The world's oceans have lost 90 per cent of prized tuna, swordfish and marlin since industrialized fishing began, Canadian scientists warned Wednesday.

Fisheries biologists Ransom Myers and Boris Worm of Dalhousie University in Halifax analyzed nearly 50 years of data on predatory fish catches worldwide.

Their findings debunk the notion that oceans are picture perfect blue frontiers teaming with life. "What we've done is sliced the head off of the world's marine ecosystem and we don't know the consequences," said Myers.

Ransom Myers: \
Ransom Myers: "oceans have lost 90% of tuna, swordfish, marlin"

The first sign of trouble began in the 1960s, when areas brimming with king-size fish immortalized in Ernest Hemingway's Old Man and the Sea dwindled.

"Although it is now widely accepted that single populations can be fished to low levels, this is the first analysis to show general, pronounced declines of entire communities across widely varying ecosystems," Myers and Worm report in Thursday's issue of the journal Nature.

The pair found it generally takes less than 15 years for commercial fishing operations to reduce the resource base to less than 10 per cent.

To measure the decline in open oceans, the researchers used data from Japanese longline catches, massive nets with thousands of hooks stretched across the ocean to catch everything in their path.

Myers said after the Second World War, longlines used to catch 10 fish per 100 hooks. Now they're lucky to catch one.

Fisheries scientist Daniel Pauly of the University of British Columbia said the longline study showed how when fishing went bad in one area, vessels simply moved on to scour another.

"For those who were interested in a quick buck, you want to go somewhere else," Pauly said. "That doesn't mean the resource was entirely gone, you could still continue, but this 'bonanza,' that was over."

Myers acknowledges some fisheries managers may find it hard to accept, but the tendency to use only the most recent data increases the problem.

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"You need to reduce fishing efforts by any means so these fish stocks and fish community can recover to anything that resembles a healthy marine ecosytem," said Worm.

The trends echo a 1994 estimate by the UN Food and Agriculture Organization that almost 70 per cent of marines stocks were overfished or fully exploited. A UN-sponsored summit in South Africa called for global fisheries to be restored by 2015.

Myers and Worm hope their data will serve as a guide.