Drifting seaweeds are utilized by some species of juvenile fishes. We collected drifting seaweeds and associated fishes off Kagawa Prefecture, Seto Inland Sea, monthly from June 2017 to June 2019. A total of 206 clumps of drifting seaweeds, comprised of 15 species, were collected. Average clump weight in each month ranged from 0.05 to 5.97 kg/clump, increasing from winter, reaching a peak in spring, and decreasing in summer and autumn. Sargassum horneri, S. muticum and S. fulvellum dominated in winter and spring, and Zostera marina did in summer and autumn. A total of 6,053 individual fish, belonging to 24 species, were collected. The density (number of individuals per kg of drifting seaweed) ranged from 0.1 to 209.0 and generally was low in winter and high in autumn. The numerically dominant species were Rudarius ercodes (54.7%), Sebastes cheni (21.7%), S. ventricosus (11.0%), Petroscirtes breviceps (4.4%) and S. schlegelii (2.8%). Rudarius ercodes was predominant between summer and autumn. Sebastes cheni and S. ventricosus were predominant species in spring, but the peak of their appearance in number was two months earlier in S. cheni than in S. ventricosus.
In the last 20 years, the population of the alien hard clam Mercenaria mercenaria in Tokyo Bay, Japan has grown. This alien species is expected to have some impacts on ecosystems, however, there are no reports on the ecosystem engineering effects of the hard clam in Tokyo Bay. In this study, we deployed a field manipulation experiment by using enclosures (including hard clam treatment, manila clam treatment, and control) to evaluate the effects of the hard clam on the physical characteristics of the sediments, microphytobenthos, and benthos communities in a sandy tidal flat. In the hard clam treatment, ORP at a depth of 5 cm and the median grain size of subsurface sediment, and chlorophyll a and pheo-pigment content of all sediment layers were significantly higher than in the other treatments at 4 and 6 weeks after the start of the experiment. However, there was no effect on the density and species composition of benthic diatom, meiobenthos and macrobenthos. These results suggest that the bioturbation of the hard clam has an ecosystem function that increases the permeability of the sediment and the biomass of benthic diatoms in Tokyo Bay.
The discoloration of Pyropia thalli resulting from a shortage of nutrients is a serious problem for the Pyropia industry and has often occurred during cultivation in the Seto Inland Sea, Japan. Therefore, a new method was developed to fertilize areas of Pyropia cultivation. Pyropia thalli growing-nets placed in a calm area were enclosed by a fence, using a Kagawa-style “nori skirt” made from polypropylene sheets and cultivation nets. Nutrients were supplied to the area enclosed by the fence. The concentrations of nutrients in the enclosed area were kept higher than those in the open area (which acted as a control). The color of Pyropia thalli was improved by this new method and the quality of the product was well recognized at market. The salinity in the area enclosed by the fence was lower following rainfall, suggesting that nutrients were effectively supplied to the cultivated thalli not only with fertilizer but also rainwater, because rainwater contains a high concentration of dissolved inorganic nitrogen.
We collected 28 and 10 Flavobacterium psychrophilum strains isolated from dead and diseased ayu Plecoglossus altivelis altivelis, respectively, from several rivers of Kochi Prefecture. A total of 38 strains were identified from 10 gene regions by using the on/off switch assay, polymerase chain reaction-restriction fragment length polymorphism, and multilocus sequence typing. Genotypes of F. psychrophilum isolated from dead ayu were mostly G-C/A/S/52 (22/28) whereas genotypes of F. psychrophilum isolated from diseased ayu were G-C/A/S/52 (6/10) and A-C/A/R/45 (4/10). The number of outbreaks of bacterial cold-water disease (BCWD) caused by F. psychrophilum in ayu in the rivers of Kochi Prefecture increased in 2015 and peaked in 2018. Most F. psychrophilum isolates from dead ayu in 2015-2018 were identified as having genotypes of G-C/A/S/52 (22/24). These results suggest that the outbreak of BCWD in ayu in the rivers of Kochi Prefecture was caused by a genotype of G-C/A/S/52.