Prolonged lake use in Atlantic salmon juveniles prior to seaward migration is associated with higher apparent mortality and faster estimated growth

Lakes can present both risks and opportunities for migratory fishes, yet their role in the life histories of anadromous salmonids remains understudied. Here, we used Passive Integrated Transponder (PIT) telemetry to examine lake‐use patterns and their associations with survival and growth in juvenile Atlantic salmon Salmo salar L. over 3 years in the Burrishoole catchment, western Ireland. A fixed antenna positioned at the transition between river and lake habitats allowed us to estimate the timing of lake entry into Lough Feeagh – the largest freshwater lake in the system through which the fish must migrate on their way to the sea. Fish entering the lake earlier took longer to reach downstream sea‐entry traps, a pattern consistent across years. Earlier entry was associated with a lower probability of detection at the downstream traps, which may reflect seasonal timing itself (e.g., seasonal variation in food availability or predation), cumulative mortality associated with prolonged lake residence (e.g., increased exposure to predators or other lake‐specific risks) or intrinsic differences among early‐ and late‐migrating fish (e.g., physiological attributes). Fish also exhibited substantially greater growth between upstream river exit and downstream recapture than inferred growth rates prior to lake entry, indicating that the lake can provide important feeding and growth opportunities during downstream migration. These findings are consistent with a potential trade‐off between growth benefits and survival costs during lake passage, although this trade‐off remains to be demonstrated directly. Overall, our results show that Lough Feeagh is more than a migration corridor for Atlantic salmon smolts and highlight the need to understand how seasonal timing, lake residence and individual characteristics jointly shape the costs and benefits of lacustrine habitat during anadromous migration.