Stress, Immunonutrition and Welfare in Lumpfish (Cyclopterus lumpus)

Lumpfish emerged as a promising ally for the Atlantic salmon industry, as a solution to the widespread sea lice outbreaks. Yet, in parallel with the rise of lumpfish farming, came the ethical and welfare questions, and the health of deployed lumpfish became a concern. This thesis set out to investigate the stress physiology, health and welfare indicators and their intersection with nutrition, as a strategy to increase stress resilience. Through a series of acute and chronic stress experiments, we simulated a common aquaculture stressor. The results show that cortisol can be a reliable indicator for stressed lumpfish, displaying a modest (~ 2-fold) increase 30 minutes post stress while secondary markers (e.g. glucose and lactate) showed minimal changes. In contrast, chronic stress exposure had a more profound impact on lumpfish lipid reserves and nutritional status. Despite an attenuation of cortisol response late in the trial, the most frequently stressed group exhibited reduced growth performance, depleted lipid reserves and altered nutritional profiles. Chronic stress decreased lysine and branched chain amino acid concentrations, indicating metabolic stress and favouring of gluconeogenic programs. Based on these metabolic insights, a final study using nutritional intervention assessed diets with different branched-chain amino acids and lysine inclusion levels. The tailored diets helped to mitigate to an extent the metabolic distress. They also enhanced antiprotease activity and helped normalize plasma amino acid profiles. Additionally, high cataract incidence was observed consistently across all trials, regardless of stress exposure or dietary treatment, pointing to broader nutritional and husbandry inadequacies earlier in the production cycle. Overall, this thesis highlights the nuanced lumpfish stress physiology, establishing cortisol as a critical stress biomarker for this species. It also provides evidence that even a mild stressor when repeated, can alter the demand for specific essential amino acids. The findings support the stance for improved welfare-centric nutritional strategies, which must go hand in hand with improvements in general husbandry.