Stress and Reproductive Hormones of Free-Ranging Dolphins Across a Natural Salinity Gradient

Bottlenose dolphins (Tursiops truncatus) inhabit environments with variable natural salinities and experience physiological imbalances from extreme salinity fluctuations. Low salinity exposure (≤10) disrupts osmoregulation and increases the production of steroid hormones aldosterone (electrolyte balance) and cortisol (stress response). The effect(s) of high salinity exposure (>35) on steroid hormone production in bottlenose dolphins has not previously been assessed. We measured stress hormones (aldosterone, corticosterone, cortisol, and cortisone) and reproductive hormones (progesterone and testosterone) in three stocks of free-ranging bottlenose dolphins inhabiting different natural salinities across the Gulf of Mexico (0-30, 22-35, 36+). Hormones were extracted from the blubber of 82 remotely biopsied dolphins and quantified using ultraperformance liquid chromatography coupled with Orbitrap Fusion mass spectrometry. A positive correlation was found between cortisol (10.38 ± 0.8 ng/g, n = 13) and salinity (r = 0.62). Aldosterone (58.9 ± 17.2 ng/g, n = 5) was quantified in dolphin blubber for the first time and was detected across the salinity gradient but was not significantly related to salinity levels. Concentrations of testosterone were seasonally variable. This study enhances our understanding of how climate changes and extensive anthropogenic stressors challenge homeostasis in a marine bioindicator species.