Abstract
Each year, hundreds of thousands of farmed Atlantic salmon escape from fish farms into the wild. Some of these escapees enter freshwater, and manage to interbreed with native populations. To hinder further genetic introgression in native populations, the use of sterile triploid salmon within commercial aquaculture is being examined. However, if triploid escapees migrate into freshwater, they may still have ecological impacts on local populations. In the present study, we used microsatellite DNA genotyping to determine the ploidy of 3794 farmed escapees captured in 17 Norwegian rivers in the period 2007–2014. Although a previous study has reported an average of 2 % triploids in Norwegian fish farms during this exact period, here, we only observed 7 (0.18 %) triploids among the escapees captured in freshwater. In addition, we identified three trisomic escapees. For the triploids where the within-river capture location was determined, they were only observed in the lower reaches and not on the spawning grounds. It is concluded that propensity for triploid Atlantic salmon to migrate into freshwater following escape from a fish farm is significantly lower than for normal diploid salmon escapees. Therefore, commercial production of triploids should not only be seen as an effective way of stopping genetic introgression, it will also significantly reduce the numbers of escapees entering rivers, which in turn limits ecological interactions and potential disease transmission.
Highlights
The Atlantic salmon (Salmo salar L.) aquaculture industry has a number of environmental challenges, of which farmed escapees and their potential genetic interaction with wild conspecifics represents one of the most significant (Taranger et al 2015)
Sample drop-out is normal in all genetic data sets, and based upon previous studies where we have genotyped triploid salmon both in a pedigree based material (Solberg et al 2013), and samples that were cross-validated against non-genetic ploidy determination methods (Glover et al 2015), it is concluded that sample drop-out is random and not connected to ploidy
A total of 7 (0.18 %) triploid, and 3 (0.08 %) trisomic farmed salmon escapees were observed among the samples captured in rivers (Tables 1, 2)
Summary
The Atlantic salmon (Salmo salar L.) aquaculture industry has a number of environmental challenges, of which farmed escapees and their potential genetic interaction with wild conspecifics represents one of the most significant (Taranger et al 2015). Hundreds of thousands of farmed salmon escape into the wild While many of these disappear never to be seen again (Skilbrei et al 2015a), some enter freshwater and spawn with wild salmon (Lura and Saegrov 1991; Saegrov et al 1997). While triploid salmon are sterile, they can still develop secondary sexual characteristics, and it has been demonstrated that farmed triploid males can successfully coax normal diploid wild females to mate with them (Fjelldal et al 2014). This could lead to many of the eggs being unfertilized depending upon the numbers of wild males present during spawning. If there are large numbers of triploid farmed salmon on the spawning grounds, genetic interactions could be replaced by ecological interactions instead
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