Atlantic salmon (Salmo salar) smolt passage at the Tobique-Narrows Generating Station, Tobique River

Abstract

My research assessed the efficiency of a hydropower bypass for endangered Outer Bay of Fundy (oBoF) Atlantic salmon (Salmo salar) pre-smolts and smolts from the Tobique River, New Brunswick. Juveniles out-migrating from the Tobique River must navigate three large hydropower stations on the Wolastoq | Saint John River (W|SJR), including the Tobique Narrows Generating Station (TNGS) near the Tobique River confluence. No dam on the W|SJR provided dedicated downstream passage until the construction of a bypass at TNGS in 2017. Accordingly, the first objective was to evaluate downstream passage efficiency, specifically bypass use by radio-tagged juveniles (pre-smolts and smolts). The second objective assessed the migration speed and estuary arrival of acoustically-tagged juveniles released into the TNGS bypass (free-swim strategy; FS) compared to juveniles released downstream of the Mactaquac Generating Station, MQGS (trap-and-haul strategy; TH). Overall, bypass efficiency was low; 10.2% (n = 9/88) of radio-tagged juveniles used the bypass to descend, whereas 70.5% (n = 62/88) descended through the turbines, and 19.3% (n = 17/88) descended through the spillways. The cumulative apparent survival to the estuary was higher for the FS strategy (51%) than the TH strategy (37%) for autumn-tagged pre-smolts. While autumn-tagged FS smolts arrived at the estuary later in the spring than their TH counterparts – due in part to experiencing delays in the Mactaquac reservoir – the majority nevertheless arrived there in May. However, when these strategies were implemented during the spring smolt migration, only fish from the TH group were observed reaching the estuary, arriving at similar times as FS smolts that began migrating the autumn prior. Together, these findings suggest that a season-specific management approach may improve smolt survival and estuary entry timing. These results suggest that Tobique smolt output from the Tobique dam – in the absence of removing the dams – would be maximized by allowing free-swim in the autumn and using trap-and-haul in the spring. It is recommended that TNGS pre-smolts be allowed to free-swim through the full length of the W|SJR, however, in the spring, the trap-and-haul strategy is recommended for the smolts captured at TNGS bypass.