The researchers estimated that an ocean temperature increase of 3 C could cause a 10 per cent decline in the population of phytoplankton- and plant-eaters, and that would reduce seafood supplies. (Canadian Press)

The oceans will likely produce less fish and shellfish as they get warmer, despite the expected increase in plant growth at higher temperatures, Canadian researchers predict.

"The way we think about the impact of warming is that we think, 'Well it's going to be warming, things will grow faster, so there should be more of them,' " said Mary O'Connor, a University of British Columbia zoologist who led the research.

To her surprise, computer simulations and laboratory experiments at different temperatures involving microscopic ocean plants called phytoplankton and phytoplankton-eating animals called zooplankton showed their populations actually decreased with temperature.

"What we found… is the phytoplankton are growing faster, but because the zooplankton are eating them more, [their populations] might not change or they might decline with warming, and that was a surprise."

The findings have big implications because half the carbon that is taken out of the atmosphere by living things undergoes that conversion in the ocean, ultimately working its way up the food chain to produce fish, O'Connor added.

The researchers estimated that an ocean temperature increase of 3 C could cause a 10 per cent decline in the population of phytoplankton- and plant-eaters and the eventual decrease in seafood supplies, especially in the world's warmer oceans.

Zoologist Mary O'Connor and her colleagues found that the growth of photosynthesizing microscopic phytoplankton, above, increased with temperature. But the appetites of phytoplankton-eating zooplankton increased even more. (Karl Bruun/Nostoca Algae Laboratory/Courtesy of Nikon Small World/Nature)

That's because cold-blooded plant-eating sea creatures consume more food and energy as temperatures increase, and the productivity of phytoplankton doesn't climb enough with temperature to keep up with the animals' growing appetites, the researchers report in an article that will be published in the peer-reviewed scientific journal The American Naturalist this week.

O'Connor acknowledged that with warm-blooded animals, the relationship between temperature and food consumption is more complex. But she added that on a global scale, 99 per cent of all kinds of animals, representing far more than 99 per cent of the world's biomass, are cold-blooded.

"And most of them live in the ocean, which means their body temperatures are identical to water," she said.

The changes would be expected to affect the way carbon is stored in the ocean, but the researchers aren't yet sure exactly what the effects would be.

Similar effects predicted on land

While the researchers' experiments involved marine creatures, O'Connor expects warming could cause similar effects on plant-eating animal populations on land.

"There are patterns in terrestrials systems that are consistent with our model," she said.

However, she noted that the predictions are very general, pertaining to global populations of all species combined.

"Individual species might behave differently," she said. But overall, she added, "we expect them all to decline."

O'Connor collaborated on the research with University of Toronto researcher Benjamin Gilbert and Christopher Brown at the University of Queensland in Australia.

A study published in Nature last year found that that the world is losing an average of one per cent of its phytoplankton each year, and the northern hemisphere has lost roughly 40 per cent since 1950. That study suggested that a warming climate affects the way nutrients mix through ocean waters, preventing them from reaching the phytoplankton near the ocean surface.