Abstract:

The 2017–2019 harmful algal bloom in southwest Florida resulted in the largest number of sea turtle deaths ever attributed to a single red tide event. The mass mortality of sea turtles associated with this event clearly demonstrates the impacts of brevetoxicosis, but there is an urgent need to characterize the sublethal, chronic effects of brevetoxins on wildlife. Marine turtles can be exposed to brevetoxins through inhalation of aerosolized toxins, but the most lethal route of exposure is through ingestion of prey items that contain accumulated brevetoxins. Sea turtles are notoriously resilient and able to withstand severe physical damage but they appear to be surprisingly susceptible to biological and chemical insults. Many studies document the acute impacts of brevetoxicosis in stranded and rehabilitated turtles showing obvious clinical symptoms, but few characterize the chronic effects of brevetoxins on wildlife health. This study evaluates the sublethal effects of red tide blooms on nesting loggerhead sea turtles and their hatchlings. Samples were taken on Sanibel Island, Florida, USA. Sanibel Island (26.4648°N, -81.1671°W) is a coastal barrier island located on the southwest coast of Florida in Lee County (Fig. 1). The island is 21 km in length with a land area of 4,429 ha. From 2018–2022, the number of loggerhead nests laid annually on Sanibel was 612 ± 55 (SD) (range: 551–671). This nesting beach has been monitored for nesting sea turtle activity since the late 1950s.During oviposition, 10 mL of whole blood was collected from the external jugular vein. Plasma, unhatched eggs, and livers from dead-in-nest hatchlings were analyzed for brevetoxins using an indirect competitive enzyme-linked immunosorbent assay (ELISA). two blood smears per turtle were made and stained using Wright’s Stain for analysis of complete blood counts with differentials (heterophils, lymphocytes, monocytes, eosinophils, basophils) and observations of red (RBC) and white blood cell (WBC) morphology by a board certified clinical pathologist. Plasma samples were shipped overnight on dry ice to the University of Miami Avian and Wildlife Laboratory (Miami, FL, USA) for biochemical analyses. Whole, formalin-fixed bodies from 45 dead-in-nest loggerhead hatchlings were submitted to Fishhead Labs, LLC (Stuart, FL, USA) for gross examination, processing, and subsequent histological evaluation. Blood samples were also sent to the Florida Fish and Wildlife Research Institute for Stable Isotope Analysis.