Fate of translocated American eel (Anguilla rostrata) in the lower Ottawa River and passage behavior at a multichannel barrier

Abstract

American eel (Anguilla rostrata) is considered endangered under the IUCN's red list.Hydropower facilities are considered a significant threat to American eel, impacting both the outmigration of adults and upstream migration of juveniles. To overcome upstream passage issues, juvenile eels may be trapped and transported around barriers as a mitigation strategy, though few studies have evaluated the efficacy of this approach. To understand the fate of transported eels, we monitored posttranslocation movements in a 110-km reach of the Ottawa River bounded by two hydropower facilities: Carillon Hydroelectric Generating Station (lower barrier) and Chaudière Falls Hydroelectric Facility (upper barrier). Additionally, we assessed the approach behavior of eels that reached the upper barrier, a multichannel facility, to assess potential fishway locations. To assess these objectives, 40 juvenile eels (440-640 mm) were implanted with acoustic transmitters and were transported and released either just upstream ($6 km) of the lower barrier or just downstream of the upper barrier ($2 km), approximately, 60 and 166 km from the capture site, respectively. Over the three-month study period, 78% of tagged eels remained upstream of the lower barrier. Of the nine eels that returned downstream of the lower barrier, seven were from the downstream release site; however, the proportion of eels that returned downstream of the lower barrier did not differ significantly between release sites. One eel passed the upper barrier despite no existing fish-passage structures. At the upper barrier, most eels visited just one of the five channels, suggesting that more than one passage structure may be necessary to allow eels entering different channels to pass upstream of the barrier. Findings from this work will help inform passage efforts for American eel, particularly in the Ottawa River where eel populations have declined severely from their historic abundance.

A contemporary estimate for the abundance of juvenile American Eel Anguilla rostrata attempting to migrate past a barrier in the Ottawa River

“…The model used estimates of survival over the study period (36%) that were believed to be highly conservative, meaning that actual survival over the study period would likely be higher. For example, there was a survival rate of 100% and a retention rate of 78% in eels translocated from the St. Lawrence River to the reach upstream of the Carillon Generating Station on the Ottawa River (Twardek, Stoot, Cooke, Lapointe, & Browne, 2021). A low survival estimate (such as the one used here) could apply, however, if high passage failure rates led to high rates of emigration.…”