Reproductive success of smolt-to-adult supplementation and wild Atlantic Salmon in an experimental stream under allopatric and sympatric conditions

Objective The reproduction success of Atlantic Salmon Salmo salar from a smolt-to-adult supplementation (SAS) program relative to that of wild Atlantic Salmon was assessed. Methods Atlantic Salmon from for smolt-to-adult supplementation were released into a fenced-off natural stream on the Miramichi River, New Brunswick, Canada, for 4 years either in sympatry with wild Atlantic Salmon or under allopatry. Reproduction success was assessed via parentage analysis of young-of-the-year progeny. Results At maiden spawning, SAS adults were significantly smaller in both length (by 13.2% for females, 16.5% for males) and weight (by 22.5% for females, 28.9% for males) than maiden-spawning wild fish of the same age. Under sympatric conditions, the number of successful SAS female spawners and their number of spawning partners were significantly lower than those for wild females. However, when SAS females were in allopatry, no differences from wild salmon under sympatric conditions were found with respect to the number of successfully spawning females or number of reproductive partners. The number of progeny per SAS female was significantly lower in both sympatric and allopatric release years than the number of progeny from wild females. The number of successful SAS male spawners was significantly lower than that of their wild counterparts in one of the two studied sympatric years. Also, the number of successful SAS males in allopatry was lower in an overall comparison with wild male salmon in sympatry, although success of males in allopatric years is not independent of the success of SAS females, restricting the comparability of males in sympatry and allopatry. For successfully spawning SAS males, no significant difference in the number of progeny was found relative to wild fish. Conclusion Our findings indicate that differences in reproduction success occur between SAS and wild Atlantic Salmon, which most likely stem from the captive-rearing environment. Theoretically, the increased survival rate of smolts that are reared to maturity in captivity could compensate for the reduced SAS offspring numbers. In an effort to continue assessing the benefits and risks of SAS programs as a salmonid population recovery tool, future work should focus on (1) better understanding how to improve the captive-rearing environment to augment the spawning success and reduce genetics risks of SAS Atlantic Salmon, (2) assessing repeat-spawning rates and success for SAS fish, and (3) remaining vigilant for unanticipated negative population effects.