From Markers to Meaning: Understanding Fish Physiological Responses to Assess Human Impacts on Rivers

The comprehensive evaluation of stress responses across various fish species occupying diverse ecological niches is highly important for unravelling the intricate impacts of stressors on aquatic ecosystems. This inclusive approach not only allows the physiology of migratory and non-migratory species to be studied but also provides a lens to understand the effects of habitat fragmentation, drawing a parallel with the social stress investigation in species. Due to their significant impact on aquatic ecosystems, migratory fish species like salmonids are a crucial focus. These species are particularly threatened by inadequate hydropower plant passage designs and are typically more vulnerable to disturbances along their migration route, such as chemical pollution or light pollution. My PhD research, however, emphasises that anthropogenic stress reactions are noticeably dependent on the species under study, demonstrating noticeable variability even among individuals of the same species. This subtle species-specificity emphasises how intricate the dynamics of stress are in aquatic environments. Regardless of these distinctions, it is essential to understand that stress can seriously impair fish welfare. Cortisol emerged as a dependable indicator of prolonged stress in fish, signifying the activation of the hypothalamic-pituitary-interrenal (HPI) axis and the consequential mobilization of energy resources. However, the susceptibility of cortisol levels in blood samples to handling and human interactions was noted, potentially obscuring the impact of other stressors. Therefore, cortisol measurement within alternative matrices, such as muscle tissue, fin and scales, was adopted as a reliable indicator of stress responses. Oxidative stress emerged as a robust marker for cellular damage and inflammation in fish. The assessment of oxidative stress was executed through varied assays, encompassing advanced oxidation protein products (AOPP) and lipid peroxidation by evaluating the malondialdehyde (MDA) level. Notably, AOPP levels demonstrated heightened consistency and conspicuity compared to other assays. Complementary evaluations of activity and gene expression of antioxidant enzyme, including superoxide dismutase (SOD), catalase (CAT), peroxiredoxin (Prdx) and glutathione peroxidase (GPx), vital components in ROS scavenging and cellular protection, were also undertaken. This induction of antioxidant defences has emerged as a credible indicator of oxidative stress risk in fish and the physiological responses to counteract it.