Abstract
Translocation and introduction of non‐native organisms can have major impacts on local populations and ecosystems. Nevertheless, translocations are common practices in agri‐ and aquaculture. Each year, millions of wild‐caught wrasses are transported large distances to be used as cleaner fish for parasite control in marine salmon farms. Recently, it was documented that translocated cleaner fish are able to escape and reproduce with local wild populations. This is especially a challenge in Norway, which is the world's largest salmon producer. Here, a panel of 84 informative SNPs was developed to identify the presence of nonlocal corkwing wrasse (Symphodus melops) escapees and admixed individuals in wild populations in western Norway. Applying this panel to ~2000 individuals, escapees and hybrids were found to constitute up to 20% of the local population at the northern edge of the species’ distribution. The introduction of southern genetic material at the northern edge of the species distribution range has altered the local genetic composition and could obstruct local adaptation and further range expansion. Surprisingly, in other parts of the species distribution where salmon farming is also common, few escapees and hybrids were found. Why hybridization seems to be common only in the far north is discussed in the context of demographic and transport history. However, the current lack of reporting of escapes makes it difficult to evaluate possible causes for why some aquaculture‐dense areas have more escapees and hybrids than others. The results obtained in this study, and the observed high genomic divergence between the main export and import regions, puts the sustainability of mass translocation of nonlocal wild wrasse into question and suggests that the current management regime needs re‐evaluation.
Highlights
Moving organisms outside their natural boundaries can cause diverse effects on the ecosystems (Atalah & Sanchez-Jerez, 2020)
If the introduced populations are genetically distinct from the local ones, hybridization and admixture can lead to altered population structure (Glover et al, 2012), lower effective population size and reduced fitness through outbreeding depression (Blakeslee et al, 2020; Glover et al, 2017; Laikre et al, 2010)
We developed and implemented a panel of informative SNPs to quantify the proportion of escaped and hybridized corkwing wrasse with a southern origin in middle and western Norway, where translocated cleaner fish are extensively used for parasite control
Summary
Moving organisms outside their natural boundaries can cause diverse effects on the ecosystems (Atalah & Sanchez-Jerez, 2020). The wrasse species utilized as cleaner fish on Norwegian fish farms inhabit shallow rocky areas along the coast from the Mediterranean Sea in the south, to the Norwegian coast in the north In recent years, their abundance has shifted northwards and diminished in the south, which has been suggested to be due to increased seawater temperatures (Knutsen et al, 2013). Recent studies suggest that populations south of the genetic break in south-western Norway are growing faster, maturing earlier, having a shorter life span and a lower proportion of sneaker males to nesting males (Halvorsen et al, 2016). Faust and colleagues showed in their study (2018) that translocated corkwing wrasse can escape and hybridize with local populations at the northern edge of the species current distribution limit in Flatanger, Norway. We addressed this by first developing an informative panel of genome-wide SNPs, and using this panel to analyse ~2000 corkwing wrasse collected from aquaculture- dense regions in western Norway and potential source populations in Skagerrak-Kattegat
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