Long-term Impacts of Red Tide Blooms on Loggerheads and Their Offspring

During red tide blooms, sea turtles and other marine animals become exposed to neurotoxins called brevetoxins, which at high concentrations can cause wildlife mortality on mass scales.

While the immediate impacts of brevetoxin exposure have been heavily studied on sea turtles that wash up ill or dead during red tide events, little is known about longer-term effects of red tide blooms on sea turtles.

Starting in 2019, SCCF launched a four-year study on Sanibel Island examining the health and reproductive impacts of brevetoxin exposure on nesting loggerheads. The research came on the heels of one of the worst red tide events to ever strike Southwest Florida in 2018, and identify correlations between brevetoxin exposure, long-term health impacts, and reproductive success. Partners included scientists, veterinarians, and pathologists from the Loggerhead Marinelife Center, Florida Fish and Wildlife Research Institute, University of Florida, Fishhead Labs, and The Everglades Foundation. when over 4 million pounds of dead marine life were collected on Lee County beaches.

Analyses revealed that nesting loggerheads had brevetoxin in their blood, and loggerheads that foraged in the Gulf of Mexico had significantly higher concentrations than those that use foraging grounds in the Caribbean.

During the study, SCCF researchers collected 428 blood samples from nesting loggerheads on Sanibel and over 1,500 samples from their eggs and offspring to determine brevetoxin concentrations and identify correlations between

Comprehensive health panels of the turtles revealed potential impacts on the turtles’ immune function and overall health.

“This provides evidence that brevetoxins potentially act as a physiological stressor that may continuously impact sea turtles and their offspring,” said SCCF Coastal Wildlife Director Kelly Sloan, lead author on the study.

The data also confirm that toxin transfer occurs from nesting female to egg/offspring and emergence success declined as hatchling brevetoxin levels increased. Hatchlings often had very high concentrations in their livers before they even emerged from the nest.

Brevetoxins were present in both hatchling livers and unhatched eggs, and these amounts were significantly correlated in samples from the same nest.

“The combination of climate change and increasing nutrient pollution will likely increase the incidence and intensity of harmful algal blooms like red tide. It’s important to consider the impacts of these blooms on threatened and endangered sea turtle health and reproduction,” Sloan said. “Prior to this study, there was a significant data gap on this topic in the policy realm and the lack of known impacts has effectively slowed federal protection.”

—Kelly Sloan, Director, Coastal Wildlife