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

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.

Model-based evaluation of the genetic impacts of farm-escaped Atlantic salmon on wild populations

“…Model-based projections following this escape event using cohort-based estimates of survival suggest negative impacts on population productivity and genetic integrity (Sylvester et al 2019). These results are consistent with evidence of genetic changes in wild Norwegian salmon populations, which show levels of introgression as high as 47% (Karlsson et al 2016), reductions in productivity (Fleming et al 2000, Skaala et al 2019, and changes in key life history traits (Bolstad et al 2017). In Atlantic Canada, Atlantic salmon aquaculture ex pansion has been proposed for several regions, in cluding those with threatened or at-risk wild salmon populations.…”

“…Simulations utilized estimates of feral fry and parr survival calculated from genetic analysis of individual cohorts following an escape event in southern Newfoundland in 2013 , Sylvester et al 2019). These estimates of survival are lower than most previous estimates of relative survival of feral parr (McGinnity et al 1997, Fleming et al 2000, McGinnity et al 2003, Skaala et al 2019, and increasing survival in freshwater has been shown to increase both genetic and demographic impacts (Sylvester et al 2019). We simulated the population consequences of invasion over a 50 yr period in a wild population of 500 individuals with the proportion of invaders varying from 0 to 100% of that of the wild population annually.…”

Timing is everything: Survival of Atlantic salmon Salmo salar postsmolts during events of high salmon lice densities

“…In River Guddal, situated in the central part of the Hardangerfjord, about 30 km North of River Etne, the smolt migration of Atlantic salmon has its peak in mid‐to‐late May (Skaala et al, 2019). If the smolt‐migration in the River Etne follows the pattern in River Guddal, most of the natural smolts probably migrate through the fjord system marginally before, or partially overlapping with the period where we observed the deadly high levels of lice in 2014.…”

Are bivalves susceptible to domestication selection? Using starvation tolerance to test for potential trait changes in eastern oyster larvae

“…Also, hatchery culture imposes a genetic bottleneck relative to wild cohorts and domestication selection further strengthens genetic drift, reducing genetic diversity to a degree that may have a fitness cost under natural conditions [8,10,14]. Evolution of domestication traits is increasingly under study to understand their rate of change and fitness impacts [5–8,21]. Not surprisingly, phenotype and performance data for oysters is mostly restricted to commercially important traits during grow-out from planted spat to adult (market size).…”