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magazine / jun09
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June 2009 issue |
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Plenty of fish in the sea?
Longliners, warming waters and the waves of change
By Steven Fick and Dan Rubinstein
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| Click to enlarge |
When Japan’s fishing fleet returned to international waters after the Second World War, ship captains
were obliged to keep detailed records of their catch. The longlines used then and now can be up to 100 kilometres
long, with as many as 3,000 hooks apiece, snagging most species of large animals in the ocean and needlessly killing scores of fish
never destined for market. But the records kept by longliners have also provided scientists with valuable long-term data.
The Future of Marine Animal Populations (FMAP) project is a Halifax-based network of scientists within the international
Census of Marine Life (see “The Transparent Oceans
Project”). FMAP’s Boris Worm, a Dalhousie University biology professor, has used
Japanese fisheries data to create a pair of maps documenting a decline in the diversity of tuna and billfish, such as marlins and
swordfish, caught on longlines between the 1960s and the 1990s
(see map). The 15 species of tuna and
billfish included in the data are all large predatory fish, and the decline they have experienced is representative of wider patterns
of change among other predatory species.
“When I see these maps, I have a sinking feeling,” says Worm. “The whole ocean is changing in a way that’s directly related to
human impact. This is worrisome, because diversity is perhaps the central component in the resilience of an ecosystem. When
you erode diversity, you’re really reducing that resilience.”
The maps document the effects of both fishing and climate change. Fishing has led to “a long, slow trend of depletion” in
predator populations, says Worm, while cyclical warming ocean temperatures have led to a redistribution of species. The optimal
water temperature for most tuna and billfish, which prefer the open ocean to coastal areas, is between 20°C and 25°C. Four
decades ago, their diversity generally peaked in subtropical regions. As these waters get warmer, which Intergovernmental
Panel on Climate Change scenarios predict, the fish could potentially migrate into more temperate water at higher latitudes.
“Will they be able to adapt and deal with the rapidly changing environment?” asks Worm. “We don’t know.”
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