Application of a hydrodynamic model to long-term monitoring data: exploring transport pathways to identify the source of high toxicity populations of Dinophysis fortii in aquaculture sites in northern Japan

Abstract

Diarrhetic Shellfish Toxicity (DST) occurs in northern Japan, Hokkaido and Tohoku, and is rarely observed in the southwest. A recent study using cultured strains of Dinophysis fortii revealed that high toxicity (HT) strains exist in northern Japan, while low-toxicity (LT) strains are found in the southwest. In this study, we analyzed 40 years of DST and plankton monitoring data from Hokkaido, eight years from Akita Prefecture, 33 years of oceanographic data from the Sea of Japan off northern Hokkaido, and the results of an ocean model experiment. The relationship between the annual DST maximum value and the cell density of D. fortii maxima in Hokkaido suggests the presence of both HT and LT strains. Results from the particle tracking experiment showed that transport of offshore populations of D. fortii to the coast is a key factor in HT DST events. Oceanographic data support that high DST along the Sea of Japan coasts in northern Hokkaido occurs in years of strong influence of the subarctic water of the Sea of Japan (SWSOJ). In northwestern Japan (Akita Prefecture), HT DST events were detected in years with strong influence of the second and third components of the Tsushima Warm Current (TWC2, 3) transporting particles from the SWSOJ. These results led to the conclusion that i) HT strains of D. fortii are transported by the SWSOJ reaching Hokkaido via its extension and Akita via the TWC2, 3, and ii) LT strains are transported to northern Japan via the first component of the Tsushima Warm Current (TWC1).