Due to discharges from the nearby Takahama nuclear power plant, tropical fish and other species were able to colonize a small coastal area in the Sea of Japan.
The findings, according to New Scientist, suggest global warming will drastically alter marine ecosystems around the temperate areas of Japan over the next few decades.
Reiji Masuda at Kyoto University and his colleagues have been carrying out underwater surveys every winter at three coastal sites near Kyoto since 2004, reported New Scientist. One of these sites is warmed by the water used to cool the Takahama nuclear power plant. This site kept winter water temperatures around 13.6°C.
Divers saw both more fish overall and a greater diversity of species, including tropical ones such as the blue damselfish and the cutribbon wrasse. There are also tropical invertebrates, including the long-spined sea urchin.
“There were so many sea urchins as they did not have predators,” according to Masuda.
The tropical species were not seen at the other two sites, however, even though winter temperatures there were only slightly lower, at 12.3°C and 11.7°C.
Operations at the nuclear plant were suspended in 2012 because of the Fukushima nuclear accident. At the time, winter water temperatures at the dive site fell by 3°C to 10.6°C. Eventually, the tropical species all disappeared, reported New Scientist.
In 2017, after two of the four units at the nuclear plant restarted, tropical species began to return.
According to Masuda, the findings show that winter water temperatures in the region are just below the critical level tropical species require to survive. Since water temperatures around the temperate parts of Japan are rising due to global warming, tropical species may soon be able to colonize vast areas of the coast, altering coastal ecosystems.
In waters whose temperatures are not near the critical level, similar increases may have less of an effect. Studies in the tropical waters around Taiwan, for example, have found no evidence that warmer water from nuclear plants there impact fish communities, according to New Scientist.
