Physiological assessment of common bottlenose dolphins (Tursiops truncatus) across a salinity gradient

Common bottlenose dolphins (Tursiops truncatus) are important bioindicators of ecosystem welfare and can inhabit environments with variable natural salinities. Anthropogenically-induced climate change exacerbates natural fluctuations in salinity and magnifies physiological imbalances in marine species. Bottlenose dolphins are well-suited model organisms to study the effects of environmental disturbances because they accumulate indices of stress in their blubber. Prolonged low salinity (< 10 ppt) exposure in dolphins elevates adrenal steroid hormones (i.e., aldosterone, cortisol) and promotes lesion development. However, the tolerances of and consequences for dolphins in hypersaline systems remain unknown. I assessed the physiological condition of three dolphin stocks in the Gulf of Mexico inhabiting areas of different natural salinities: Mississippi Sound, MS (0 – 30 ppt), Redfish Bay, TX ( 22 – 35 ppt), and Upper Laguna Madre, TX (37 + ppt). Steroid hormones were measured in remotely biopsied dolphin blubber using high-resolution liquid chromatography-mass spectrometry. Skin lesions were assessed using images of the dorsal fins and bodies of dolphins photographed from a research boat. There is a positive relationship between cortisol and salinity, indicating high salinity may impose physiological stress in dolphins. Testosterone concentrations in males are seasonal, with peaks in the fall and winter months. Progesterone levels in females were highest in the spring and summer and were indicative of gestation events. Skin lesions are most prominent on dolphins in the fall and winter, and a negative correlation between lesion prevalence and water temperature suggests cold water has a strong effect on epidermal integrity and lesion susceptibility in dolphins. I present here the first physiological assessment of free-ranging dolphins in a natural hypersaline bay. The dolphin health data collected from this research can help fill national data gaps for GoM dolphin stocks outlined by the National Oceanic and Atmospheric Administration, fill local data gaps for RB and ULM dolphin stocks, inform coastal communities of local marine ecosystem health and potential impact on human health by utilizing dolphins as bioindicators, and contribute to the understanding of how global climate change impacts the ability of marine organisms to adapt to highly variable environmental conditions. Additionally, this research will contribute to the planning of environmentally sustainable infrastructure (e.g., desalination plants) and promote environmental stewardship, ecotourism, and appreciation for natural resources.