While the impact of salinity on euryhaline fish species has been well-documented, there remains a significant knowledge gap concerning the effects on hatchery-derived brown trout (Salmo trutta), particularly regarding their physiology and swimming performance. To address this gap, 30 freshwater reared S. trutta from a landlocked population were divided into 4 treatment groups, acclimatized to brackish (9.5 – 15.0 ppt) or full-strength seawater (31.3 – 34.6 ppt) and individually tested via critical swimming speed (UCrit) trials within an inter-flow closed respirometry chamber in changing salinity regimes. The effect of salinity was measured on oxygen consumption (MO2), and swim performance - quantified by total trial time (minutes) and maximum swim speed (m/s) – and blood biochemical markers, specifically osmolality, chloride, glucose, lactate, and cortisol. Significant differences in MO2 responses to treatments post-trial were observed across brackish water and full-strength seawater treatment groups. Full-strength seawater groups did not exhibit reduced swimming performance during UCrit trials compared to brackish water groups. Plasma osmolality and chloride levels showed no significant differences across the experimental timeline and treatment groups, suggesting that the trout adapted well to the altered salinity among the treatments. Glucose, lactate, and cortisol levels suggested that exercise induced stress, or exhaustion did not significantly contribute to the observed results, implying alternative stressors most likely related to handling and/or the inherent stress response to salinity changes. The results suggest a complex interplay of factors influencing the physiological responses of these hatchery-derived S. trutta. More comprehensive studies are therefore necessary to better understand the underlying factors driving salinity tolerance for S. trutta in both hatchery and wild conditions.
Acknowledgement
Thanks to Thomas Weiseth at Lundamo hatchery for providing the fish and fish rearing information. Further, a big thanks to Dag Altin, Tora Bardal and Iurgi Salaverria at NTNU Sealab for helpful assistance with the equipment setup, technical troubleshooting, and all-around support. Daniel Morgenroth from the University of Gothenburg, Sweden, is thanked for his hands-on help and advice in conducting UCrit swim trials.