Post-release survival, movement and marine migration of smolt-to-adult supplemented Atlantic Salmon

Atlantic salmon (Salmo salar) populations are declining across much of their range, prompting the use of supplementation strategies, such as smolt-to-adult supplementation (SAS), to enhance adult returns. However, little is known about how captivity while maturing to adulthood influences post-release survival, behaviour and migration of salmonids. This study evaluated SAS as a conservation approach by using acoustic telemetry to assess survival, freshwater movement, river exit timing, acceleration activity and marine migration of SAS salmon released into Clyburn Brook, Cape Breton Highlands National Park, Canada. Wild adult salmon from the nearby Chéticamp River were also tagged to provide a regional comparison for overwintering and marine movements of SAS salmon, but differences in spawning state at tagging and population specific life-history traits (e.g., spawn timing, marine distribution) limited direct behavioural comparisons. SAS salmon had 53% freshwater mortality and exhibited elevated acceleration activity, frequent high-energy bursts, lack of upriver movement and premature river exit. Notably, SAS salmon held in captivity for 2 years had higher survival than those held for 1 year, but they exhibited little upstream movement, raising concerns of whether any spawning activity occurred. In contrast, SAS salmon held for 1 year had higher mortality, but more moved upstream toward presumed spawning areas prior to final detection, suggesting potential spawning opportunities. Wild Chéticamp River salmon had a large marine distribution route through the Gulf of St. Lawrence and north to Labrador, whereas SAS salmon were detected regionally in the Atlantic Ocean off Cape Breton, including St. Ann's Bank Marine Protected Area and Sable Island. Contrasting marine distributions likely reflect intrinsic population differences rather than effects of SAS rearing alone. Our findings highlight the complex effects of adult captivity on post-release survival and movement in Atlantic salmon, and underscore the need for further research to refine SAS management approaches, including consideration of captivity duration, feeding strategies and release locations.