Swordfish, tuna, and other predatory fish species have plunged to 10% of their abundance before industrialized fishing, according to a new global analysis. It confirms smaller studies showing that overfishing has decimated certain fish species and gives a baseline estimate of earlier population numbers that should aid in conservation.
Many studies have documented the decline of coastal marine species, from sea turtles to rockfish, but pinning down large-scale changes of fish populations in the open ocean has proved more elusive. And those oceanic studies have usually focused on individual species fished beyond recovery, such as Canada's Atlantic cod. Partly because of the difficulty of estimating population size for far-traveling fish, few studies have looked for global changes. Many have relied on estimates of unexploited abundance that are too low, because they were calculated after fishing had already begun to impact stocks.
To get at global trends, fisheries biologists Ransom Myers and Boris Worm at Dalhousie University in Nova Scotia, Canada, analyzed a range of existing data, they report in the 15 May issue of Nature. They got hold of records from Japanese oceanic fishing expeditions that use kilometer-long lines with hundreds of baited hooks. These trips have covered all open ocean except the polar seas. For fish that live in the shallower waters over the continental shelf, the researchers used data from research cruises. They found that while the vessels caught six to 12 large fish per 100 hooks whenever longliners began fishing, the catch dropped to .5 to two fish per 100 hooks in the first 10 years of fishing in a given region. On average, populations have plummeted to a tenth of their former abundance from prefishing times.
"I think what this paper gives is a magic number," says Jeremy Jackson, a marine ecologist at the Scripps Institution of Oceanography. By giving a solid estimate of worldwide declines, Jackson says, the study makes a case for aggressive, international conservation of remaining stocks.