Fish populations in the world's oceans are depleting at an alarming rate, new research has found, with worrying consequences for those higher up the food chain – which includes humans. If we don't act fast, the situation is going to get worse.
Data collected from 1930 to 2010 has shown that sustainable fish stock declined 4.1 percent on average over that time period. In some regions, including the East China Sea and the North Sea, the drop was as high as 15-35 percent.
Both climate change and overfishing are to blame, the team of researchers says. They did also find that a small number of fish populations actually increased - because previously colder waters became more habitable for them.
"Fish populations can only tolerate so much warming, though," says one of the team, Olaf Jensen from Rutgers University in New Jersey.
"Many of the species that have benefited from warming so far are likely to start declining as temperatures continue to rise."
The researchers looked at how ocean warming affected 235 populations of fish worldwide, covering 124 species across 38 ecological regions. As well as fish, they also included some crustaceans and molluscs.
Warming waters are usually bad news for fish: not only does warmer water contain less oxygen, it also impairs bodily functions, which in many fish occur at the same temperature as the water. If you've ever felt the oppression of a hot summer day, you've got an idea of what we're talking about.
That 4.1 percent figure the scientists arrived at refers specifically to the maximum sustainable yield – the amount of fish we can catch without depleting population numbers in the long term.
"That 4 percent decline sounds small, but it's 1.4 million metric tons of fish from 1930 to 2010," lead researcher Chris Free, from the University of California, Santa Barbara, told Kendra Pierre-Louis at The New York Times.
With fish stocks supplying a significant chunk of animal protein in the world's diet – especially in coastal, developing countries – the trend is a worrying one. Add to that the 56 million people worldwide who work in the fisheries industry or rely on it, and the situation looks even bleaker.
There is some good news from the study though: many fisheries around the world have already started responding to ocean warming, and well-managed fisheries allow populations to take a break and recover numbers.
However, reducing overfishing is essential to minimising the potential impacts of warming oceans – otherwise the situation is likely to worsen, with increases in water temperature predicted to continue.
"We recommend that fisheries managers eliminate overfishing, rebuild fisheries and account for climate change in fisheries management decisions," says Free.
"Policymakers can prepare for regional disparities in fish catches by establishing trade agreements and partnerships to share seafood between winning and losing regions."
The research has been published in Science.