Citation: Guinn, M.A.; Toms, C.N.; Sinclair, C.; Orbach, D.N. Seasonal Prevalence of Skin Lesions on Dolphins across a Natural Salinity Gradient. Sustainability 2024, 16, 4260. https://doi.org/10.3390/su16104260 Academic Editor: George P. Kraemer Received: 22 March 2024 Revised: 11 May 2024 Accepted: 16 May 2024 Published: 18 May 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Seasonal Prevalence of Skin Lesions on Dolphins across a Natural Salinity Gradient Makayla A. Guinn 1, *, Christina N. Toms 2,3 , Carrie Sinclair 4 and Dara N. Orbach 1 1 Department of Life Sciences, Texas A & M University—Corpus Christi, Corpus Christi, TX 78412, USA; dnorbach@gmail.com 2 Psychology Department, New College of Florida, Sarasota, FL 34243, USA; ctoms@ncf.edu 3 Sarasota Dolphin Research Program, Brookfield Zoo Chicago, Sarasota, FL 34236, USA 4 National Oceanic and Atmospheric Administration, Southeast Fisheries Science Center, Marine Mammal and Turtle Division, Pascagoula, MS 39567, USA; carrie.sinclair@noaa.gov * Correspondence: makaylaguinn18@gmail.com Abstract: Bottlenose dolphins (Tursiops truncatus) inhabit waters across a broad natural salinity gradient and exhibit changes in skin condition based on the quality of their environment. Prolonged exposure to low salinities (≤10–20 ppt) degenerates the epidermal barrier and causes cutaneous lesions in dolphins, while the role of high salinity exposure (>35 ppt) in lesion development remains unknown. We assessed seasonal lesion prevalence in three free-ranging dolphin stocks inhabiting coastal Gulf of Mexico (GoM) waters of different salinities (0–30 ppt, 22–35 ppt, and 36+ ppt) using images of dolphin bodies. Lesions were documented on 44% of the dolphins photographed (n = 432), and lesion occurrence was significantly related to cold seasons and water temperatures but not salinity. Cold water temperatures may heighten dolphin susceptibility to infectious pathogens and disease and compound the effects of anthropogenic pollutants in the GoM. As dolphins are a bioindicator species of marine habitat welfare, natural studies assessing dolphin skin may reveal environmental degradation with potential impacts on marine ecosystems and human health. Keywords: bioindicator; bottlenose dolphin; epidermal disease; Gulf of Mexico; hypersaline 1. Introduction Bottlenose dolphins (Tursiops truncatus; hereafter “dolphins”) residing in bays, sounds, and estuaries of the Gulf of Mexico (GoM) can experience physiological changes resulting from exposure to a broad range of natural salinities [1–3]. The salinity gradient of the coastal GoM is driven by fluctuations in global precipitation and the hydrological cycle, which are often intensified by anthropogenic stressors [4,5]. Free-ranging dolphins generally live in marine waters with an average salinity of 35 parts per thousand (ppt), although some dolphins utilize lower salinity habitats (e.g., <20 ppt) [6]. Skin disorders (hereafter “lesions”) in dolphins are commonly associated with low salinity exposure, which increases the permeability of dolphin skin and fluid accumulation in the superficial epithelial layer [1]. Dolphins that frequently utilize low salinity habitats may be physiologically adapted to local salinity fluctuations or may use stratified water columns with higher salinities at depth [7]. Dolphins can also inhabit hypersaline lagoons (≥36 ppt) where salinities can double that of the GoM [8], although site fidelity in these hypersaline systems is poorly understood and the effects of hypersaline exposure on lesion development have not been assessed. Lesions on dolphins can be highly variable in origin and manifestation and are often context-dependent. Exposure to salinities < 10 ppt (often resulting from tropical storms and freshwater inundation in shallow systems) for several days or weeks generally induces skin pallor and cutaneous lesions in dolphins of varying degenerative stages [6,9]. Despite the continuous sloughing (i.e., shedding) of dolphin skin, fungi, algae, and bacteria may Sustainability 2024, 16, 4260. https://doi.org/10.3390/su16104260 https://www.mdpi.com/journal/sustainability