Partitioning survival during early marine migration of wild and hatchery-reared Atlantic salmon (Salmo salar L.) smolts using acoustic telemetry

The marine migration of Atlantic salmon (Salmo salar) has been prioritised for research internationally as populations of S. salar have declined significantly throughout the species' range. The main objectives of this study were to use acoustic telemetry to partition survival during the early migration phase, investigate potential causes of mortality and establish diurnal and tidal influences on movements. In 2017 and 2018 wild (n = 49) and hatchery (n = 81) S. salar smolts were tagged with acoustic transmitters. Migration was monitored through a brackish tidal lake, which discharges through a short estuary into northeast Clew Bay in Ireland situated in the northeast Atlantic. Partitioned survival through each area was similar for both wild and hatchery smolts and both groups followed the same migration routes, travelling along the main current out of Clew Bay. Total survival was high within the brackish lake (wild = 98% +/- 1.21 and hatchery = 100%) and estuary (wild = 98% +/- 1.01 and hatchery = 89% +/- 9.28) compared to the marine environment (wild = 67% +/- 4.39 and hatchery = 66% +/- 7.91). Leaving the brackish lake during an ebb tide and entering the marine environment during daylight increased the probability of survival through the early marine period. The majority of smolts transited through the study area during ebb tides. Migration of hatchery smolts occurred mainly during hours of daylight while wild smolt migration showed no diel patterns. High mortality rates during the initial stages of the marine migration have consequences for the persistence of salmon populations and should be addressed through regionally tailored management measures and conservation efforts aimed at increasing the resilience of salmon stocks.