Tuna, swordfish and other species favored by fishing fleets gather in "hotspots" across the world's oceans -- but these are in serious decline, according to a survey published Thursday.
WASHINGTON Tuna, swordfish and other species favored by fishing fleets gather in "hotspots" across the world's oceans -- but these are in serious decline, according to a survey published Thursday.
The hotspots -- off the east coasts of the United States, Australia, and Sri Lanka; south of Hawaii; and in the southeastern Pacific -- seem to be linked to a type of zooplankton, the researchers found to their surprise.
Writing in the journal Science, they said overfishing had clearly caused the biggest declines in both numbers and diversity, although climate change also apparently played a role.
Boris Worm of Dalhousie University in Halifax, Nova Scotia, and colleagues studied data from Japanese fishing fleets.
"It is like solving a giant puzzle and seeing the night sky in constellations for the first time -- even as the stars are blinking out. It's beautiful and tragic at the same time," Worm said in a statement.
The researchers studied data going back 50 years from Japanese longline fisheries, as well as data from the United States and Australia.
Longlines are the most widely used fishing gear in the open ocean, with baited lines up to 60 miles long that target tuna or billfish but catch many other species too.
Not only are the numbers of fish shrinking, but the variety of species is as well, they found.
"Diversity declined between 10 percent and 50 percent in all oceans, a trend that coincided with increased fishing pressure, superimposed on strong El Nino Southern Oscillation- driven variability across the Pacific," they wrote.
"Everywhere you go, in every ocean basin, our "hotspots" today are only relics of what was once there," Worm said in a statement.
"It really hurts to see this."
The loss of diversity means that where 10 different species might be found in an area, now just five can be caught. The more diverse an ecosystem, in general, the more it thrives, researchers have found.
"It's not yet extinction -- it's local fishing-out of species," added Ransom Myers, also of Dalhousie University. "Where you once had a range of a species in dense numbers, now you might catch one or two of a certain species."
The fish gather in the same spots as a type of zooplankton called foraminifera -- single-celled animals that float in the seas and are eaten by a range of animals.
"Our paper suggests there is a solution -- while some hotspots have already disappeared, there are still some very special places where species concentrate," Worm said.
"We have the chance and the political measures to protect some of these areas. To me, it's the most important thing in the world right now -- to keep as many pieces of the puzzle as we can before we destroy it."