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Abstract:
Interactions between domesticated escapees and wild conspecifics represent a threat
to the genetic integrity and fitness of native populations. For Atlantic salmon, the recurrent
presence of large numbers of domesticated escapees in the wild makes it necessary
to better understand their impacts on native populations. We planted 254,400 eggs
from 75 families of domesticated, F1‐hybrid, and wild salmon in a river containing upand
downstream traps. Additionally, 41,630 hatchery smolts of the same pedigrees
were released into the river. Over 8 years, 6,669 out‐migrating smolts and 356 returning
adults were recaptured and identified to their families of origin with DNA. In comparison
with wild salmon, domesticated fish had substantially lower egg to smolt survival
(1.8% vs. 3.8% across cohorts), they migrated earlier in the year (11.8 days earlier across
years), but they only displayed marginally larger smolt sizes and marginally lower smolt
ages. Upon return to freshwater, domesticated salmon were substantially larger at age
than wild salmon (2.4 vs. 2.0, 4.8 vs. 3.2, and 8.5 vs. 5.6 kg across sexes for 1, 2, and 3
sea‐winter fish) and displayed substantially lower released smolt to adult survival (0.41%
vs. 0.94% across releases). Overall, egg‐to‐returning adult survival ratios were 1:0.76:0.30
and 1:0.44:0.21 for wild:F1‐hybrid:domesticated salmon, respectively, using two different
types of data. This study represents the most updated and extensive analysis of
domesticated, hybrid, and wild salmon in the wild and provides the first documentation
of a clear genetic difference in the timing of smolt migration—an adaptive trait presumed
to be linked with optimal timing of entry to seawater. We conclude that spawning and
hybridization of domesticated escapees can lead to (i) reduced wild smolt output and
therefore wild adult abundance, through resource competition in freshwater, (ii) reduced
total adult abundance due to freshwater competition and reduced marine survival
of domesticated salmon, and (iii) maladaptive changes in phenotypic traits.