An extensive common-garden study with domesticated and wild Atlantic salmon in the wild reveals impact on smolt production and shifts in fitness traits.

Øystein Skaala,Anne Grete Sørvik,Reidar Borgstrøm,Francois Besnier,Michael Møller Hansen,Kevin Alan Glover,Britt Iren Østebø,Bjørntorgeir Barlaup,Eirik Normann

doi:10.1111/eva.12777

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 up‐ and 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.

Highlights

Atlantic salmon (Salmo salar) has been the subject of research for over a century, generating knowledge of its extensive biological and life‐history variation within and among populations
The results demonstrate that spawning and hybridization of domesticated salmon in native populations can (a) reduce the production of wild salmon smolts and wild adult abundance, through resource competition in freshwater, (b) reduce the total adult abundance through a combination of resource competition in freshwater and reduced marine survival of domesticated salmon, and (c) influence the recipient population's phenotypic and phenological characteristics
When taken together with data from the low number of studies previously conducted in the natural environment, we demonstrate that domestication introgression and hybridization may lead to less productive and more fragile wild populations

Summary

| INTRODUCTION

Atlantic salmon (Salmo salar) has been the subject of research for over a century, generating knowledge of its extensive biological and life‐history variation within and among populations. As a result of nearly 50 years of directional selection regimes for traits of economic importance, inadvertent domestication selection, and relaxed natural selection, domesticated salmon display a wide variety of genetic differences to wild conspecifics (Glover et al, 2017) Of these differences, the documented lower survival of domesticated salmon offspring in the natural environment (Fleming et al, 2000; McGinnity et al, 2003; Skaala et al, 2012) provides some of the most compelling evidence that introgression of domesticated escapees in native populations is likely to lead to negative consequences. In the fourth study (Jonsson & Jonsson, 2017), releases of hatchery‐produced domesticated, hybrid, and wild salmon smolts from the River Imsa revealed lower marine survival, increased straying rates, and generally larger size at age in comparison with wild salmon In addition to these experimental studies, Bolstad et al (2017) investigated life‐history differences between naturally recruited wild and domestication‐admixed salmon in 62 Norwegian salmon populations. The overall aim of the study was to provide extensive up to date data on the type and magnitude of genetic differences in a range of key fitness‐related traits between domesticated and wild salmon in the natural environment—including both the freshwater and the poorly studied far marine stage

| MATERIALS AND METHODS

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Findings

| DISCUSSION