Global warming is likely to shrink the size of fish by as much as one-quarter in the coming decades, according to a groundbreaking study of the world’s oceans.
The reduction in individual fish size will be matched by dwindling overall fish stocks, scientists warned, at a time when the growing human population is putting ever greater pressure on fisheries.
“We were surprised, as we did not think the effects would be so strong and so widespread,” said William Cheung, from the University of British Columbia in Canada, who led the research.
His team examined the effect of rising ocean temperatures on the growth and distribution of more than 600 fish species around the world and found that they are likely to shrink in size by between 14 percent to 24 percent by 2050, most notably in tropical regions.
“It could be worse than that,” said Callum Roberts, at the University of York in northern England, who said the research as the most comprehensive yet.
Roberts, who was not one of the study’s authors, said additional impacts of climate change such as the acidification of the oceans and reduction of nutrients in surface waters could decrease fish stocks even further, as would continued overfishing.
“We will see dramatic changes in the oceans likely to reduce productivity,” Roberts said. “One billion people rely on fish for primary animal protein and that is going to increase, especially in developing countries. We have to get to grips with our dependence on fossil fuels, otherwise we are stuffed.”
The fish shrinkage predicted by the new research results from two effects: the difficulty of growing in warmer, oxygen-poor waters and migration.
“The metabolic rate of fish in the warm oceans increases and therefore they need more oxygen,” said Cheung, whose work is published in Nature Climate Change.
However, warm water holds less oxygen and so their growth is limited.
In addition, there are more small-bodied fish in the tropics and these will migrate to temperate or polar regions as the ocean warms, reducing the average fish size. The two effects are similar in impact, said Cheung, who used computer models to project the effect of warming on fish physiology, distribution, migration and population.
“We are already seeing the effects,” he added, pointing to a study last year that showed the reduction in the size of haddock in the North Sea correlated closely with increasing temperature.
Cheung’s researchers projected temperature increases using data from the Intergovernmental Panel on Climate Change, based on a high-emissions scenario that matches the trajectory of greenhouse gases.
Roberts said the research showed the impact of climate change on the oceans seemed likely to be greater than previously predicted.
The reduction in the size of fish owing to overfishing was well known, Roberts said, as for the last century fish had increasingly “lived fast and died young,” preventing them from reaching full size. However, if overfishing continued, this effect would add to the shrinkage caused by warming, he said.
Furthermore, rising carbon dioxide in the atmosphere leads to more of the gas dissolving in the ocean, increasing its acidity.
“That makes life much tougher for animals that make a chalky skeleton,” Roberts said. “We need to worry about these tiny animals — such as coccoliths and foraminifera — which are an important part of primary production: the base of the food chain.”
Lastly, Roberts said the heating of the oceans meant that the warmer layer at the surface mixed less with the colder layer below. As the colder layer contains most of the nutrients, that means less food for fish.
“We are already seeing some evidence of this, as oceanic ‘deserts’ are getting larger,” he said. “All this is yet one more reason to do something to cut greenhouse gases urgently.”