Opening the Door: Alosa pseudoharengus (Alewife) Passage through Tide Gates in Two Adjacent Rivers on the Chignecto Isthmus, Bay of Fundy, Canada

We assessed Alosa pseudoharengus (Alewife) upstream passage during 2016-2019 spawning runs using radio frequency identification (RFID) tracking through a periodically submerged and a constantly submerged top-hinged tide gate in the LaPlanche and Missaquash rivers, respectively, on the Chignecto Isthmus, Canada. First detection of individuals tagged in previous years (i.e., returnees) revealed synchronous upstream run timing in both rivers during 2015 and 2018. Greater delays of migrants attempting passage through the periodically submerged tide gate prompted temporary removal of 1 flapper door during 2016-2019 runs. During a 7-day period following door removal, the number of individuals passing per day increased 4- to 6-fold during 2016-2017 and less than 2-fold during 2018-2019. More than 80% of returnees passed the periodically submerged gate following door removal, decreasing mean delay to less than that of migrants attempting passage of the constantly submerged gate. Most individuals passed the periodically submerged gate during mid-flood daily tide phases and none passed during low-tide outflow due to high water velocities. Passage of the constantly submerged gate occurred during all tide phases, with 77% passing during ebb and low tides. The constantly submerged tide gate provided greater windows of opportunity, resulting in relatively shorter delays compared to the periodically submerged tide gate. Long-term monitoring in this study highlighted the effectiveness of temporary and/or adaptive mitigation when addressing issues in river connectivity and fish passage.