GLOBAL warming is likely to shrink the size of fish by as much as a quarter in coming decades, according to a groundbreaking study of the world's oceans.
The reduction in individual fish size is predicted to coincide with a dwindling of international fish stocks as the world's growing human population puts greater pressure on fisheries.
Researchers modelled the effect of rising ocean temperatures on the growth and distribution of more than 600 fish species around the world and found they were expected to shrink by 14-24 per cent by 2050.
The biggest changes were predicted for tropical regions, particularly the Indian Ocean.
Fish sizes are expected to shrink for two reasons. Warmer oceans hold less oxygen, slowing growth rates. And fish from the tropics, which are generally smaller-bodied, are likely to spread away from the equator to what are now temperate and polar regions, reducing average fish size.
"We were surprised as we did not think the effects would be so strong and so widespread," said lead researcher Professor William Cheung, from the University of British Columbia.
The study, published by the journal Nature Climate Change, relied on computer models to project the effect of warming on fish physiology, distribution, migration and population.
But Professor Cheung said the results were already being matched by real-world observations, pointing to a 2011 study that showed the size of haddock in the North Sea correlated with rising temperatures.
Nathan Bindoff, a research program leader on climate change and ocean processes at the University of Tasmania, said there was documented evidence of several fish species moving south along the Australian east coast from New South Wales to Tasmania in recent years.
Professor Bindoff said the new study was "relevant and important". "This is the sort of information we need to inform policy about tackling climate change," he said.
He said the researchers had acknowledged they had not taken into account the level to which fish might adapt and evolve to avoid shrinking.
Professor Cheung's team projected temperature rises using data from the Intergovernmental Panel on Climate Change, based on a high-emissions scenario that matches the current trajectory of greenhouse gas emissions.