Red tide and seasonal occurrence of the harmful raphidophyte Heterosigma akashiwo in Hakodate Bay, Hokkaido
2019
Natsuike, M. (Hakodate Fisheries Institute, Hokkaido Research Organization, Hakodate, Hokkaido (Japan)) | Kanamori, M. | Shimada, H.
Seasonal sampling at the Hakodate Bay, Hokkaido, from April to November 2018 revealed the occurrence of the harmful raphidophyte, Heterosigma akashiwo, detected first in early July at a density of 5 cells m/L, which subsequently increased rapidly until mid-July, with the maximum density, reaching 6,000 cells m/L. H. akashiwo was observed until August at low densities(< 1 cells m/L) and not detected from September to November. When H. akashiwo occurred in high density, surface water temperature ranged from 14.8℃ to 18.1℃, which was reported to be the optimal growth temperature for H. akashiwo in culture experiments. Lower salinity (< 30) at the surface layer, caused by precipitation, was observed in Hakodate Port when the density of H. akashiwo increased rapidly. A red tide of H. akashiwo was observed in Hakodate Bay in mid-July 2018. Fish mortality was not reported during the occurrence of the red tide, even at the maximum cell density (113,000 cells m/L) in the bay. Some other red tide-causing species that are lethal for fishes, such as Fibrocapsa japonica, were detected from July to November. Thus, continuous monitoring of harmful flagellates is important in the Hakodate Bay, to detect the occurrences of harmful red tides.
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