Tracking hatchery and wild Atlantic salmon ( <i>Salmo salar</i> ) smolt during river exit and early marine migration to assess survival, predation risk and migration speed

Significant declines in Atlantic salmon (Salmo salar) populations across their range have prompted an increased use of hatchery supplementation programmes to support population recovery. However, concerns persist regarding the effectiveness of these programmes due to limited understanding of how hatchery-reared salmon perform post-release in natural river systems compared to their wild counterparts. This study examined survival, predation risk, migration speed, river exit timing and early ocean migration patterns of hatchery-reared and wild Atlantic salmon smolts in the LaHave River, Nova Scotia, where populations are among the most dramatically reduced in Canada. Using acoustic telemetry, we tracked the movements of 100 smolts (50 hatchery origin, 50 wild origin) during their freshwater downstream migration and into the ocean. Compared to wild smolts, hatchery smolts exhibited significantly lower survival during river outmigration, with a third dying before reaching lower river sections. Predation was a major driver of mortality, with hatchery smolts experiencing over four times higher predation risk than wild smolts following release. Hatchery smolts had significantly slower migration speeds in the upper river and lake sections, and they also exhibited significantly greater reverse movements near the head of tide compared to wild smolts. However, river exit timing and early ocean migration routes were comparable between smolts' origins, indicating potential synchrony and environmental cue retention for hatchery smolts. Critical empirical evidence of early marine migration for the salmon population was captured, with both hatchery and wild smolts following the expected eastward migration along the southern coast of Nova Scotia. Taken together, these results indicate that hatchery smolts experience higher predation risk and less efficient migration than wild smolts, likely due to predator naivety and captive rearing conditions. This study also identified a predation hotspot in Wentzells Lake, the one lake smolts migrate through during river exit, highlighting an opportunity to increase smolt survival by releasing hatchery smolts downstream of high-risk predation areas. Implementing targeted release strategies and innovative captive rearing conditions may help mitigate post-release challenges for hatchery smolts.