Large-scale migration patterns of silver American eels from the St. Lawrence River to the Gulf of St. Lawrence using acoustic telemetry

Downstream migration of silver American eels (Anguilla rostrata) from the St. Lawrence system was examined using acoustic telemetry. One hundred and thirty six silver American eels were tagged, and their passage was recorded using fixed acoustic arrays covering a 420 km distance along the St. Lawrence River and Estuary. Eighty-nine percent of the tagged eels were detected. All migrant eels (111) exhibited unidirectional and downstream movements, but the migration was not completed in one continuous direct movement. High individual variability in migratory longitudinal profiles was documented as well as in individual speed with no apparent relation to river discharge or morphological traits. Migration speed increased over the season. Our observations demonstrated that migrating silver American eels are largely nocturnal and demonstrated the use of noctur- nal, ebb tide transport to leave the estuary. With 44 additional eels tagged and released in the maritime estuary, escapement of 180 silver American eels from the Gulf of St. Lawrence system was monitored along a 125 km acoustic line that entirely covered Cabot Strait in 2011. Surprisingly, only four of the tagged eels were recorded escaping the Gulf of St. Lawrence. Resume : La migration de devalaison de l'anguille d'Amerique argentee (Anguilla rostrata) a ete etudiee grâce aun systeme de telemetrie acoustique. Cent trente-six anguilles argentees ont ete marquees et leur passage ou presence a ete enregistre grâce a ` des lignes acoustiques fixes couvrant une distance de 420 km le long du fleuve et de l'estuaire du St-Laurent. Quatre-vingt neuf pourcent des anguilles marquees ont ete detectees. Toutes les anguilles migrantes (111) montrent des mouvements unidirection- nels vers l'aval mais la migration n'est pas menee d'une seule traite. Une tres grande variabilite individuelle au niveau des profils de migration a ete demontree, ainsi qu'au niveau des vitesses de migration sans que ces dernieres ne puissent etre reliees ni au debit du fleuve ni ades traits morphologiques. La vitesse de migration augmente au cours de la saison. Nos observations suggerent l'utilisation du transport de jusant nocturne pour quitter l'estuaire. Avec 44 anguilles supplementaires marquees et relâchees en estuaire maritime, l'echappement de 180 anguilles argentees du systeme St-Laurent a ete suivi grâce aune ligne acoustique couvrant les 125 km du detroit de Cabot en 2011. Etonnamment, seulement quatre des anguilles marquees sortant du Golfe du St-Laurent ont ete detectees.