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Most know that climate change is changing the world today and will change it more tomorrow. But what’s not always as well-examined is how much it’s altered already. A new study out of Rutgers University gazes back at shifts that have taken place since the 1930s in global fish stocks, finding losses of 4.1 percent—1.4 billion metric tonnes of fish—since that time.

“Much of the work on climate change and fisheries is forecast into the future,” says study author Chris Free, now at the University of California, Santa Barbara.

Free did the research with Rutgers when he was a postdoc. He and his colleagues wanted to figure out what changes had already taken place, both to gain a better understanding of historical climate change and to better forecast what trends may emerge when we look back at today. Fish stocks are a vital measure of the health of the oceans as a whole, but they’re also an important indicator of long-term food and economic security.

Although the health of global oceans affects everybody, the impacts from declining stocks are especially felt in tropical countries, which rely more on fish as a food source and fishing as part of the economy, says Rashid Sumaila, director of the University of British Columbia’s Fisheries Economic Research Unit.

The researchers relied on maps of historical ocean temperature and the most comprehensive database available of historical catches. By relating the ocean temperature with fish catch, they were able to produce a “hindcast” (think forecast, but reversed) of fish stocks.

“There have been enormous regional discrepancies,” says Free. While losses of fish biomass could be as high as 30 percent or more in East Asian ecoregions like the Sea of Japan, other regions, like the Newfoundland-Labrador area of Canada, saw an increase in available harvest.

This has taken place over less than a century, and changes to the oceans are only speeding up as warming intensifies. Free and his colleagues found that overfishing dramatically contributes to whether fish stocks could remain sustainable or not in a warming ocean, so they recommend curtailing overfishing as a major priority for ensuring sustainability going forwards. The hindcast may be an unusual approach, but it can yield insights for the future. “Looking back is always a good thing,” says Rashid Sumaila, “because they’re doing a descriptive analysis of what has happened.”

Sumaila is the lead author of a recent paper that forecasts the potential economic benefits of meeting Paris Agreement climate targets. His team found that keeping warming below 1.5 degrees Celsius would massively benefit fisheries, particularly those in developing tropical countries like Indonesia and the Maldives.

What both Free’s and Sumaila’s findings highlight is that the effects of climate change are being disproportionately felt by people around the equator, rather than the nations responsible for the greatest emissions. But curtailing warming, and keeping the fish alive, is not just an ethical duty, Sumaila says: it’s also in everybody’s best financial (and culinary) interests.

“I think one of the takeaways for the consumer is just the importance of reducing greenhouse gas emissions now,” says Free.

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