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,” said conservation biologist Dee Boersma of the University of Washington, who was not involved in the research. The findings demonstrate the widespread reliance of seabirds on small forage fish, she said.
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,” said co-author Philippe Cury of the Institute of Research for Development in Ste, France. Worldwide, about 25 percent of forage fish stocks have collapsed, he said.
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 percent.
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 years to 47 years.
All bird species showed the same response, the team reported in the December 23 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,” said co-author Ian Boyd, a mammalogist at the University of St. Andrews in the United Kingdom. “It bowled us all over.”
Boyd said that once prey become too scarce, hunting probably becomes inefficient and the birds lack enough food to successfully raise as many young.
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