Biological oceanographic study on method for predicting the occurrence of paralytic shellfish toxin along the Okhotsk Sea coast off Hokkaido

The scallop fishery along the Okhotsk Sea coast of Hokkaido is known as 'sowing culture', harvesting after three years of scallop seed sowing on the fisheries grounds, and is one of the largest bivalve fisheries (catch slightly less than 300 thousand tons per year) in the world. However, the scallop fishery has experienced economic damage due to incidental occurrence of paralytic shellfish toxin accumulation by shellfish (PST) caused by the toxic dinoflagellate Alexandrium tamarense in summer once every several years. An intensive PST occurrence recorded in summer 2002 stopped the scallop fishing for more than one month, leading to a steep fall in the scallop market value and serious economic losses. To minimize economic damage due to PST occurrence caused by A. tamarense, the transportation mechanism of water mass contaminated A. tamarense from the oceanic area to the scallop fishing ground along the coastal area was revealed and a method for predicting the occurrence of PST was established in the present study. Seasonal changes in occurrences of vegetative cells of A. tamarense and PST toxicity were researched along the coast of Hokkaido during 2005-2006. Vegetative cells occurred in the Okhotsk Sea and the Pacific coast (cold current area affected by the East Sakhalin Current and the Oyashio) but not detected in the Sea of Japan and the Tsugaru Strait (warm current area affected by the Tsushima Warm Current and the Tsugaru Warm Current). Occurrences of PST over the quarantine level (4 MU /g scallop whole meat) were recorded in Funka Bay during blooms of A. tamarense (> 10E2 cells /L) were observed. Horizontal distribution of resting cysts of toxic Alexandrium spp. was investigated around Hokkaido during 1999-2000. Resting cysts of A. tamarense were widely distributed in the Okhotsk Sea and the Pacific Ocean. Regarding the relationship between past PST occurrences and cyst abundance in the sediment of each area, positive correlations were found between the frequencies of PST occurrence years and the cyst abundances and between the annual maximum PST toxicities and the cyst abundances. Therefore the cyst abundance implies important information about the past PST occurrences (frequency and magnitude) of each area. The occurrences of vegetative cells of A. tamarense during 2005-2006 (above mentioned) were considered to reflect the cyst abundance of each area. However the occurrences of vegetative cells and cyst abundance in the Okhotsk Sea off Hokkaido, showed that despite the low occurrences of vegetative cells in the coastal area, large cyst abundances were found on the continental shelf from Hokkaido to Sakhalin. Thus the oceanic area of the Okhotsk Sea off Hokkaido was considered to have a high potential for initiation of A. tamarense blooms. Spatial distribution of vegetative cells of A. tamarense was examined in the Okhotsk Sea off Hokkaido in summer during 2002-2007. The vegetative cells frequently occurred in the surface low salinity water (LSW, salinity < 32.5) in the oceanic area and rarely appeared in the Soya Warm Current water (SWC, salinity > 33.6) along the coastal area and the dichothermal water (DTW, temperature < 2 degC) below 30 m deep in the oceanic area. Nutrient concentrations were respectively higher in the DTW than the LSW and the SWC. Despite the low nutrient concentrations of the LSW, A. tamarense can be considered to utilize nutrients originating from the DTW just below the LSW due to the effects of diel vertical migration. A continuous diatom bloom was observed along the front area between the LSW and the SWC where a belt-shaped upwelling area occurred with higher nutrient concentrations. Bloom of A. tamarense tended to be found in the LSW just outside the front area, because of interspecific competition with the diatom bloom. Lower DIP concentration of the SWC is supposed to restrict formation of a A. tamarense bloom in addition to the absence of the DTW with higher DIP concentration below the SWC. Therefore environmental conditions of the SWC were concluded to be severe for bloom formation of A. tamarense. Regarding the interannual relationship between the abundance of A. tamarense in summer and the relative frequency of each water mass in spring and summer, the abundance tended to be higher in years when higher frequencies were recorded of the SWC in spring and of the LSW in summer. The results suggest that warming by the SWC in spring prompts germination of A. tamarense cysts in the sediment and domination of the LSW in summer gives optimum medium for bloom formation of A. tamarense. Toxin profiles of 103 culture strains of A. tamarense isolated from sediment or seawater samples collected from the coast of Hokkaido and Aniva Bay (southern Sakhalin) were analyzed using HPLC during 2005-2009. As a result of cluster analysis of the toxin profiles of culture strains, 101 culture strains were classified in the same cluster, producing C-toxin-2, gonyautoxin-4, gonyautoxin-3 and neosaxitoxin as dominant toxin components excluding two culture strains. The toxin profiles of the 101 culture strains were almost the same as past reports on toxin profiles of A. tamarense from Japan and Sakhalin. Cellular toxin contents of culture strains varied from 1 to 1128 fmol cellsup(-1), and were inversely proportional to cell densities. As a result of the estimations if a bloom of A. tamarense (cellular toxin content: 10E3 fmol cellsup(-1), cell density: 10E2 cells /L) was fed on by scallops (filtration rate: 10E2 L /day, accumulation ratio of toxin: 35 %), toxification rate of scallop is calculated as 0.4 MU /g digestive diverticula /day. Result of the estimation suggests that scallop become toxic over the self-imposed quarantine level (20 MU /g digestive diverticula) after 50 days from the initial occurrence of a A. tamarense bloom, scientifically proving the empirical data, 'Bloom of A. tamarense exceeding ca. 10E2 cells /L causes shellfish toxification over the quarantine level'. To clarify the transportation mechanism of A. tamarense in the Okhotk Sea coast off Hokkaido, area-wide sampling in the oceanic area, time-series monitoring in the coastal area and current velocity measurements of the SWC using ADCP were conducted in 2004, 2007 and 2008. These surveys were organized based on the hypothetical scenario, 'PST occurrence is caused by the inflow of LSW contaminated with A. tamarense to the scallop fishing ground at the temporal weakening of SWC indexed by the decrease of the sea-level difference (SLD) between Wakkanai and Abashiri'. It was revealed that A. tamarense blooms appeared in the coastal fishing ground simultaneously with the weakening of SWC indexed by the SLD. Retrospective analysis on time-series relationship between the weakening of SWC and the PST toxicity in PST occurrence years also elucidated that the toxicity increased just after the weakening of SWC. Therefore the hypothetical scenario was verified, and a method for predicting the occurrence of PST was constructed as follows; (1) Sampling in the oceanic area in June (before PST occurrence) and July (during annual peak of PST occurrence) to monitor horizontal distribution of A. tamarense. (2) Monitoring of the weakening of SWC indexed by the SLD using internet. (3) If A. tamarense bloom (> 10E2 cells /L) has been found in the oceanic area, and the weakening of SWC is observed, warning of potential PST occurrence within a few weeks should be provided. Semi-realtime data of the prediction method is available to the public for controlling shipping plan of scallop since 2009. The prediction method gives high cost effectiveness since the essential part of prediction can be simply constructed with the twice a year sampling in the oceanic area and the monitoring of SLD. The present study provides important information about feeding environment of main fishery resources, not only scallop but also fish such as salmon, since the high contrast structure and dynamics of water masses has been revealed by biological oceanographic studies in the Okhotsk Sea off Hokkaido, focusing on A. tamarense as a biological tracer. Recently, it had been reported that warm-water, non-armored flagellates causing harmful red tides have been detected for the first time in the coast of the Sea of Japan and the Tsugaru Strait of Hokkaido. Once a harmful red tide appears in the scallop fishing ground, serious damage may occur to the scallop fishery. Therefore monitoring of non-armored red tide flagellates is necessary to start in addition to armored harmful dinoflagellates such as A. tamarense in northern Japan, since the northward expansion of warm-water harmful flagellates is increasingly possible due to ocean warming in the future.