Reinforcement of American eel Anguilla rostrata populations in eastern Lake Ontario tributaries, Ontario, Canada: revisited a decade later.

A dramatic decline in American eel Anguilla rostrata abundance led to the species being assessed as Endangered in Ontario (Canada) and the closure of fisheries. As part of efforts to recover populations, four million eels (glass eels and elvers) were batch marked and released over a five-year period (2006-2010) into the upper St. Lawrence River and Lake Ontario. Over 2019-2022, we repeated an earlier large-scale electrofishing survey of eastern Lake Ontario tributaries done to assess the distribution and survival of the released eels. A decade later, we collected two hundred and fifteen eels from 19 sites along four rivers near the release site. Eels were absent from three tributaries occupied in 2010-2011. Along the Trent River, navigational locks have likely permitted migration past seven large dams previously assessed as probably impassable. Growth of translocated eels was variable among rivers. As well, many eels were still smaller than yellow eels naturally migrating into Lake Ontario. Our study indicates high, long-term post-release survival in these rivers. However, timing of out-migration and potential for successful spawning migrations remain unknown.