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Aoki K.,Japan National Research Institute of Fisheries Science | Onitsuka G.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Shimizu M.,Japan National Research Institute of Fisheries Science | Kuroda H.,Hokkaido National Fisheries Research Institute | And 7 more authors.
Estuarine, Coastal and Shelf Science | Year: 2012

A harmful bloom due to the raphidophycean flagellate, Chattonella antiqua, was found in the Yatsushiro Sea, western Kyushu, Japan, from the end of July to the beginning of August 2009. The bloom resulted in enormous economic damage to cultured finfish production in aquaculture farms concentrated in the southwestern area. To investigate the factors controlling the spatio-temporal distribution of the bloom, data analysis and numerical simulations were conducted using field monitoring data and a three-dimensional hydrodynamic model coupled to a Lagrangian particle-tracking model. Results of the monitoring data analysis showed that the initial development of the C. antiqua bloom occurred in Kusuura Bay and the northeastern area near the mouth of the Kuma River, and subsequently the bloom expanded rapidly to the whole area. The simulation results indicated that the source region of the widespread bloom was not Kusuura Bay but the northeastern area. The southwestward evolution of the bloom was primarily controlled by the passive transport due to the surface residual current driven by fresh water discharge from the Kuma River and northeasterly winds. On the favorable conditions of river discharge and wind, the massive bloom of C. antiqua that formed in the northeastern area was quickly transported southwestward within a few days. © 2012 Elsevier Ltd. Source


Aoki K.,Japan National Research Institute of Fisheries Science | Onitsuka G.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Shimizu M.,Japan National Research Institute of Fisheries Science | Kuroda H.,Hokkaido National Fisheries Research Institute | And 2 more authors.
Fisheries Science | Year: 2015

Mass mortality of Seriola quinqueradiata was caused by a Chattonella bloom in summer 2010 in the southern area of the Yatsushiro Sea. We analyzed the diurnal records of the mortality occurrence and cell density of C. marina to examine the short-term mortality dynamics. In addition, a hydrodynamic model and Lagrangian particle tracking method were used to examine interregional differences in the mortalities. Mortalities were concentrated along the southern coast of Shishi Island and western coast of Nagashima Island in early July. In late July, mortalities occurred along the southern coast of Shishi Island and eastern coast of Nagashima Island. Severe mortalities occurred where the bloom (>100 cells ml−1 of Chattonella marina) appeared in both early and late July. Model results show that interregional difference of the bloom was controlled by the advection system in the Yatsushiro Sea. The consistent spatio-temporal pattern between fish mortalities and bloom dynamics indicates the potential benefits of real-time monitoring of harmful algae and assessing factors controlling hydrographic conditions to mitigate mortalities. © 2015, Japanese Society of Fisheries Science. Source


Aoki K.,Japan National Research Institute of Fisheries Science | Onitsuka G.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Shimizu M.,Japan National Research Institute of Fisheries Science | Kuroda H.,Hokkaido National Fisheries Research Institute | And 6 more authors.
Marine Pollution Bulletin | Year: 2014

The dynamics of river plume in relation to harmful blooms of the raphidophycean flagellate, Chattonella antiqua in summer 2008-2010 in the Yatsushiro Sea, Japan were studied using a hydrodynamic model and monitoring data. In the southern area, the bloom formed in the waters stratified by a halocline caused by the southward expansion of riverine water from the Kuma River after the bloom initially forming in the northern area. The timing of the southward riverine water advection can be explained by the balance between the wind stress term and the pressure gradient term calculated from the horizontal density difference between the northern and southern areas. The wind stress and pressure gradient terms were evaluated using the sea surface temperature, salinity, wind speed and direction at two stations. Real time monitoring or continuous observations in these areas will enable nowcasts of bloom expansion when a bloom develops in the northern area. © 2014 Elsevier Ltd. Source


Onitsuka G.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Shikata T.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Kitatsuji S.,Japan National Research Institute of Fisheries And Environment of Inland Sea | Abe K.,Japan National Research Institute of Fisheries And Environment of Inland Sea | And 3 more authors.
Journal of Oceanography | Year: 2016

Short-term growth dynamics of diatoms were examined by daily sampling and microcosm experiments under in situ environmental conditions in the late rainy season (July) for 10 days and by weekly sampling during summer (June–September) at a fixed point in the Yatsushiro Sea, Japan. During the daily sampling period, a bloom of diatoms such as Skeletonema spp., Thalassiosira spp., and Chaetoceros spp. (>104 cells mL−1) occurred in the surface and subsurface layers during the first half of the period, but the population of diatoms declined and sank to the bottom by an average of about 1 m day−1 during the second half of the observation period. Daily changes in net growth rates of the three dominant diatom genera (ranging from −3.8 to 3.0 day−1), which were estimated by daily 24-h microcosm incubations, roughly corresponded with those in ambient cell densities in the surface layer. The relationship between net growth rates and environmental factors suggest that the growth dynamics of diatoms were regulated by irradiance and river discharge (i.e., salinity and phosphate concentration) during the observation period. The field observations using daily microcosm experiments revealed that the maintenance and decline of a diatom bloom on a daily/weekly time scale are controlled by the rapid growth and sinking responses of diatoms following abrupt changes in coastal environments in the Yatsushiro Sea. © 2016 The Oceanographic Society of Japan and Springer Japan Source

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