Ballan wrasse (L. bergylta) has become a popular choice of biological control against sea lice infestation in salmon farms in the UK and in other salmon-producing countries in Europe. They are stocked in cages with salmon where they stay throughout the ongrowing period of up to 2 years. This consequently subjects ballan wrasse to seasonal changes in environmental conditions specifically water temperature which affects their delousing performance and welfare (Brooker et al, 2018). During winter, ballan wrasse has been observed to enter dormancy, exhibiting stunted growth and increased susceptibility to diseases (Erkinharju et al 2020). However, the basic understanding of thermal tolerance limits and tolerance range of ballan wrasse is lacking. In the present study, we determined the thermal tolerance limits of ballan wrasse and analysed the molecular changes following acclimation to different temperature and exposure to their thermal limits.
Materials and Methods
Hatchery-bred juvenile ballan wrasse were acclimated at a range of water temperature (6°C, 10°C or 14°C) for one week. Critical Thermal Methodology (CTM) was then performed wherein ballan wrasse from each group were individually placed in a challenge container and subjected to either increasing or decreasing water temperature until fish exhibit loss of equilibrium, indicating their upper (CTMax) and lower (CTMin) thermal tolerance limits. Gill tissues were collected from fish before and after they were subjected to CTM and analyzed for changes in transcriptome and methylation analyses.
Results and Discussions
CTM has shown that thermal tolerance of ballan wrasse shifts depending on acclimation temperature (Fig. 1). CTMax and CTMin values are lowest in fish acclimated to 6°C and highest in those acclimated to 14°C. Consistently, the thermal tolerance range was positively correlated with acclimation temperature. No prior estimate is available for the thermal tolerance limits of ballan wrasse although it has been shown that decrease in temperature is associated with significant decline in metabolic performance and aerobic scope which limits their tolerance to environmental conditions (Yuen et al 2019).
Preliminary results from transcriptome analysis show that acclimation temperature has an effect on the overall gene expression profile in the gills of ballan wrasse. Differential gene expression analysis between fish exposed to CTMax and CTMin is ongoing. In addition, DNA methylation data is being analysed to determine the epigenetic effects of acclimation temperature and exposure to thermal tolerance limit to ballan wrasse. This study will provide relevant information to improve our understanding of the physiological responses associated to temperature changes in ballan wrasse.
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Erkinharju, T., Dalmo, R.A., Hansen, M. & Seternes, T. (2021), Cleaner fish in aquaculture: review on diseases and vaccination. Reviews in Aquaculture, 13: 189-237. https://doi.org/10.1111/raq.12470
Yuen, J. F., Dempster, T., Oppedal, F., & Hvas, M. (2019) Physiological performance of ballan wrasse (Labrus bergylta) at different temperatures and its implication for cleaner fish usage in salmon aquaculture. Biological Control, 135:117-123. https://doi.org/10.1016/j.biocontrol.2019.05.007.