The great predators of the seas – tuna, swordfish, marlin and others – could be on the way out. Canadian researchers who surveyed the catches from ocean fishery “hotspots” warn that not only are numbers in decline, but also the variety of species in any region.
The research, published in Science today provides fresh ammunition for conservationists who want to see the creation of large, internationally protected marine parks where fish populations can breed and recover.
Boris Worm and Ransom Myers of Dalhousie University, who showed in 2003 that shark populations in the north Atlantic had fallen by 90% in 15 years, combed fisheries data for the past 50 years to discover that catches were becoming less diverse.
Where fishermen might once have caught 10 different species, they now haul in only five. “It’s not yet extinction – it’s local fishing out of species,” Dr Myers said. “Where you once had a range of species in dense numbers, now you might catch one or two of a certain species.”
Oceans cover 70% of the planet: they are also exploration’s last frontier. The research highlights a pattern of hotspots in the open ocean where tuna, swordfish and other predators congregate, presumably to hunt smaller fish attracted by local surges of zooplankton in the high seas.
For the first time, marine scientists have begun to understand why sea surface temperatures and other conditions make some fishing grounds richer than others.
But for some species of commercial fish, it may already be too late. Cod catches are in sharp decline, the Atlantic halibut has all but disappeared, and bluefin tuna catches are now controlled.
“This is the great joy of science,” Dr Worm said. “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. Beautiful and tragic at the same time.”
The two started with catches logged by Japanese long liners. Pelagic long liners pay out baited hooks on lines up to 60 miles long. Though the fishermen may be after bluefin tuna, or swordfish, they also catch other kinds of tuna and billfish as well as sharks, sea turtles and even albatrosses.
They matched this evidence against data collected independently by Australian and US researchers, who counted more than 140 species in the same regions in the past 15 years. They concluded that the diversity of big predators had fallen by up to 50% in the last five decades.
“In every ocean basin, our hotspots today are only relics of what was once there,” Dr Worm said.
“While some hotspots have already disappeared, there are still some very special places where species concentrate. We have the chance and the political measures to protect some of these areas.”
He added: “To me, it’s the most important thing in the world: to keep as many pieces of the puzzle as we can before we destroy it.”
Source: The Guardian (UK)
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