Introduction
Increasing water temperatures arising from heat waves and global warming can lead to stressful conditions for farmed fish. Fish have developed mechanisms to adapt to challenging external environments and the ability to cope with stress, is a major health trait that can determine fish susceptibility to opportunistic pathogens and disease. However, rapid temperature changes can exceed the organism capacity to adapt to new homeostatic states. Stress resilience and fish welfare, within a context of changing environmental conditions, is rapidly becoming a major research subject in aquaculture. The present work aimed to assess the effects of high temperature-induced chronic stress on stress resilience as well as immune and oxidative stress responses of rainbow trout (Oncorhynchus mykiss) juveniles.
Material & Methods
A trial with rainbow trout juveniles was devised. Three different groups were created, namely: CTR (16 °C); MED (19 °C); HIGH (22 °C). In each group, fish were randomly distributed into 6 tanks with 20 fish each (approx. 20 g initial body weight). Following an acclimatization period, fish were hand fed a fixed amount of feed (3% body weight) 2 times a day with a commercial-like diet, for 4 weeks. After this period, blood samples were collected for hematological procedures while plasma was isolated and used for cortisol, metabolites and humoral immune parameters. Liver was sampled for the analysis of metabolites and oxidative stress responses. Head-kidney, brain and hypophysis were sampled for gene expression analysis. Skin mucus and water were sampled both for proteomics and transcriptomics (miRNA) analyses, while skin tissue was collected solely for RNAseq analysis.
Results and Discussion
Results from the analysed plasma humoral parameters showed no differences between experimental groups regarding anti-proteases activity, after 4 weeks. However, the HIGH temperature group showed higher lysozyme activity than CTR. Regarding the head-kidney panel of selected genes, glucorticoid receptor 1 (gr1) and arginase 2 (arg2) showed significantly higher gene expression in HIGH temperature group in comparison to MED, although no differences were perceived relative to CTR.
Chronic stress is associated with glucocorticoid receptor loss of sensitivity, impairing corticosteroids regulatory effects, namely the anti-inflammatory function. It has also been linked with increased circulating pro-inflammatory biomarkers in blood. Ongoing data analysis on the above referred tissues and matrices will allow a better understanding of stress and health related mechanisms involved in chronic stress response, namely the relation between the HPI axis and the immune mechanisms. The integration of all the generated data, is expected to provide the establishment of reliable biomarkers under chronic stress conditions.
Work Co-funded by UKRI and by the European Union’s Horizon Europe research and innovation programme (GA No. 101084651 - project IGNITION).