Current status of expansion of HAB species in Hokkaido, northern Japan, with special reference to large-scale dispersal of Karenia mikimotoi via ocean current system

To reveal the recent situation of the expansion of HAB species around Hokkaido, northern Japan, we investigated the past occurrences of HAB events since 1970s. Several HAB species were newly detected after 2013 as well as the toxic dinoflagellates Alexandrium catenella (Group I) and Dinophysis spp., that produce shellfish toxins. Notorious red tide species Chattonella marina, Karenia mikimotoi and Margalefidinium polykrikoides, and toxic species Ostreopsis sp. and Prorocentrum lima had been newly recorded in 2010s. These species are supposed to be introduced to the west coast of Hokkaido from western Japan via the Tsushima Warm Current system. In autumn 2021, the harmful red tide caused by Karenia selliformis occurred on the Pacific coast of eastern Hokkaido, and the death of large amounts of fishery species (e.g., sea urchin Strongylocentrotus intermedius, chum salmon Oncorhynchus keta and sea snail Neptunea spp.) were related to this HAB event. The harmful event is assumed to be caused by the introduction of K. selliformis from the offshore area of north Pacific via ocean current systems. It is necessary to elucidate geographical distribution of HAB species and to monitor further expansion of HAB species via ocean current system in subarctic regions where new HAB species are at risk of expansion.