Investigating genetic change in wild populations: modelling gene flow from farm escapees

Fish-farm escapees threaten the genetic integrity of wild populations. Because significant 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 commercial 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 multiple 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.