Response to capture stress involves multiple corticosteroids and is associated with serum thyroid hormone concentrations in Guadalupe fur seals (Arctocephalus philippii townsendi)

Guadalupe fur seals are a threatened species with few breeding locations, which potentially makes them sensitive to environmental or anthropogenic stressors. We present the first study to quantify adrenal and thyroid function in this species in an effort to measure their stress response to capture. We analyzed a suite of corticosteroid hormones released over time during capture in both adult females (n=10) and weanling pups (n=26) during March 2016. Multiple corticosteroids were released during capture, and aldosterone was associated with the response to stress in adults only. These results suggest the regulation of aldosterone secretion in association with the HPA axis in otariids as reported in other marine mammals. Individuals varied markedly in the magnitude of their endocrine response to capture. A lower total integrated stress response to capture for both cortisol and corticosterone was associated with decreased concentrations of thyroid hormone T3 and elevated concentrations of reverse T3 (rT3), suggesting parallel downregulation of adrenal and thyroid endocrine axes in some individuals. A scaled body condition index was negatively associated with T3 and positively associated with rT3 in adults. Together these findings suggest utility in using endocrine responses to capture stress to evaluate individual and population health.