The “large-scale harmful red tide” that
occurred in the Pacific coastal area of Hokkaido in the fall of 2021 caused the
death of salmon in fixed nets and the mass mortality of short-spined sea
urchins. The damage amounted to 27,900 salmon and 2,800 tons of sea urchins,
and the total amount of fishery damage in the entire Hokkaido region was
reported to be 8.19 billion yen as of February 28, 2022.
The dinoflagellate Karenia selliformis
is believed to be the causative algae of this large-scale harmful red tide. So
far, red tides caused by Karenia species in Japan have been reported
mainly in the Seto Inland Sea and Kyushu coastal areas of western Japan, where
damage was caused by Karenia mikimotoi. The occurrence of this K.
mikimotoi in Hokkaido has been reported in Hakodate Bay in southern
Hokkaido and is believed to have been transported by the Tsushima Warm Current
water moving northward in the Japan Sea. On the other hand, K. selliformis,
the algae causing the large scale harmful red tide of 2021, is a species
described from the South Island of New Zealand in 2004. Red tide formation has
been reported in the Gulf of Mexico, New Zealand, Australia, Tunisia, and
Kuwait, suggesting that K. mikimotoi and K. selliformis probably
have a pan-global distribution.
For the 2021 large harmful red tide, genetic
analysis and cell size of K. selliformis, as well as estimation
oceanographic characteristics of the red tide formation area and of the origin
area using particle tracking models, are being conducted. For the large-scale
harmful red tide K. selliformis, a red tide caused by a genetically
identical strain was reported to have occurred on the eastern coast of the
Kamchatka Peninsula, Russia, in the fall of 2020. In the coastal areas of east
Hokkaido, outbreaks of red tides caused by dinoflagellates have been reported
in 1972, 1983, 1985, and 1986. The mechanism of these red tide outbreaks in the
coastal areas of east Hokkaido is considered to be a “rainfall-type red tide”.
However, the large-scale harmful red tide outbreak in 2021 is not confined to
one particular area of ocean, and the damage area covers a wide geographical
area from off Nemuro to Cape Erimo, so it is necessary to reconsider the cause
of its formation and the mechanism of its occurrence.
In this study, we clarified the cell number
density of K. selliformis and the horizontal distribution of
phytoplankton communities that appeared in samples collected from the west
coast of Cape Erimo to offshore Akkeshi over a wide area from October 6-12,
2021, and summarized previously reported red tides in the east Hokkaido coastal
area, and discussed conditions under which large-scale harmful red tides occur.
