Diel and Seasonal Activity of the American Eel (Anguilla Rostrata) in the Upper St. Lawrence River Determined By Acoustic and Radio Telemetry

American eels are catadromous, inhabiting fresh, brackish, and oceanic waters; many factors have affected its decline, including a unique migratory life cycle. Little is known about diel and seasonal activity of the mysterious eel. Acoustic and radio telemetry in the upper St. Lawrence River has been used to fill this important knowledge gap of this now rare and endangered species. The study area (23 km²) is mainly shallow but has a 13-m-deep channel; 82% is <5 m and has supported a historically productive eel fishery. Submersible receivers and mobile locating were used to study 28 transmitter-implanted eels for 1½ years. Greatest monthly diel activity occurred in July between dusk and dawn and was influenced by water temperature and changes in length of the nocturnal period. The greatest monthly movement occurred when eels moved to (October, 29%, 4.2 km) and from (April, 17%, 2.5 km) the wintering grounds, where they were dormant (December-March), and only 3% (0.4 km) movement per month from June to September. A water temperature of 10°C in a changing thermal regime elicits the greatest seasonal movement, substantiated by commercial hoop-net catch. Telemetry confirms that this elusive burrowing fish has specific, consistent diel and seasonal activity patterns.