Now we know how much global warming is reducing the world’s seafood harvest

photo of worker sorting fish in japanese fish market
REUTERS/Issei Kato
A majority of the world’s most important fishing stocks continue to decline.

Among all the knotted problems in global food supply, it’s hard to think of one that has received more focused attention than global fisheries and the challenges of overexploitation, ecological intricacy, regulatory responses and failures.

And yet, after decades of international treaties and sustainability studies and harvest limits  — some of the latter volunteered by industry  — a majority of the world’s most important fishing stocks continue to decline.

Overfishing remains the key driver; other factors include pollution and habitat destruction. A typical status report will mention climate change, too, always as an afterthought, an emerging force whose impact cannot yet be calculated.

That changed at the end of February with publication in the journal Science of groundbreaking research that filtered out all other factors, then measured the influence of a warming ocean all on its own. Its unusual approach was to generate a “hindcast,” looking backward through nearly a century of data on seawater temperature and laying these against a standard measure of abundance for fish and shellfish  — maximum sustainable yield  — that has been in use since 1930.

The result is a set of trends, for the period 1930-2010, that of course can reasonably be expected to continue and even accelerate. And while the patterns of past change are varied, the overall picture they form is rather grim.

The basic unit in fisheries management is the stock, essentially a geographical subpopulation of a particular species. For this research, led by scientists at Rutgers University, data were considered for 235 fish stocks, representing 124 species and 38 ocean regions, that happen to account for roughly one-third of the world’s fish harvest  — so, a large sample.

Of those 235 stocks, half showed declines ranging from 8 percent to 35 percent, with considerable variability among regions and some among species. One marquee example: Atlantic cod, whose numbers in the northeast Atlantic Ocean showed a 34 percent decline.

On the other hand, roughly one-quarter of the stocks showed no significant change, and another quarter showed some growth. Among these “winners” is the black sea bass, which seems to be benefiting from warmer temperatures in the shallows off New England and Canada. (This would be better news if black sea bass didn’t happen to eat a lot of baby lobsters.)

Loss: 3 billion pounds per year

Collating all the changes, the study finds an overall decline in sustainable fish catch of 4.1 percent worldwide. Doesn’t seem like much, you say? Well, it represents an annual loss in seafood production of about 1.4 million metric tons, or a bit over 3 billion pounds. It’s a loss that cannot be reversed without reversing global warming. And the pace of the decline appears to be magnified by overfishing, which has itself proved resistant to regulatory control.

The impact of these shrinking fisheries can be measured not just in higher prices for cod fillets at a Minneapolis fish counter, but also in scarcities of essential protein for people whose diets rely on fish as a local produced staple.

A Rutgers announcement of the results observes that 56 million people throughout the world are critically dependent on fisheries,  because seafood provides essential dietary protein, or their livelihood, or both. This is especially so in the coastal populations of developing countries and isolated island economies, where seafood can provide as much as 50 percent of the animal protein available for human consumption.

In thinking about climate impacts on that resource, Rutgers’ Christopher Free and Malin Pinsky realized that two enormous sets of historical data could be studied in tandem. One was the record of ocean temperature measurements. The other was maximum sustainable yield, or MSY, a measure enshrined in various laws, treaties and recovery programs starting in 1930.

MSY is the product of complex calculations to arrive at a simple figure: the portion of a current fish stock that can be harvested without long-term harm to population health, expressed in catch weight. (It’s often likened to bank interest  — how much you can withdraw from a savings account without reducing the principal.)

The paper acknowledges some controversy over the measure, but these arguments seem to be about its usefulness in measuring specific population characteristics and dynamics rather than overall sizes and trends. I saw no criticism on this point in science journalists’ coverage of the work, nor in the admiring comments that Free and Pinsky’s peers offered for quotation. An example from Kendra Pierre-Louis’s piece in the New York Times:

“This is going to be one of those groundbreaking studies that gets cited over and over again,” said Trevor Branch, an associate professor at the University of Washington’s School of Aquatic and Fishery Sciences, who was not involved in the study. “Most of what I’ve seen before in terms of climate-change impacts have been speculative, in terms of, ‘We think this is what’s going to happen in the future.’ This one’s different.”

Geography is critical

Patterns of seawater warming have shown considerable geographic variation, so it may be unsurprising that the study found ocean region to be the variable most strongly linked to a fish stock’s fate. From the Free/Pinsky paper:

The greatest losses in productivity occurred in the Sea of Japan, North Sea, Iberian Coastal, Kuroshio Current, and Celtic-Biscay Shelf ecoregions, whereas the greatest gains occurred in the Labrador-Newfoundland, Baltic Sea, Indian Ocean, and Northeast U.S. Shelf ecoregions. The East Asian ecoregions experienced some of the largest warming-driven declines in MSY (8 to 34%) and support some of the largest and fastest-growing human populations in the world.

Interspecies differences in such factors as body size, preferred depth and habitat type didn’t seem to make much difference, although fish with faster growth rates and earlier ages of maturity were likely to respond more negatively to rising temperatures (unless they could move conveniently to cooler waters nearby).

Some of the biggest declines were found in stocks whose home waters were near the upper end of the species’ tolerable range, like Atlantic cod and herring; Pinsky has likened a species’ temperature needs to Goldilocks’ insistent that the oatmeal be neither too hot nor too cold, but just right. And one human-controlled factor was quite important:

Populations that had experienced intense and prolonged overfishing were more likely to be negatively influenced by warming, especially when they had also experienced rapid warming (>0.2°C per decade).

This interaction likely arises through several mechanisms. First, fishing can truncate age distributions and select for earlier maturation or reduced body sizes, both of which can decrease reproductive output. Fishing can also reduce intraspecific diversity, alter species interactions, and damage habitat. As a result, overfishing can magnify fluctuations in abundance due to environmental variability and interact with life history and climate variability to increase the likelihood of population collapse.

Thus, overfishing has reduced the resilience of populations to climate change, and climate change will likely hinder efforts to rebuild overfished populations….

Although the trends measured by Free and Pinsky are sure to continue, their paper stops short of making detailed projections of how this may show up in fish stocks of the future. But an especially fine piece by Erik Stokstad for Science magazine offered some thoughts on the subject:

The overall decline will most likely steepen, as forecasts have previously suggested. Since 1930, average sea surface temperatures have risen by about 0.5°C. By the end of this century, more than three times that amount of warming will likely happen, and marine heat waves will become more frequent. Although temperatures will become more favorable to fish in higher latitude waters, “those benefits can’t last forever,” Free says. “There probably is a tipping point.”

Fishery managers can help the situation. The analysis suggests stocks are harder hit by rising temperature if they have been heavily overfished. That is surprising, [German marine ecologist Rainer] Froese says, because fishing tends to selectively remove larger fish and heavily fished stocks evolve to be smaller and mature faster. These smaller fish, which are more efficient at using oxygen, might, in theory, be better able to cope with warmer water that has less oxygen. But the new study suggests these stocks were less resilient to temperature increases.

One reason could be that excess fishing wiped out the genes for coping with warmer temperatures, Froese says. Whatever the mechanism, fisheries scientists know that curbing overfishing leads to larger and more sustainable harvests. “Reducing overfishing,” he says, “is a no-brainer.”

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The full paper, “Impacts of historical warming on marine fisheries production,” can be found here but access is not free.

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