Physiological and behavioral responses of Atlantic Salmon smolts to jet injector flow for downstream guidance

Downstream guidance of migrating fish is a major conservation challenge in fragmented riverine systems, where orientation depends on fine-scale environmental cues and physiological readiness. Flow manipulation is increasingly explored to improve behavioral guidance at hydropower facilities, particularly for salmonids during the smolt stage. We investigated the effects of laterally injected jet flow on stress physiology and behavior of Atlantic salmon (Salmo salar) smolts in a large-scale flume. Fish were exposed to three treatments: jet flow from the left (Left Jet), from the right (Right Jet), and no flow (Control). Physiological analysis revealed that the Right Jet treatment significantly elevated malondialdehyde (MDA) levels and increased the activity of the antioxidant enzymes catalase (CAT) and glutathione peroxidase (GPx) in erythrocytes, indicating the presence of oxidative stress. No significant effects were detected for cortisol (plasma), hemoglobin and lactate (whole blood), or advanced oxidation protein products (AOPP, erythrocytes). Behaviorally, smolts showed a consistent left-side preference and swam predominantly near the bottom, regardless of treatment. Nevertheless, jet flow did not significantly alter lateral or vertical swimming distribution. Together, these results demonstrate that sublethal physiological strain can occur without overt behavioral change, underscoring the value of integrative, non-lethal approaches for evaluating fish responses to hydrodynamic conditions in guidance systems.