by Chris Free with The Conservation
Climate change has been steadily warming the ocean, which absorbs most of the heat trapped by greenhouse gases in the atmosphere, for 100 years. This warming is altering marine ecosystems and having a direct impact on fish populations. About half of the world’s population relies on fish as a vital source of protein, and the fishing industry employs more the 56 million people worldwide.
My recent study with colleagues from Rutgers University and the U.S. National Oceanic and Atmospheric Administration found that ocean warming has already impacted global fish populations. We found that some populations benefited from warming, but more of them suffered.
Overall, ocean warming reduced catch potential – the greatest amount of fish that can be caught year after year – by a net 4% over the past 80 years. In some regions, the effects of warming have been much larger. The North Sea, which has large commercial fisheries, and the seas of East Asia, which support some of the fastest-growing human populations, experienced losses of 15% to 35%.
The reddish and brown circles represent fish populations whose maximum sustainable yields have dropped as the ocean has warmed. The darkest tones represent extremes of 35 percent. Blueish colors represent fish yields that increased in warmer waters. Chris Free, CC BY-ND
Although ocean warming has already challenged the ability of ocean fisheries to provide food and income, swift reductions in greenhouse gas emissions and reforms to fisheries management could lessen many of the negative impacts of continued warming.
How and why does ocean warming affect fish?
My collaborators and I like to say that fish are like Goldilocks: They don’t want their water too hot or too cold, but just right.
Put another way, most fish species have evolved narrow temperature tolerances. Supporting the cellular machinery necessary to tolerate wider temperatures demands a lot of energy. This evolutionary strategy saves energy when temperatures are “just right,” but it becomes a problem when fish find themselves in warming water. As their bodies begin to fail, they must divert energy from searching for food or avoiding predators to maintaining basic bodily functions and searching for cooler waters.
Thus, as the oceans warm, fish move to track their preferred temperatures. Most fish are moving poleward or into deeper waters. For some species, warming expands their ranges. In other cases it contracts their ranges by reducing the amount of ocean they can thermally tolerate. These shifts change where fish go, their abundance and their catch potential.
Warming can also modify the availability of key prey species. For example, if warming causes zooplankton – small invertebrates at the bottom of the ocean food web – to bloom early, they may not be available when juvenile fish need them most. Alternatively, warming can sometimes enhance the strength of zooplankton blooms, thereby increasing the productivity of juvenile fish.
Understanding how the complex impacts of warming on fish populations balance out is crucial for projecting how climate change could affect the ocean’s potential to provide food and income for people.
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