Assessing biomarkers in common dolphin: Insights from novel blubber cortisol analysis

Monitoring the health of protected marine mammals is crucial for conservation. Cortisol, a hormone involved in the stress response and metabolism, is a recognised biomarker for physiological states across species. Previous studies assessing blubber cortisol in marine mammals predominantly relied on an extraction methodology using numerous steps with different solvents. In this study, we applied a simplified extraction method from blubber tissue, followed by a commercial enzyme-linked immunosorbent assay to quantify the cortisol content. Assay validations included intra- and interspecific coefficients of variation, as well as tests for parallelism and accuracy. While relatively low and variable extraction efficiencies suggest that protocols still need refinement before future application, our results indicate potential for improvement in blubber cortisol studies. This could lead to optimisation for a faster and more cost-effective protocol. While a decline in the nutritional health of common dolphins in the Celtic Seas ecoregion has been identified based on morphometric data, physiological markers have not yet been applied. To further validate the method and potentially disentangle the multitude of stress factors affecting the species we applied blubber cortisol analysis to stranded common dolphins along the Irish coastline (2017–2019, n = 71). Using generalised linear models with causes of death, body condition, body length, and maturity status as predictors, cortisol concentrations were found to be negatively correlated with body length and significantly higher in sexually immature individuals. The higher cortisol concentrations in younger animals may result from a combination of developmental, metabolic, or stress-related factors, underscoring the need for further investigation. • New method for cortisol extraction from dolphin blubber tissue with potential for a faster, more cost-effective protocol. • Elevated cortisol levels indicate physiological stress in younger dolphins. • Cortisol analysis can complement conventional measures of nutritional health.