The effects of age, sex, season and geographic region on circulating serum cortisol concentrations in threatened and endangered Steller sea lions (Eumetopias jubatus)

The role of stress in the decline of Steller sea lions has become of interest in the identification of factors leading to their drastic decline and subsequent failure to recover. Serum cortisol was validated as a potential indicator of stress in three sub-populations inhabiting the majority of the Steller sea lions range (n = 941). Additionally, seasonal variability in serum cortisol measured in captive animals (n = 9) of different age classes and sex was investigated. Significant interactions were found between region and year, as well as year and sex (p = 0.001 and p = 0.005, respectively). Samples from pups in Russia (151.5 ± 3.2 ng ml−1) and southeast Alaska (152.7 ± 2.7 ng ml−1) were consistently higher than concentrations measured in pups from southwest Alaska (133.0 ± 3.2 ng ml−1). Serum cortisol concentrations measured in 2001 (134.9 ± 2.0 ng ml−1) were significantly lower than 2002 (156.5 ± 2.4 ng ml−1, p < 0.001) for all regions. Seasonal effects indicated significant interactions between age, year and sex (p = 0.011). Serum cortisol was elevated in months associated with the breeding season and annual molt (86.5 ± 5.0 and 92.4 ± 5.1 ng ml−1, respectively) compared to the other seasons. Our results suggest that cortisol concentrations in Steller sea lions can be a useful diagnostic tool to compare the physiology between groups of sea lions, but factors such as animal sex, age, season and year must be considered.