Couch, C.E., T.N. Neal, C.L. Herron, M.L. Kent, C.B. Schreck, and J.T. Peterson. 2023. Gut microbiome composition associates with chronically elevated corticosteroids and morbidity in Chinook salmon (Oncorhynchus tshawytscha) Scientific Reports 13(1), 2567.
Pacific salmon experience prolonged elevation in corticosteroid hormones during important life history events including migration, reproduction, and senescence. These periods of elevated corticosteroids are known to correspond with changes to immunity and energy metabolism, therefore fish may be particularly vulnerable to mortality due to disease, thermal stress, and other external stressors at these times. Understanding and predicting survival and performance of fish with chronically elevated corticosteroids is important for understanding the potential impacts of anthropogenic stressors and management decisions on salmon population health. Numerous studies in human and laboratory systems have demonstrated mechanistic links between corticosteroids and host-associated microbial communities, and recent studies suggest that stress-induced increased cortisol associates with microbial community shifts in salmonids. In this work, we experimentally evaluated the relationships between gut microbiome composition, chronically elevated corticosteroids, and mortality in juvenile Chinook salmon (Oncorhynchus tshawytscha). We found that treatment with slow-release implants of the corticosteroids cortisol or dexamethasone resulted in significant changes to the gut microbiome that persisted up to 7 weeks post-treatment. We also found that morbidity in cortisol- and dexamethasone-treated fish strongly associated with microbiome composition, suggesting that the gut microbiome reflects individual differences in how fish respond to chronic elevation of corticosteroids. Additionally, we analyzed a small number of samples from adult fish in various stages of senescence and found that the microbiome of corticosteroid treated juveniles resemble those of senescent adults. Overall, findings from this work point toward the gut microbiome as a potential biomarker of mortality risk during periods of chronic corticosteroid elevation.