This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Ocean fish stocks have been increasingly exploited over time, and remote sensing shows they have declined in the open ocean over the last three decades, particularly in the tropics. But it's uncertain how fish populations will fare in the future, as climate models show both increases and decreases in global trends by the end of this century.
Climate change, warmer oceans, ocean acidification, weakened upwelling and increasing ocean stratification all play a part in determining the trend. A new paper looks at changes in fish biomass during a past episode of global warming and finds that sustained warming can stimulate open-ocean biomass growth as well as carry carbon taken up by marine plants and algae into the deeper ocean. But it takes a long while to happen.
The study is published in the journal Nature Communications. The prior episode of global warming considered is the Paleocene-Eocene Thermal Maximum (PETM) that occurred about 55.9 million years ago. Large amounts of greenhouse gases such as carbon dioxide and methane were released into the atmosphere, most likely from large volcanic traps—very large regions of stacked volcanic flows that spread over a major region.
In the case of the PETM, the most likely culprit was the North Atlantic Igneous Province (NAIP), a large, solidified magma region in the North Atlantic, centered on Iceland. (Research on the exact cause(s) of the PETM has been going on for decades and continues today.) In any case, the beginning of the PETM saw rapid emissions occurring over only a few thousand years. About 3,000 to 10,000 billion metric tons of carbon were released into the atmosphere. Open-ocean temperatures rose by about 5°C, though varying with latitude and depth, and the global-average near-surface air temperature rose 5–6°C.
The warming lasted for about 100,000 years. The PETM is considered perhaps the best analog to today's global warming. The PETM's overall carbon emission rate is about an order of magnitude lower than ours in the fossil fuel era; today's rate of warming is about 100 times faster, so far, than the PETM's average rate over its estimated 100-millennium lifetime.
To investigate past stocks of pelagic (open ocean) fish, the researchers—four from China, including lead author Xiuwen Zhou of the Southern University of Science and Technology in Shenzhen, and one in Canada—examined proxy indicators that are a function of fish stocks. They combined measurements of the accumulations of fossil fish teeth and dermal denticles, together called ichthyolites—the second in the pair being tiny, flat V-shaped scales that are more like teeth than fish scales, found only in cartilaginous fish like sharks and rays—from six core sites in ocean sediment. Ichthyolites are deposited on the ocean floor through natural shedding or excretion of indigestible parts by predators.
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