Genomic and physiological studies on thermal tolerance in ballan wrasse (Labrus bergylta)

Abstract

Ballan wrasse (Labrus bergylta) is increasingly used in salmon farming as a biological control agent and a sustainable alternative to chemical treatments against sea lice. Field observations have suggested that its delousing performance and overall welfare are affected by seasonal changes in water thermal conditions, especially during winter, wherein dormancy occurs. Despite its wide use as a cleaner fish, the thermal biology of this species remains poorly understood. Thus, this thesis aimed to investigate the physiological and genomic basis of thermal tolerance in ballan wrasse. Chapter 2 of this thesis determined the thermal tolerance limits of ballan wrasse and found the intrinsic thermal tolerance to be 7.9 to 16.8°C, with dormancy occurring at temperatures below this range. The winter thermal conditions also prompted physiological adaptations that conserve energy as demonstrated in Chapter 3. This included reallocation of energy towards storage in the liver, reducing basal transcriptomic activity, and relying on inflammation as a form of immunity. However, prolonged exposure (~2.5 months) to low temperatures triggered tertiary stress responses, including elevated circulating levels of primary and secondary stress markers, as well as stunted growth. Through genome-wide association studies described in Chapter 4, this thesis found that distinct genomic regions regulate growth under summer and winter conditions as well as for thermal tolerance, highlighting the polygenic nature of these traits. Several growth- and thermal tolerance-related single nucleotide polymorphisms were identified. Moreover, this thesis demonstrated that both seasonal growth and thermal tolerance have moderate heritability values, indicating that genetic selection for these traits will yield genetic gains. Overall, this thesis contributes novel insights into the species thermal tolerance and provided the basis for management and production strategies in order to improve seasonal delousing performance.