EVALUATION OF IMPACT ON DIADROMOUS FISHES BY CHANGES IN BOTH SEA AND RIVER WATER TEMPERATURE

<p> Changes in water temperature (WT) due to climate change occurs in sea and river areas, therefore, the impact on diadromous fishes that use both as habitats is concerning. However, river ecological impact assessments considering WT in both sea and river have not yet been conducted. In this study, public WT data in sea and river estuaries in class A river systems in Japan was obtained to estimate the gradients of these long-term changes. Using this data, a statistical model for habitat probability was built with a classification tree model for three species of representative diadromous fishes widely, but unevenly, distributed in the Japanese archipelago. The importance of habitat condition variables, including WT, was evaluated for each species using the random forest model. In addition, the probability of habitat in the future WT environment was assessed by assuming the obtained WT fluctuation gradient in the sea and river areas and compared it between the rivers. Consequently, the gradient based on the observation was more significant in the river estuary area (+3.93℃/100 yrs) than in the sea area (+1.29℃/100 yrs). Sea WT was selected as the variable most strongly influencing the estimated population probability for Rheopresbe kazika and Gymnogobius petschiliensis, however, the river WT was selected for Gymnogobius opperiens, which has a more river-like lifecycle. Assuming the observation-based WT gradient, WT in the river will be a more substantial control factor for the target habitat due to greater increases in WT in the river.</p>