Abstract
The genetic impact of farmed fish escaping aquaculture is a highly debated issue. However, non-target species, such as cleaner fish used to remove sea lice from farmed fish, are rarely considered. Here, we report that wild corkwing wrasse (Symphodus melops), which are transported long distances to be used as cleaner fish in salmon farms, escape and hybridize with local populations. Recently, increasing numbers of corkwing wrasse have been reported in Flatanger in Norway, north of its described distribution range, an area heavily relying on the import of cleaner fish from Skagerrak. Using genetic markers identified with 2bRAD sequencing, we show that, although the Flatanger population largely is a result of a northward range expansion, there is also evidence of considerable gene flow from southern populations in Skagerrak and Kattegat. Of the 40 corkwing wrasses sampled in Flatanger, we discovered two individuals with clear southern genotypes, one first-generation hybrid, and 12 potential second-generation hybrids. In summary, we provide evidence that corkwing wrasse escape from fish farms and hybridize with local populations at the leading edge of an ongoing range expansion. Although the magnitude and significance of escapees warrant further investigation, these results should be taken into consideration in the use of translocated cleaner fish.
Highlights
Marine species display a range of levels of genetic divergence among populations, from panmictic species to species with marked genetic structure, as a consequence of reduced gene2018 The Authors
We investigate the origin of wild corkwing wrasse captured in Flatanger, amid salmon farms where wrasses are currently used as cleaner fish and rely heavily on the import of wrasse from southern Norway and Sweden
Our objective is to investigate whether the wild corkwing in Flatanger represents: (i) the leading edge of an ongoing northward range expansion [24], (ii) escaped wrasse from aquaculture with origin from Skagerrak and Kattegat or (iii) a mix of both
Summary
Marine species display a range of levels of genetic divergence among populations, from panmictic species to species with marked genetic structure, as a consequence of reduced gene2018 The Authors. Transferring individuals between spatially separated 2 populations that are genetically distinct is likely to result in genetic changes to native populations. Such changes could involve shifts in allelic composition, loss of genetic variation, erosion of local adaptation and/or breakdown of population structure [2]. Human-mediated releases of genetically different individuals to native populations are increasingly common. There are many examples from open-pen farming of salmonids, where escapees have hybridized with local river populations, leading to genetic swamping and reduced fitness [4,5]
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