Precision of behavior of migrating juvenile American eels (Anguilla rostrata) utilizing selective tidal stream transport

Abstract

A three-year study was conducted to determine how precisely the semidiurnal vertical migrations of juvenile American eels ( Anguilla rostrala ) are timed to slack water, to determine the current velocities during which the migrations occur, and to measure the length of time the eels spend in the water column on the flood tide. Eels were sampled on 20 dates in the Penobscot River estuary, Maine, USA, with paired plankton nets in a series of short tows which encompassed the flood tide. Vertical profiles of water current velocity, salinity, and temperature were made concurrently. There was a statistically significant trend in the timing of the eels' ascents off the bottom. The ascents occurred after slack water in salinity-stratified conditions (≥1), clustered around slack water in partially stratified conditions (≥1 during final 1–2 h of flood tide), and occurred before or clustered around slack water in tidal freshwater conditions (>1). Descents to the bottom relative to the time of slack water were not significantly different in the three water conditions. Further, the length of time eels spent in the water column on the flood tide was not significantly different in the three water conditions. These findings are more consistent with the hypothesis that the eels use endogenous rhythms rather than exogenous cues to time their sojourns.