Fish are shrinking due to global warming, a new study reveals.
Warmer water means that smaller plankton – the microorganisms that fish feed on – float to the surface. This means fish get less nutrition from what they eat, researchers found.
The University of Tokyo researchers analyzed the individual weight and total biomass of thirteen fish species, including mackerel, anchovies and sardines, and looked at long-term data for six fish populations of four species between 1978 and 2018.
Sea surface temperature data between 1982 and 2014 were also studied to see if changes in the ocean surface and subsurface layers may have had an impact.
The results, published in Fish and Fisheries, showed two periods of decline in fish body weight, first in the 1980s and again in the 2010s.
The results, published in Fish and Fisheries, showed two periods of decline in fish body weight, first in the 1980s and again in the 2010s (photo: sardines underwater)
This initial weight loss was originally attributed to increased numbers of Japanese sardines, which increased competition with other species for food.
However, further analysis found that climate change’s effect on warming the ocean appears to have resulted in increased competition for food, as cooler, nutrient-rich water could not easily rise to the surface.
The findings in the Pacific support previous research in other parts of the world, which has shown that trophy fish caught in fishing competitions are also becoming smaller, and that smaller fish species are also increasing in numbers at the expense of larger ones.
Professor Shin-ichi Ito from the University of Tokyo said: ‘At higher temperatures the upper layer of the ocean becomes more stratified, and previous research has shown that larger plankton are replaced by smaller plankton and less nutritious gelatinous species, such as jellyfish.
‘Climate change may alter the timing and length of phytoplankton blooms, and the explosive growth of microscopic algae at the ocean surface, which may no longer correspond with key periods in the fish life cycle.
Warmer water means smaller plankton, which means fish get less nutrition from what they eat, researchers found (photo: freshly caught herring at a fish market)
‘Other studies have also found that fish migration is affected, which in turn affects fish interactions and competition for resources.’
The western North Pacific accounted for almost a quarter of the global total of fish caught and sold in 2019, according to the UN Food and Agriculture Organization.
The team adds that their results have implications for fisheries and policymakers trying to manage ocean resources under future climate change scenarios.
Professor Ito said: ‘Fish stocks need to be managed differently than before, given the increasing impact of climate-induced conditions.
‘The situation with fish is much more serious than decades ago. If we cannot stop global warming, the quality of fish may decline.
“So we need to take action so we can enjoy a healthy ocean and delicious fish.”