Health of Wild Atlantic Salmon ( <i>Salmo salar</i> ) Smolts Along Coastal Migratory Routes in Newfoundland, Canada With and Without Salmonid Aquaculture as Assessed Using Sentinel Cages

ABSTRACT Declines in wild Atlantic salmon populations in the North Atlantic with migratory routes adjacent to commercial salmonid aquaculture operations in coastal marine environments have raised concerns about the potential for direct or indirect effects this industry may have on local wild populations. To better understand potential impacts on wild Atlantic salmon in Newfoundland, Canada, we used six moored sentinel cages stocked with wild Atlantic salmon smolt to compare smolt health in an area with Atlantic salmon and steelhead trout aquaculture (Conne River/Bay d'Espoir) and without (Campbellton River/Notre Dame Bay). Average survival for wild Conne River smolt stocked in sentinel cages in Bay d'Espoir was 64.1% but varied from 35.5% to 84.8%. Average survival for wild Campbellton River smolt stocked in sentinel cages in Notre Dame Bay was higher at 73.5%, less variable and ranged from 69.7% to 78.1%. Differences in environmental variables and body condition loss among sentinel cages showed no significant association with survival. Health assessments revealed no external parasites, including the absence of sea lice on the smolt. Kidney and heart tissue samples from both locations were negative for infectious salmon anaemia virus. Prior to stocking, histological analysis revealed no epicarditis in the Conne River smolt and mild epicarditis in 6.7% of the Campbellton River smolt. Following deployment, histological analysis revealed mild epicarditis in 5.7% of the Conne River smolt and 7.7% of the Campbellton River smolt. Results suggest that local stressors, where present, are likely transient and spatially variable, highlighting the need to consider exposure duration and microhabitat influences when interpreting sentinel cage experiments. This research supports the use of sentinel cages in Newfoundland, and likely elsewhere, as a method for evaluation of wild smolt health along coastal migratory routes, whereas outcomes must be interpreted relative to short‐term site‐specific environmental conditions.