Atlantic salmon (Salmo salar) migrations in a large hydropower reservoir and the regulated Saint John River

Abstract

My research focused on evaluating the impacts of the large Mactaquac Generating Station (MGS) reservoir on the migrations of the endangered Outer Bay of Fundy Atlantic salmon (Salmo salar). Salmon respond to flowing waters to determine the direction, timing, and speed of their migrations. A large reach of the Saint John River (SJR) was impounded by the MGS in 1968, transforming the habitat from a freeflowing river to a lacustrine environment with altered and slower flow. I examined all migratory lifestages of Atlantic salmon in the SJR using acoustic telemetry, including pre-smolts, smolts, adults, and post-spawned adults (kelts), as they navigated these environments. Migration rates were compared between the lentic MGS reservoir and the more lotic reaches upriver and downriver of the MGS to assess whether migration is delayed in the reservoir. Nearly all of the tagged salmon experienced migratory delay within the reservoir (medians: smolts 1.3-6.4 d, kelts 3.5-10.5 d, adults 1.5-5.7 d) due to suppressed migration rates (medians: smolts 5.0-13.3 km d[-1 superscript] vs. 15.4-29.3 km d[-1 superscript], kelts 4.4-8.9 km d[-1 superscript] vs. 14.9-36.8 km d[-1 superscript], adults 8.5-20.1 km d[-1 superscript] vs. 19.3-46.9 km d[-1 superscript]). Migration success through the reservoir was higher for downstream migrants (smolts > 81 % and kelts > 82 %) than upstream migrants (adults 47 %). Recommendations informed by these findings with the aim of aiding recovery of this endangered population are given to hydropower and fisheries managers, including: i) changing the spill regime to allow a greater proportion of downstream migrants the option of spillway passage since all but a few smolts and even the earlier kelt migrants were sometimes forced to pass via turbines; ii) constructing a downstream surface-bypass facility which is more economically feasible than increasing spill and is supported by the observed variable passage timing; iii) allowing the free-swim of downstream migrants through bypasses in comparison to a trap-and-haul strategy that was not found to increase survival of smolts; and iv) maintaining trap-and-haul operations for adults migrating upstream of the MGS due to the high proportion of fallbacks and increased energy expenditure from superfluous movements in the reversed direction to the intended migration observed in the reservoir.