According to sources including the Japan Fisheries Agency (JFA)’s Seto Inland Sea Fisheries Management Office, as far as records are available, a large number of yellowtail farmed in the Seto Inland Sea in west Japan died from red tide in 1972, causing over 7.1 billion yen (about $63 million) in damage.
Though the red tide did occur along the coast of Hakodate in southwest Hokkaido in 2015, there have not been many red tide cases in east Japan except for in Tokyo Bay, and the JFA does not compile the extent of damage across the country. For that reason alone, the latest phenomenon can be viewed as rare. The Hokkaido Prefectural Government has said that damage this time could cost as much as 17 billion yen (roughly $150 million).
The presence of Karenia selliformis, a type of phytoplankton that can propagate even under low water temperatures, has been pointed out as a cause of red tide. This plankton is said to have caused red tide in the waters off the southern Kamchatka Peninsula of Russia last year, and it has also been found in waters off the Hokkaido coast damaged this time.
Research by Hokkaido University’s School of Fisheries Sciences found that, “While it is unclear where the source of the breakout is, it can be thought that the plankton moved southward on the Oyashio Current from Russian waters.”
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https://mainichi.jp/english/articles/20211113/p2a/00m/0op/021000c