Hydrographic data off the Pacific coast of Tohoku and off the Boso Peninsula were analyzed using newly developed software that conducts a statistical cluster analysis to reveal the spatial distribution and temporal variation of occurrence frequency of water-masses in surface and subsurface layers. The interannual variation of water-mass occurrence frequency off the coast of Tohoku in summer was most associated with the interannual meridional shift of the Oyashio first branch, and the cold, fresh (warm, saline) clusters were more frequently observed when the Oyashio first branch shifted southward (northward). This tendency was also detected at a 10-m depth, where the previous water-mass classification method was difficult to apply. At a 100-m depth, multiple clusters were classified as Tsugaru Warm Current water, but those that could not be formed by isopycnal mixing between Kuroshio and Oyashio waters were limitedly distributed along the coast of Tohoku. It was suggested these clusters originated from the Tsugaru Strait. Off the coast of the Boso Peninsula in spring, the occurrence frequency of cold water-mass tended to increase when the Kuroshio axis departed from the coast. To relate the water-mass variations to fisheries catch variations, the number of clusters may need to be determined by considering the physiological characteristics of each fish species, such as optimum temperature range. By expanding the analysis to different regions and fish species and accumulating knowledge, the software is expected to provide a useful tool for extracting characteristic water-mass variations and clarifying the relationship between water-mass and fisheries catch variations.
To clarify the seasonal variations in the phytoplankton community and the mechanism in Tokyo Bay, the concentrations of size-fractionated (>10 μm, 2–10 μm, <2 μm) chlorophyll ɑ (Chl-ɑ) and nutrients, and the abundance of large-sized phytoplankton species (>10 μm) were investigated at the surface of the inner part and mouth of the bay from April to December 2012. In spring, fall and winter, the seasonal variations in the concentrations of size-fractionated Chl-ɑ and the abundance of large-sized phytoplankton species were similar between the inner part and mouth of the bay. In April, the total Chl-ɑ concentration was high during the observation periods at both stations, and the >10 μm fraction accounted for more than 50% of the total Chl-ɑ concentration. Further, the large-sized diatom accounted for a high proportion of the phytoplankton community. However, in May, the <2 μm fraction and raphidophyte (Heterosigma akashiwo) were dominant at both stations. Consequently, it is probable that the succession of phytoplankton occurs synchronously in the inner part and mouth of the bay from April to May. Nitrate+nitrite concentrations rapidly decreased from April to May at both stations. Therefore, it is likely that the size-fractionated Chl-ɑ concentration and the phytoplankton community in the bay were strongly affected by the nitrate+nitrite concentration. On the other hand, a difference was observed with the seasonal variations in the size-fractionated Chl-ɑ concentrations and species composition of the phytoplankton community at both stations during summer. It is possible that the difference was caused by the nutrient concentrations.
Nagatsura-ura Lagoon is the main sea area producing oysters in Miyagi, Japan. However, there is little information regarding the nutrient supply process or the primary production process that maintains high productivity of oysters in the lagoon. Seasonal changes in nutrients and chlorophyll a concentration (as an indicator of primary production) were surveyed in the lagoon and the surrounding area from 2014 to 2015. Chlorophyll a concentration was found to be high from late March to September, and especially high from June to September. The amount of river water containing rich nutrients( NO3–N and SiO2–Si) flowing from the Shin-Kitakami River into the lagoon increased from March to September due to increases in both the river flow rate and the frequency of northeast winds blowing from the mouth of the river to the lagoon. Moreover, the release rate of certain nutrients( PO4–P and NH4–N) from the sediment of the seafloor increased due to rising water temperature and decreasing dissolved oxygen concentrations at the bottom layer of the lagoon after June. It was concluded that the high chlorophyll a concentration over the long period from March to September is due to the supply of nutrients from the river and from the sediment in the lagoon.
We proposed a new forecasting method of annual (May–December) landing of immature fresh skipjack tuna at Kesennuma port (northeastern Japan) in May, whose annual landing of fresh skipjack tuna has been the largest in Japan over the past 22 years. Monthly landing of fresh skipjack tuna by commercial size categories at Kesennuma during May–December and Boso Katsuura (middle-eastern Japan, hereafter called Katsuura) during January–April, 2002–2018 were converted into putative seasonal cohorts: large-sized (XA group), middle-sized (BC group), and small-sized (DE group), by using estimated growth curves for these groups. In order to examine relationships among annual/seasonal landing of these groups, considering an observed declining trend in fishing effort of Japanese skipjack pole-and-line fishery, which is the major fishery for fresh skipjack tuna, original landing data for each group was detrended as year-specific residuals from a fitted linear regression line. The BC group, the most abundant among the three groups, migrates from subtropical waters to northeastern waters off Japan, i.e., Kuroshio-Oyashio Transition Zone (KOTZ), during early summers. There was a statistically significant positive relation between detrended annual landing of BC group at Kesennuma and detrended landing during January–April at Katsuura. The XA group is supposed to represent DE groups in a previous year according to recently estimated growth curves, and it migrates to KOTZ in summers. There was also a statistically significant positive correlation between detrended annual landing of XA group and that of DE group in a previous year at Kesennuma. The best multiple regression model suggested that annual landing of the BC group at Kesennuma in a given year can be forecasted with two explanatory variables: landing of BC group at Katsuura during January–April and mean March sea surface temperature around the southern Izu Islands, south of Katsuura, in the same year.