Adaptation or plasticity as response to large scale translocations and harvesting over a climatic gradient in the marine ecosystem?

Norway leads the world production of farmed Atlantic salmon (Salmo salar); however, sea lice infestation remains a major challenge for the salmon aquaculture industry. In the last decade, the use of wild-caught wrasses as cleaner fish has emerged as a cost-effective alternative against sea lice infestation. A unique feature of the newly developed wrasse fishery in Norway is the fact that thousands of wild fish caught in warmer regions in the southern Skagerrak coast are annually moved (translocated) to colder regions in northern Norway. The impact of the intensive fishing pressure on wrasses and the adaptive response of translocated wrasses to the new environmental conditions encountered in northern regions represented major gaps of knowledge demanding urgent research. The project "Adaptation or plasticity as response to large scale translocations and harvesting over a climatic gradient in the marine ecosystem?" aimed at gaining knowledge of the relative importance of plasticity (ability to cope with environmental variability) versus local adaptation (changes in gene composition over generations) in marine fishes in response to climatic (e.g. global warming) and human stressors (e.g. translocations and harvesting). Using corkwing wrasse (Symphodus melops, Linnaeus, 1758), grønngylt in Norwegian, as model species the project applied genomics (entire set of DNA) and transciptomics (complete set of RNA) approaches under natural and experimental conditions. The genomic characterization of corkwing wrasse populations in Norway showed limited mixing between Skagerrak and western wrasses and suggested the existence of several populations adapted to specific environmental conditions along the Norwegian coast. Population sizes in western fjords are larger than in the Skagerrak. Meanwhile, genomic profiles of wrasses from fishing and non-fishing marine protected areas showed no evidence of adaptive divergence in response to fishing pressure. In addition to the genomic characterization of natural populations, the project incorporated an experimental common garden set-up to further investigate reproductive fitness and the adaptive response of corkwing wrasse to temperature. At the beginning of the project, a total of 285 adults from Skagerrak and 401 adults transported alive from the west coast of Norway were kept at the experimental facilities of IMR at Flødevigen and used as broodstock to produce the offspring for the downstream analyses. During the spawning season, the broodstock were gently stripped mixing the eggs and milt to rear the offspring in captivity at different temperature regimes resembling those registered during the spawning season of the species in Norway. Eggs exposed to cold water (12 °C) showed a significant delay in hatching time, 372 h, compared to those placed at warmer water (18 °C), 144 h. In an additional experiment, a set of 298 juveniles collected from the basin were transferred to three tanks indoor and kept for one-month period at 12, 15 and 18 °C. After this period, all individuals were examined for pedigree analysis and classified as "South" when both parental fish were from the Skagerrak, "West" when both parents were original from the west coast or "Hybrids" when each parental fish was from different origin. All juveniles were grouped according to their parental origin and water temperature and subjected for transcriptomic (RNA) analysis. The transcriptomic characterization revealed significant differences in gene response between the three groups of offspring exposed at different temperature and suggested that native wrasse may present advantage fitness compared to translocated wrasse populations. Arguably, the main concern regarding the use of large numbers of wrasses by the salmon aquaculture industry is to understand whether translocate wrasses who escaped from the nets can adapt and interbreed with native wrasses. To answer this question, we traced the pedigree of 651 offspring collected from the basin and confirmed the presence of offspring from all three origins; South, West and Hybrid origin. These findings confirmed that in case of escapees, translocated wrasses are able to adapt and interbreed with native populations. In summary, using corkwing wrasse as model species, this project has contributed to clarify major issues related to the adaptive potential of coastal fish species under global warming and intensive human pressures.

The use of wrasses as cleaner fish has become a cost-effective alternative for the biological control of sea lice infestation in salmon aquaculture. However, while catches and translocations of wrasses have rocketed in the last years, our knowledge on fun damental ecological and evolutionary aspects of the species remains largely unknown. This project aims at applying state-of-the-art approaches in Next-Generation Sequencing (NGS), i.e. genomics and transcriptomics, under natural and experimental condition s to gain fundamental scientific understanding regarding the relative importance of plasticity versus local adaptation in marine fishes in response to climatic (e.g. global warming) and human stressors (e.g. translocations and harvesting). Corkwing wrasse (Symphodus melops, Linnaeus, 1758) is a unique model species to accomplish this kind of studies. First, its sedentary behavior and genetic structure point towards the existence of locally adapted populations. Second, translocations are accomplished on ad ult wild fish. Since escapees or intentional releases are common, this species offers excellent possibilities to investigate interactions between native and "foreign" stocks, removing the selective effects of domestication under a hatchery-released scheme . Third, in contrast to other wrasses, the draft genome of corkwing wrasse is currently being sequenced and completed shortly, opening huge possibilities to deepen into our understanding of adaptive fitness in the marine realm. Hence, genomic scans of Sca ndinavian populations and gene expression profiling under controlled temperature gradients will: 1) characterize the genomic resources of Scandinavian stocks; 2) identify those genes/genomic regions regulating adaptation to temperature; 3) discriminate na tive versus translocated fish; 4) compare their reproductive success and fitness performance; and 5) insight into the effects of harvesting pressures on genes/genomic regions associated to life-history traits.