Experience of eutrophication and harmful algal blooms in the Seto inland sea, Japan
Imai I1, Yamaguchi M2, Hori T3
1 Laboratory of Marine Environmental Microbiology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan.
2 Red Tide Research Division, National Research Institute of Fisheries and Environments of the Seto Inland Sea, Ohno-cho, Saeki-gun, Hiroshima 739-0452, Japan.
3 Hyogo Prefectural Fisheries Experimental Station, Futami, Akashi, Hyogo 674-0093, Japan.
The incidents of noxious red tides had dramatically increased in frequency and scale in Japanese coastal waters, especially in the Seto Inland Sea, along with serious eutrophication in 1960s and 1970s. The maximum incident 299/yr was recorded in 1976. The most important red tide organisms causing huge fishery damages by fish-kill were Chattonella antiqua, C. marina and Heterosigma akashiwo (Raphidophyceae), and Gymnodinium (Karenia) mikimotoi (Dinophyceae). The maximum fisheries damage (death of 14 million yellowtails) was 7.1 billion yen caused by C. antiqua in Harima-Nada in 1972, which is the world record. Average economic loss has been estimated to be about 1 billion yen per year thereafter in the Seto Inland Sea. "Law Concerning Special Measures for Conservation of the Environment of the Seto Inland Sea" was legislated in 1973 and industrial loading was decreased to half of the level of 1972. The level of nutrient salts had been lowered and red tide incidents had consequently decreased thereafter and reached about 100/yr in late 1980s, but this level of red tides has been kept so far with the level of nutrients supporting red tides. In 1990s, however, new red tide dinoflagellate species, Heterocapsa circularisquama appeared and has killed bivalves of both nature and culture recurrently. Moreover, the toxic dinoflagellate Alexandrium tamarense has become dominant in the Seto Inland Sea in spring season, and made cultured oyster toxic almost every year. More efforts are needed for understanding, forecasting, and preventing harmful algal blooms.
