Environmental Correlates of the Freshwater Migration of Elvers of the American Eel in a Rhode Island Brook

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Abstract
The freshwater migration of elvers of the American eel Anguilla rostrata was continuously monitored at the freshwater interface and three locations in the lower stretches of a Rhode Island brook in 1980. Most elvers arrived at the freshwater interface in May and were incompletely pigmented. Although their initial arrival coincided with a large increase in water temperature, subsequent captures at the interface did not correlate with any measured environmental variable including water temperature, time of day, and tidal condition. Elvers appeared to delay upstream migration at the interface while experiencing behavioral and physiological changes. Two hundred meters above the interface, elvers were caught primarily at midday and capture records showed a significant 14.8‐d periodicity corresponding to, but lagging behind, that of spring tides. The onset of upstream migration may have been triggered by changes in water chemistry caused by the intrusion of estuarine water during high spring tides. Further upstream, captures persisted through October and lacked both diurnal and 14.8‐d periodicity. Migration in fresh water appeared highly stochastic; light and water temperature showed a weak positive correlation with some capture rates, and rain occasionally influenced migration at one location by altering river flow. The behavioral changes that elvers undergo during their entry into fresh water, coupled with the apparent influence of local hydrographic conditions on migration, may explain discrepancies among prior studies. The behavior of elvers of the American eel is similar to that of other species of freshwater eel.

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