Wild individuals of the minami-medaka (Oryzias latipes) San-in Subpopulation were collected from freshwater and brackish-water areas in the Hii River system in eastern Shimane Prefecture and measured in standard length (SL). Population dynamics were compared between stations using cohort analysis. The population was divided into several cohorts at stations where many individuals were collected during the study period. It was revealed that individuals with an average SL of < 20 mm in early spring comprised primary breeding cohorts, while various cohorts enabled them to have a long reproductive season. Little difference was observed between the San-in Subpopulation of O. latipes, the Eastern Subpopulation of O. latipes, and kitano-medaka (O. sakaizumii) in Japan during the recruitment season of the under-yearling population and the decreasing season of the larger fish (yearling fish). The size distribution of the San-in Subpopulation during the winter period was slightly broader than that of O. sakaizumii inhabiting the Sea of Japan side. The cohort structure of the San-in Subpopulation occasionally showed yearly fluctuations and differences among stations within the freshwater and brackish-water areas. Within the two observed years, the movement of the freshwater and brackish-water populations was similar; however, the growth rate from spring to early summer varied from year to year in the brackish-water population.

　To examine the possibility that neonicotinoid insecticides in river water may affect arthropods, water samples were collected once a month, both upstream and downstream, throughout the year along the Sendai River in Tottori Prefecture, Japan. These samples were then analyzed for concentrations of neonicotinoid insecticides. Dinotefuran was detected both upstream and downstream over the entire period. Downstream, dinotefuran was the most abundant species, except in June 2022. Dinotefuran had the highest concentration in August at 24 ng L^-1 upstream and 134 ng L^-1 downstream. The total imidacloprid concentration, converted from the molecular weight of each neonicotinoid, was 254 ng L^-1 downstream, with an average of 44 ng L^-1. The results of the monthly survey of neonicotinoid concentrations suggest that arthropods, such as aquatic insects (which are food for fish), may have decreased in the lower reaches of the Sendai River in August due to acute toxicity and from June to September due to chronic toxicity.