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Hwang, S.D.; Lee, T.W.; Choi, I.S., and Hwang, S.W., 2014. Environmental factors affecting the daily catch levels of Anguilla japonica glass eels in the Geum River estuary, South Korea. The Japanese eel, Anguilla japonica, is a catadromous fish whose recruitment stage glass eels enter estuaries and rivers after being born offshore in the ocean. The daily variation in glass eel catch was analyzed in relation to environmental factors that affect upstream migration. The first glass eels of the 2003 season were caught in the Geum River estuary in early March, when the water temperature exceeded 5°C. The catch rates peaked in mid-March, when the temperature reached 6-7°C. Few glass eels were caught when the temperature was above 14-16°C in early May. Water temperature may have triggered the timing of glass eel immigration. The catch fluctuated with tidal cycle, since the catch level was relatively high during each new and full moon throughout the sampling period. A generalized additive model indicated that the glass eel catch was significantly related to tidal range, temperature, and salinity. To estimate the effect of minor factors, such as salinity and wind speed, on catch rate, we calculated the standardized residuals of catch rate. The standardized residuals of catch rate did not show any correlations to freshwater inflow or cloud cover, but occasionally increases were seen as a result of drops in salinity and 2-3 days after strong winds, suggesting that these factors may also trigger upstream migration of glass eels in the estuary. [ABSTRACT FROM AUTHOR]