Climate change and rise in sea temperature may see fish shrink

FISH could shrink by almost a quarter of their current size by the middle of this century because of global warming, scientists have warned.

Experts using computer simulations to predict the effects of climate change on hundreds of species of ocean fish, found the animals’ maximum body weight could drop by up to 24 per cent by 2050.

The impact was linked to rising sea temperatures, which are causing oxygen levels in the world’s oceans to fall, with even small changes causing dramatic reductions in fish size.

Researchers from the University of British Columbia, in Vancouver, Canada, admitted that the extent of the impact was far greater than expected.

It has serious implications for managing fish stocks internationally, with quotas limiting catches typically set by weight rather than number.

Lead researcher Dr William Cheung said: “We were surprised to see such a large decrease in fish size.

“Marine fish are generally known to respond to climate change through changing distribution and seasonality.

But the unexpectedly big effect that climate change could have on body size suggests that we may be missing a big piece of the puzzle of understanding climate change effects in the ocean.”

Outlining the significance of the study, which was published in the journal Nature Climate Change yesterday, they said: “Although the projected rate of change in environmental temperature and oxygen content seems to be small, the resulting changes in maximum body size are unexpectedly large.

“This study shows that both the tropics and temperate regions will... be impacted by reduction in body size.

“Other human impacts, such as over-fishing and pollution, are likely to further exacerbate such impacts. Consequently, these changes are expected to have large implications for trophic (food chain) interactions, ecosystem functions, fisheries and global protein supply.”

Fish size is largely dependent on seawater oxygen levels.

If predatory fish are stunted due to lack of oxygen, this will also have a knock-on effect on the rest of the ecosystem, researchers added.

Two computer models were used in the study to forecast the impact of climate change on more than 600 different species of marine fish around the world.

They predicted that temperatures at the sea floor in the Pacific, Atlantic, Indian, Southern and Arctic oceans would increase by between 0.029C and 0.037C per decade.

As a result, the amount of oxygen in the water was expected to fall by between 0.8 and 0.1 millimoles per cubic metre per decade.

Scientists concluded that the maximum body weight of fish could decline by anything from 14 per cent to 24 per cent between the years 2000 and 2050.

The effect was found to be strongest in the tropics and intermediate latitudes, where fish dropped in size by more than 20 per cent.

Changes in distribution, abundance and the physiology of fish also contributed to the trend.

Yesterday’s warning follows a study by Scottish experts published last year, which blamed global warming for haddock getting significantly smaller over the past few decades.

Experts at Aberdeen University reported in 2011 that the average haddock was about a third of the size of the fish in the 1970s.

Scientists said that a one degree rise in the temperature of the North Sea had encouraged juvenile fish to mature earlier, giving them less time to grow.