.
"Keep one-third for the birds"—that would be a useful guide to managing forage fisheries so that seabird populations remain resilient, the authors say. Fisheries biologist Steven Murawski of the University of South Florida in Tampa says that a properly managed fishery will maintain at least this much anyway in order to ensure the sustainability of the fish themselves. Nevertheless, he likes the motto, as it "gets at the notion that we need to leave a healthy margin of fish in the water. It surely is a great bumper sticker!"