Environmental Factors Regulating the Seaward Migration of European Silver Eels (Anguilla anguilla)

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Abstract
We examined the connection between descent of European silver eels (Anguilla anguilla) and water discharge, water temperature, turbidity, photoperiod, light intensity, and moon phase based on 10 yr of field data and field experiments. The eel migration takes place between August and December, and mean water temperature for July–August and mean water discharge for August–October explained 91% of the total variation in the start of the yearly silver eel run. Low mean water temperatures during July–August and high mean water discharge during August–October resulted in an early start of the yearly silver eel run, whereas high mean water temperature and low water discharge gave the opposite effect. There was no significant correlation between the duration of the entire yearly silver eel run and the environmental variables investigated. But the duration of a part of the run, e.g. number of days from 5 to 25% cumulative eel descent, was significantly correlated with mean water temperature and increase in water discharge for the period in question (R2 = 0.84). Maximum silver eel descent was at a water temperature of 9 °C. Few silver eels descended at temperatures below 4 °C or above 18 °C. The migration speed of transplanted silver eels in the River Imsa was correlated with water discharge and water temperature (R2 = 0.88). Water discharge alone explained 85% of the total variation in migration speeds. The recapture rate of silver eels transplanted within the River Imsa was highest at 9 °C and decreased at higher and lower temperatures (R2 = 0.95). Illumination of 20 lx upon the river reduced the descent of silver eels.