Excess carbon in the atmosphere has prompted warmer ocean temperatures and altered productivity, causing disturbances to fish populations in which some win and some lose. Do the wins outgain the losses? According to a new paper published in Science (Free et al. 2019)—no, climate change has already reduced the amount fish we can sustainably harvest from the ocean and will continue to impact global fisheries.
Free et al. 2019 controlled for used climate and fisheries models to control for fishing pressure, management, natural fluctuations, and other factors to isolate the effect of ocean temperature on fisheries over time. Researchers studied 235 populations of 124 commercially important species that compose roughly a third of modern fisheries catch. They found that, solely due to warming water, total maximum sustainable yield (MSY) of these 235 populations decreased by 4.1% from 1930 to 2010, representing over a million tons of potential lost catch.
The losses are not universal across populations or regions, though. Some populations have benefited from warmer waters, but more have declined. Surprisingly, there was little regional consistency. For example, the North Sea saw a dramatic decline in MSY, but the neighboring Baltic Sea saw a significant increase in MSY.
It may seem that most areas increased or decreased only slightly, but those few percentage points equal thousands of tons of fish and the jobs/income that come with them.
Factors that determine fishery outcomes under warming temperatures
Geographic location of a population within its species-specific temperature range was found to be a significant factor in the outcome of the population. For example, Atlantic cod live as far south as New England and as Far north as the Arctic, covering a large temperature range. The populations on the colder end of their temperature range (closer to the Arctic) benefited slightly from warming temperatures, while those on the warmer end (further from the Arctic) suffered. (We chronicled the changes in several Atlantic Cod populations here).
Because overfishing reduces reproductive output, it was also a significant factor, “populations that had experienced intense and prolonged overfishing were more likely to be influenced by warming.” Overfishing magnifies natural fluctuations in population size which compromises the resilience of fish populations to climate change. Essentially, climate change makes it harder to rebuild overfished populations.
What will the future look like?
Researchers expect the ‘winning’ fish populations from global warming to become further outweighed by the ‘losers.’ Fish that benefited from warming water likely did so because most fish are ‘cold-blooded,’ meaning they take the temperature of the water they live in; warmer fish are more active and have more energy. But those gains in energy are marginal—the benefits slow as they get closer to their temperature limit.
However, a recent paper cited by Free et al. 2019, shows that many of the negative effects of climate change can be offset with improvements in global fisheries management. If we rebuild depleted fisheries and manage for climate change impacts, there is potential for more fish to be sustainably harvested than there is today. We wrote about that paper here.
Max Mossler
Max is the managing editor at Sustainable Fisheries UW.
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