Physiological and Endocrine stress in Atlantic salmon (Salmo Salar): Validation of alternative matrices for Evaluating fish welfare in Captivity - focus on less invasive matrices of mucus and feces

Abstract

The rapid growth of the aquaculture industry has underscored the critical importance of ensuring fish welfare in captivity. This research aimed at validating less-invasive methods for assessing stress in Atlantic salmon (Salmo salar) by utilizing alternative matrices, such as mucus and feces. This approach provides both ethical and practical advantages for monitoring fish welfare. In the first study, we evaluated the use of feces and mucus to assess stress responses in Atlantic salmon, complemented by gene expression analysis to understand the underlying physiological mechanisms and impacts. Stress was induced through crowding in reduced water volume. Additionally, three different water flow rates (0.5, 1-, and 1.5 body lengths per second) and multiple tissues were applied to examine the impact of swimming speed on fish stress. Samples of blood, mucus, and feces were collected at multiple time points: pre-stress, and 1, 6-, and 24 hours post-stress. The results demonstrated that cortisol levels in mucus along with cortisol (and/or its metabolites) in feces increased in response to rising plasma cortisol levels, confirming the effectiveness of these matrices for stress evaluation. including steroidogenic acute regulatory protein (star), Cytochrome P450 17A1 (cyp17a1), 11β-Hydroxysteroid dehydrogenase (11β-hsd2), and steroid 5α-reductase 1 (srd5a1). These changes were tissue-dependent and influenced by water flow, underscoring its critical role in fish stress experiments and responses. In the second study, we investigated the metabolic impacts of stress, focusing on hepatic glucose metabolism. We examined the mRNA expression patterns for key enzymes involved in glycolysis and gluconeogenesis, including glucose-6-phosphatase (gp6ase), pyruvate kinase (pk), glycogen phosphorylase (glp), 6-phosphofructo-2-kinase (6pf2k), and glucokinase (gk). Although variability was observed in mRNA levels, the results underscored the need for frequent sampling and larger sample sizes to accurately capture enzyme activity patterns. Furthermore, water flow emerged as a significant factor influencing metabolic responses. Additionally, we observed that lactate is actively utilized either as an energy source or as a substrate for glycogen replenishment during post-stress recovery periods. In conclusion, this research highlights the potential of less-invasive methods, such as mucus and feces, for stress monitoring in aquaculture. By minimizing harm to fish, these methods offer practical solutions for improving welfare. Furthermore, the findings emphasize the importance of optimizing water flow in both aquaculture practices and experimental designs.