The hearing survey was conducted to clarify the present status and the changes in the last 10 years
on seaweed beds and fishery organisms related to seaweed on the coast of Japan Sea, Aomori Prefecture,
Japan. We interviewed the 29 fishermen at 21 coastal areas about the extents of sargassum beds,
ISOYAKE caused by feeding of sea urchins, and the resources of sea urchin, top shell, abalone, edible
algae, rockfish and sailfin sandfish in April and May 2017.
As the result, it became clear that the extents of the ISOYAKE and the amount of sea urchin
Mesocentrotus nudus remarkably decreased, while the extents of sargassum beds turned to increase on
the Japan Sea coast of Aomori Prefecture. The amount of red/black rock fish which lives in sargassum
beds, and of sailfin sandfish that spawn on sargassum also increased. Top shell and abalone which feed on
seaweed increased especially on the southern coast. On the survey coast from 2010 to 2014, the highest
water temperature exceeded 26℃ as the lethal temperature of sea urchin, and in the same period its fish
catch declined. The feeding behavior of Mesocentrotus nudus prevents the growth of seaweeds and is
known as the main sustained factor of ISOYAKE along the coast of Aomori Prefecture.
From these results, it was considered that the high seawater temperature in the summer that
continued for several years would have contributed to the reduction of ISOYAKE and the expansion of
the sargassum beds.
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