A HEARTY fillet of fish, already a rare treat because of over-trawled oceans, will become even more infrequent in the future when global warming starts to reduce fish size, scientists say.

Researchers looked at computer models to see how warmer, and thus less oxygenated, seas affected more than 600 species of fish.

Compared to 2000, the maximum attainable body weight of these fish declined by between 14 and 24 per cent by 2050.

Fish inhabiting the Indian Ocean were the most affected, being reduced by 24 per cent, followed by counterparts in the Atlantic (20 per cent) and then the Pacific (14 per cent), with tropical waters worst hit.

"It's a constant challenge for fish to get enough oxygen from water to grow, and the situation gets worse as fish get bigger," Daniel Pauly of the University of British Columbia in western Canada, who raised the warming-and-growth link 30 years ago, said on Sunday.

"A warmer and less oxygenated ocean, as predicted under climate change, would make it more difficult for bigger fish to get enough oxygen, which means they will stop growing."

The investigation appears in the journal, Nature Climate Change.

The model used the so-called A2 scenario, in which global atmospheric - not sea - temperatures rise by 3.4C by 2100 compared to 2000.

Until recently, this would have been considered a pessimistic scenario, but many climatologists today say it is realistic in the light of a relentless rise in fossil-fuel emissions.

Under the A2 scenario, the sea bottom temperatures of the Pacific, Atlantic, Indian, Southern and Arctic oceans would rise at average rates of between 0.012C and 0.037C per decade up to 2050.

As those oceans warm, there would be slight but progressive declines in oxygen levels, a measurement expressed in a unit called millimoles.

The average fall, per decade, would range from 0.1 millimoles per cubic metre in the Arctic to 1.1 millimoles per cubic metre in the Atlantic.

"Although the projected rate of change in environmental temperature and oxygen content appears to be small, the resulting changes in maximum body size are unexpectedly large," said the paper.