Gill net fishing vessels in Alaska. Eighty-one types of birds have been reported killed by the nets.
Fishing vessels that deploy gill nets snare and drown at least 400,000 seabirds every year, and the actual figure could be considerably higher, according to research published in the June edition of an academic journal devoted to conservation.
The study, in the journal Biological Conservation, uncovered reports of 81 species of birds killed by gill nets, including penguins, ducks and some critically endangered ones like the waved albatross. One of its three authors, Cleo Small, called the estimated toll a bare minimum, saying that data on the deaths from many areas were either nonexistent or too old to be useful.
“It’s quite startling,” said Dr. Small, who heads the global seabird program at the British conservation group BirdLife International, which sponsored the study. “In Japan, for example, some populations have already been extirpated on islands. Some colonies have disappeared where there is gill net fishing, and in other areas they have dramatically declined.”
Other academic studies have estimated that a minimum of 160,000 additional birds are killed each year by longline fishing, she said. Longlines, which dangle baited hooks from lines that can stretch for miles, are widely used in commercial fishing.
Gill nets, mesh nets that are much smaller, are used both by commercial and small local fishermen. Anchored in the water by weights and buoys, they are designed to snare fish by their gills, but they can catch any creature that is too large to swim through the mesh. Conservationists say that includes large numbers of sea turtles and mammals like porpoises, seals and even whales.
Nearly half of the seabirds killed by gill nets were in a section of ocean stretching from the northern tip of Africa to north of Greenland and Scandinavia. The bird catch was said to be especially high around Iceland, a prolific seabird habitat, where researchers estimated that 100,000 birds die every year.
But at least 140,000 die annually in the northwest Pacific, an area extending from the Aleutian Islands west and south to Russia and China, the study said.
Losses in United States coastal fisheries appeared to be smaller. In a 2011 survey of 28 coastal fisheries using data from 2005, the National Oceanic and Atmospheric Administration counted fewer than 7,800 birds caught in all types of nets, although some academic studies suggest that gill nets may kill tens of thousands of one species, the marbled murrelet.
Seabird populations are falling faster than other types of birds, and gill nets have long been identified as a main reason, the researchers said. Huge drifting gill nets were estimated to be killing 500,000 birds annually before a United Nations moratorium on large driftnets took effect in 1992.
Most of the gill net toll involves seabirds that dive into the ocean in search of fish. Many birds were once able to see the nets from above and avoid them, but modern nets made of monofilament are all but invisible.
Reducing seabird deaths is hard, as there are few proven ways to deter them. California has sharply cut the toll by banning gill nets in some shallow waters. Other efforts, like setting gill nets at night and placing lights and sonar-style “pingers” on them, have shown some success.
But many of the options are either too expensive or unworkable for local fishermen, who are major users of gill nets, said George Wallace, a vice president at the American Bird Conservancy. And there are so many small fishing enterprises that merely reaching and educating the proprietors is a monumental task, he added.
Most fishermen, nevertheless, want to reduce bycatches if it is economically possible, Mr. Wallace said. “Fishermen want to catch fish,” he said. “That’s what they can sell. They don’t want to tie up their lines with seabirds and sea turtles.”