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Warming oceans have already reduced fish populations over past 70 years

A new study suggests we look to the past for guidance on how to cope going forward.

There is much discussion about how climate change will affect the world's oceans and fish in years to come, but less about how how they have already been affected. This retrospective information, however, can help us to understand just how serious the situation is and what we need to focus on right now to avert further devastation.

A new study, published last week in Science, offers this valuable historical perspective. Researchers tracked 235 fish populations in 38 ecological regions around the globe and found that, between 1930 and 2010, global fish populations have declined by 4.1 percent, due to warming waters. In fact, it's been estimated by the National Oceanic and Atmospheric Administration that “more than 90 percent of the earth’s warming over the past 50 years has occurred in the ocean.”

Four percent may seem minor, but that adds up to a very significant 1.4 million metric tons of fish. Certain regions, such as the Sea of Japan and the North Sea, have seen the highest losses of over 34 percent. The East China Sea (8.3 percent), Celtic-Biscay Shelf (15.2), Iberian Coast (19.2), South Atlantic Ocean (5.3), and Southeast U.S. Continental Shelf (5) also saw significant dips (via NY Times).

Fish in colder regions tended to fare better than those in warmer ones, where the change was often too much for them to handle. In the words of Malin Pinsky, study co-author and associate professor at Rutgers University, “Fish are like Goldilocks: They don’t like their water too hot or too cold.”

Some fish species did better in warmer waters, such as black sea bass off the Northeast U.S. Continental Shelf, which increased 6 percent over the study period, but this singular example is not cause for celebration. From Mother Jones' writeup, published on Grist:

“Many more of the populations studied had a negative versus positive reaction to the warming. And even for the species currently thriving in warmer waters, as the warming increases — as it’s expected to — these benefits could run out when species reach their temperature threshold.”

In the words of lead study author, Chris Free, “These populations that have been winning aren’t going to be climate winners forever.”

The researchers found that overfishing, another major global threat, compounds the harmful effects of warmer waters. It makes populations more vulnerable to temperature changes by hurting their ability to reproduce and harming their ecosystems.

These declines, if allowed to continue, will have a big impact on the 3 billion people who rely on fish as their primary source of protein and the 10 percent that relies on fisheries for their livelihood. It's a $100-billion industry whose collapse would have a huge ripple effect around the world.

Consider that these observed changes have occurred in waters warmed by a half-degree Celsius. And yet, “projections for the future expect more than three times that increase.” It's more important than ever to take drastic measures now to ensure it doesn't get worse.

The study authors recommend better-managed fisheries to start, as stable populations are at an advantage when it comes to coping with changing temperatures. Dr. Free would like to see adaptive regulations: “Fishery managers need to come up with new innovative ways of accounting for those shifts. That includes reducing catch limits in warm negative years, but it can also include increasing catch limits in cooler positive years.”

A new study suggests we look to the past for guidance on how to cope going forward.