Abstract
Fish-farm escapees threaten the genetic integrity of wild populations. Because sig- nificant genetic differences often occur among aquaculture strains, gene flow from multiple farmed sources may compromise the ability to detect genetic change in wild populations. Here, we investigate this situation by simulating genetic change based upon data from 9 microsatellite loci in 4 wild Atlantic salmon populations receiving variable mixtures of escapees from 5 commer- cial Norwegian strains. As expected, neutral markers detected genetic change in wild populations when gene flow came from a single and distinct farmed strain. However, the genetic change detected in the wild population was significantly lower when gene flow was simulated from mul- tiple farm strains simultaneously ('concealing effect'). Although the degree of concealing varied among the wild populations, in one of the cases, no significant genetic differentiation was detected when 20% effective migration was simulated from 2 or more farmed strains for up to 10 generations. While individual admixture analysis succeeded in detecting introgression, it was nevertheless strongly underestimated. Where gene flow is expected to originate from multiple farmed sources, analysis of selectively neutral genetic markers is thus likely to underestimate the true level of genetic introgression.
Highlights
Fish-farm escapees represent a significant threat to the genetic integrity of wild populations where domesticated and wild forms co-exist
Moving from the simulations of gene flow from single farmed strains into the Vosso population (Fig. 2A,B) to the more complicated situation where gene flow was simulated from multiple farmed strains simultaneously (Fig. 2C−F), we observed that, despite the fact that each additional strain included in the migrating pool was more distinct from Vosso than the previous one, the resultant genetic change did not increase past the level caused by migration from the genetically most similar strain, DOM-2.96, alone (Fig. 2A,F)
This indicates a concealing effect, where genetic change detected after genetic introgression from a heterogeneous pool of migrants appears smaller than the genetic changes detected after genetic introgression from the same number of migrants originating from 1 single farm
Summary
Fish-farm escapees represent a significant threat to the genetic integrity of wild populations where domesticated and wild forms co-exist (http://genimpact. imr.no/). Results of field studies conducted in both Irish and Norwegian rivers have demonstrated that offspring of farmed salmon display significantly reduced survival when compared to offspring of native wild salmon (McGinnity et al 1997, 2003, Fleming et al 2000). Considering these differences together with the fact that Atlantic salmon displays considerable population genetic struc-
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