Effects of extreme environmental variation on the physiological responses of European seabass, Dicentrarchus labrax

Global climate change is causing significant challenges for aquaculture productivity and fish welfare. Despite climate-change challenges, for most aquaculture species, physiological responses to extreme temperature events while at different salinities remain unknown. To predict future susceptibility to extreme climatic events, a detailed understanding of extreme temperatures and precipitation effects on European seabass, Dicentrarchus labrax is essential in different ways: 1) fish responses to environmentally realistic extreme low and high-temperature events, 2) fish performance during extreme ambient heatwaves and cold spells while inhabiting at precipitation induced different salinities, 3) Can dietary manipulation techniques be used to mitigate temperature stress in fish? The overall objective of my Ph.D. was to assess the effects of extreme temperatures and precipitation events on growth performance, metabolic, physiological, and molecular stress responses in European seabass, Dicentrarchus labrax. Moreover, nutritional mitigation options were also evaluated if fish could fare better during climate change-induced stress. This doctoral thesis is a cumulative work where the core scientific outcome is divided into four chapters. Each chapter representing published scientific articles is finally entrenched in the general introduction and discussion. To simulate extreme cold and heatwave events on fish, European seabass were exposed to 8 °C, 16 °C, 24 °C, 32 °C for variable periods (10-30 days). To understand extreme precipitation-induced osmotic stress, fish were exposed to different environmentally realistic salinities (3, 6, 12, and 30 PSU).  Combined impacts of stressful salinities and temperatures were also tested under different temperature and salinity combinations. Fish growth, physiology, immunity, and molecular stress response parameters were repeatedly evaluated during the experimental periods to understand the thermal and osmotic stress responses over the study period. Results indicated that climate-change-induced extreme temperature and precipitation stress alters growth and physiology, and bio-physiological processes in fish from genes to cells. A concurrent relationship among growth, physiological, and molecular responses has been observed in fish exposed to environmentally realistic stressful temperatures and salinities. The overall findings show that European seabass are subjected to significant physiological stress during extreme ambient cold and warm events while in stressful osmotic environments. None of the repeatedly evaluated parameters indicated acclimation capacity to cope with tested salinities during extreme warm and cold exposure. However, during simulated extreme ambient warm and cold events, European seabass acclimatized at intermediary salinities (6 to 12 PSU) showed relatively better physiological plasticity compared to fish at low (~3 PSU) and full-strength (~30 PSU) saline water. Finally, to mitigate stress on fish exerted from extreme temperatures and precipitation events, a range of dietary supplements were tested to see if these ingredients can ameliorate stress during stress events. Results demonstrated that fish fed on Vitamin C, E, propolis, and phycocyanin supplemented diets could fare better during extreme warm and cold temperature events exposure, suggesting a possible insight into future aquaculture management options in the changing climate.