Environmental correlates of freshwater migration of elvers of the American eel in a Rhode Island brook

The freshwater migration of elvers of the American eel (Anguilla rostrata) wascontinuously monitored at the freshwater interface and three locations in the lower stretches of aRhode Island brook in 1980. Most elvers arrived at the freshwater interface in May and wereincompletely pigmented. Although their initial arrival coincided with a large increase in watertemperature, subsequent captures at the interface did not correlate with any measuredenvironmental variable including water temperature, time of day, and tidal condition. Elversappeared to delay upstream migration at the interface while experiencing behavioural andphysiological changes. Two hundred meters above the interface, elvers were caught primarily atmidday and capture records showed a significant 14.8-d periodicity corresponding to, but laggingbehind, that of spring tides. The onset of upstream migration may have been triggered bychanges 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-dperiodicity. Migration in fresh water appeared highly stochastic; light and water temperatureshowed a weak positive correlation with some capture rates, and rain occasionally influencedmigration at one location by altering river flow. The behavioural changes that elvers undergoduring their entry into fresh water, coupled with the apparent influence of local hydrographicconditions on migration, may explain discrepancies among prior studies. The behaviour of elversof the American eel is similar to that of other species of freshwater eel