"A cleaner break" - Genetic analysis identifies two major groups of ballan wrasse (Labrus bergylta) within Scandinavia and evidence of phenotype derived divergence

For a long time, sea lice have represented the major financial and fish welfare challenge of the salmonid farming industry. In the late 1980s, the use of parasite-eating cleaner fish, first and foremost wrasses (family Labridae), was put into practice. The use of wrasse declined in the 2000s as reliance on chemotherapeutants grew. However, sea lice quickly evolved a high level of resistance to chemotherapeutants, and therefore, the salmonid farming industry needed to come up with alternative control measures. This included, among other things, a resurge in the use of cleaner fish. These cleaner fish are primarily harvested from wild populations, and often translocated over large distances, such as from Sweden to mid-Norway. This provides at least two potential environmental challenges. First, possible overharvest in the source populations, and second, genetic interactions with recipient populations via inadvertent translocation (i.e., losses or deliberate release of non-local wrasse from farms into the surrounding coast). To ensure sustainable use of cleaner fish in aquaculture, a better understanding of the population genetic structure of the key species, including ballan wrasse, is needed. In this thesis, an extensive population genetic analysis based on newly developed single nucleotide polymorphisms (SNPs), and previously existing microsatellites (MSATs), was conducted on >1000 individuals collected from 18 locations along the Scandinavian coast as well as one location in Galicia, Spain. Some of these samples were also morphotyped as plain or spotted. The following main results and conclusions were obtained: 1. Within Scandinavia, ballan wrasse are characterized by two distinct genetic groups, a north-western (NW) and south-eastern (SE) component, with very little within-cluster variation. The clusters are divided by a clear genetic break located in the south-western part of Norway; an area associated with a long continuous stretch of sandy substrate. It is concluded that the two genetic groups are likely to be shaped by differences in colonization histories and are held partially isolated by a combination of restricted adult movement, restricted benthic egg dispersal (i.e., non-pelagic), and habitat discontinuities in south-western Norway. 2. Less genetic diversity was observed within samples from Scandinavia than in the Galician sample. Additionally, Galician ballan wrasse were more similar to ballan wrasse from NW than SE Scandinavia. Furthermore, NW Scandinavian ballan wrasse were genetically more similar to the plain morphotype wrasse than to the spotted morphotype wrasse from Galicia. In contrast, this was not the case for SE Scandinavian ballan wrasse where no difference in the magnitude of genetic differentiation was observed with any of the Galician morphotypes. These results suggest that founder effects and genetic bottlenecks have characterized the colonization process of Scandinavia from southern latitudes, and that NW Scandinavia was colonized with ballan wrasse of both spotted and plain morphotypes from southern latitudes prior to southern Norway and western Sweden. This route of colonization is also in agreement with glacial retreat models after the last glacial maxima (~21 kya), that propose an eastward melting pattern, with western Norway being liberated from the ice prior to eastern parts of the Scandinavian peninsula. 3. Very distinct genetic differentiation was identified between the spotted and plain wrasse in Galicia. This was not observed in Scandinavia where the phenotypic variation was also less distinct. It is therefore concluded that in Galicia, spotted and plain ballan wrasse are genetically distinct and are thus may represent sympatric sub-species as has been suggested earlier. However, the mechanisms and processes that could maintain the clear genetic differences between morphotypes in Galicia, for example assortative mating, are at best cryptic or non-existent in Scandinavia where these two potential sub-species appear undifferentiated. From these findings it is proposed that care should be taken, and subsequently changes should be made to the current management of ballan wrasse in Scandinavian waters to minimize possibilities of future impact of escaped fish on local populations.Masteroppgåve i biologiBIO399MAMN-BI