Evaluating Smolt Stocking Tradeoffs for Landlocked Atlantic Salmon: Imprinting-Potential, Dam Passage, Avian Predation, and Genetic Strains.

Heim, Kurt¹, Jonah L. Withers¹, Theodore Castro-Santos², ¹U.S. Fish and Wildlife Service, Lake Champlain Fish and Wildlife Conservation Office, Essex Junction, VT, ²U.S. Geological Survey, Leetown Science Center, S. O. Conte Anadromous Fish Research Center, Turners Falls, MA

Whereas stocking salmon smolts far upstream may increase imprinting to suitable spawning sites, the resulting longer migrations can lead to higher migratory failure. Ideal stocking locations thus reflect a tradeoff between imprinting potential and survival. We assessed to-lake survival of two broodstocks of landlocked Atlantic salmon smolts (one tolerant to thiamine deficiency, one with higher genetic diversity) stocked at two sites (27 km from mouth and upstream of two dams, downstream of all dams at 16 km) in the Winooski River, VT. Surprisingly, we found no difference in to-lake survival among release locations, broodstocks, or years (2021, 2022) but a strong influence of tagger (survival = 0.39 tagger B, 0.20 tagger A) and a positive effect of fish size. There was limited delay at dams, generally high dam passage (0.94 - 0.99), but a notable decline in survival in a 1-km reach below a dam. Both release groups - those that had passed the dam and those stocked in the lower river (i.e., no dam passage) had very low survival here that is in part due to avian predation. Here, stocking production smolts (22,000) attracts avian predators to the dam tailrace. We recovered 42/331 (12%) of our transmitters at Gull and Double Crested Cormorant nesting colonies, confirming avian predation is a major source of mortality. Our study demonstrates impacts of predators immediately after stocking events, underscores the importance of considering stocking and tagger effects in passage studies, and represents an example of focused research to guide tradeoff evaluation for smolt stocking.