Blubber cortisol in four Canadian beluga whale populations is unrelated to diet

Changing conditions in the Arctic have had severe consequences for many marine mammals. In this study, we examined blubber cortisol using radioimmunoassay in 4 Canadian beluga whale Dephinapterus leucas populations. The endangered Cumberland Sound population had higher cortisol levels (mean +/- SE: 0.65 +/- 0.11 ng g-1) than populations not at risk: Eastern Beaufort Sea (0.31 +/- 0.03 ng g-1; p < 0.001), Eastern High Arctic-Baffin Bay (0.32 +/- 0.09 ng g-1; p = 0.004), and Western Hudson Bay (0.44 +/- 0.04 ng g-1; p = 0.004). To evaluate if measured cortisol differences were due to differences in diet, we compared stable isotope ratios of carbon and nitrogen (delta 13C and delta 15N) and dietary fatty acids among populations. Beluga whales from Eastern Beaufort Sea had lower delta 13C (p <= 0.017) and higher delta 15N (p < 0.001) values than other measured populations, while Western Hudson Bay beluga dietary fatty acid profiles differed from other measured populations (p < 0.001). Population and sex were significant predictors of blubber cortisol (p <= 0.017). Females exhibited higher cortisol than males. Despite diet differences among populations, neither stable isotopes nor fatty acids were significant predictors of cortisol, suggesting differences in cortisol levels were unrelated to diet. Other factors, such as increased risk of predation, hunting pressure, vessel traffic, or differences in baseline blubber cortisol concentrations may be contributing to elevated cortisol levels in Cumberland Sound beluga whales. Measuring blubber cortisol in combination with chemical indicators of diet provides a useful method for monitoring population health and can be used to inform management and conservation.