A new study published Thursday in the journal Science outlines the impacts warming waters are having on commercially important fish species.
The world's fishing industry relies on what's called fisheries, the clusters of regional fish populations that people can catch economically. And on average, the researchers found that the numbers of fish in critical fisheries around the world have decreased by four percent since 1930.
Fisheries located in the Sea of Japan and the North Sea were the worst off. They experienced as much as a 35 percent drop in their numbers. Other fisheries, however, benefitted from warmer waters, and their populations grew, an expansion scientists warn could create unsustainable competition for resources.
“We were surprised at the strength the impact of warming has already had on fish populations,” says the study's lead author, ecologist Chris Free at the University of California Santa Barbara.
Taking the ocean's temperature
To measure the effect of warming and overfishing, Free started by looking at temperature data from the past 80 years and comparing it to how productive a given fishery was during periods of higher-than-average temperatures. The team looked at 235 populations of 124 species of fish spread across 38 different regions.
Warmer waters can make some fisheries smaller by putting metabolic stress on the fish, making it harder for them to reproduce or find food. Warm waters can also cause zooplankton, essential fish food, to decline. The impacts on smaller organisms then have cascading impacts on the rest of the food chain.
In the North Sea and the Sea of Japan where they measured increased water temperature, scientists found overfishing had further made the fisheries even more vulnerable.
“It's like a one-two punch,” says study co-author Malin Pinksy, an ecologist at Rutgers. “If fishing already knocks them down, they're more likely to respond poorly when it's hot.”
“When Chris brought me these figures, I was really stunned,” Pinksy adds. “We knew that animals were moving into new locations, but I didn't realise it already affected the ability of these populations to produce fish.”
Mitigating warming impacts
Fisheries ecologist Will White from Oregon State University was not involved with Free and Pinsky's study, but he says their conclusions highlight the importance of managing fisheries.
“With a lot of fisheries off the West Coast, we have historically had pretty good fishery management, which has given us resilience,” says White.
From 2014 to 2016, the West Coast felt the deadly presence of a patch of warmer water called the “blob.” When it warmed Pacific waters, it killed off marine creatures like zooplankton that salmon feed on, jeopardising the health of Oregon's lucrative salmon fisheries.
INSIDE THE STRUGGLE TO SAVE AN ENDANGERED GROUPER SPECIES
“I'm not sure there's a way to manage our way out of that,” White says of acute ocean warming, but on a less dramatic, global scale, he says taking warming oceans into consideration should be an important fishery management tool.
Pinksy cautions against viewing growing populations as a good sign.
“Fish are a bit like Goldilocks,” says Pinsky. “For some it's too cold, but warming will make it too hot.”
Growing populations of one fish could also encroach upon the territory of another species. Off the coast of New England, populations of black sea bass have increased.
“Turns out,” he adds, “they like to eat lobster. As they become more abundant, they may start impacting American lobster. There are all these ripple effects.”
Feeding the planet
If current population trends continue at the same rate, the world will need to double its food production by 2050. To compensate, world leaders are consistently looking toward fisheries to be a critical source of protein for millions of people.
In 2016, 155 million tonnes of fish were taken from the sea, and that number is expected to rise to 182 million over the next ten years.
“Food security is a big concern,” says Pinksy. An estimated three billion people use fish as their primary source of protein.
“Beyond that,” Pinsky adds, “we also know that it has very important local impacts for those who make their livelihoods catching these fish.”
He thinks better management can help mitigate the impacts of warming. Establishing “no-take” zones, for instance, allow fish time to reproduce and regrow overfished populations.
Ultimately, Free says his study highlights the wide-ranging impacts of burning greenhouse gases. Unabated, he says some fisheries will likely continue their decline.
He adds, “We'll just have to be adaptive.”