Fish transfer nutrients as they migrate, enhancing the primary productivity of the destination ecosystem. Although nutrient subsidy by a migratory fish species is temporally limited, nutrient subsidy may be prolonged in rivers where diverse fishes migrate up sequentially. In this study, we documented the seasonal migration pattern of diverse fish species in an inflowing river of Lake Biwa and investigated how that supported prolonged nutrient subsidy.
To document the seasonal dynamics of fish migration, nutrient subsidy, and response of primary producers, we conducted biweekly surveys from February to December 2021 in the Chinai river, one of the major inflow rivers to Lake Biwa.
As result, migration by six fish species was observed for eight months from April to November. During the migration period, river ammonium and SRP concentration increased about 15.3 times and 1.5 times, respectively, and the density of attached periphyton increased about 5.0 times in the areas inhabited by fish. The increases in nutrient subsidies and periphyton biomass of migratory fish seen in this study lasted much longer than previous studies of only one migratory fish species. The results of this study suggest that nutrient subsidy by migratory fish may be longer term and wider spread than previously recognized.