The sea turtle nesting beaches of Sanibel have been monitored since 1959, first led by sea turtle researcher Charles LeBuff and his organization Caretta Research, Inc. Sanibel was initially monitored for loggerhead (Caretta caretta) turtles and, more recently, a growing green turtle (Chelonia mydas) population.

The SCCF sea turtle team continues to analyze nest-monitoring and night-tagging datasets from Sanibel to identify trends in the reproductive metrics of our local nesting populations.

“These trends can be used to assess how Sanibel’s sea turtle population is responding to environmental and anthropogenic pressures,” said Sea Turtle Biologist Savannah Weber.

The metrics analyzed included nest counts and dates of first and last nesting emergences from 1980 to 2023, as well as hatch success from 1998 to 2023. Data from the 2024 nesting season is now being analyzed and will be added to the long-term dataset.

“The results found that nest counts for both species significantly increased over time,” she said.

In the early 1980s, the number of loggerhead nests recorded each year was around 70 nests. In recent years, the team’s data averages approximately 600 loggerhead nests each year.

“This likely reflects the effectiveness and success of early conservation efforts, as there has been a reduction in major threats like poaching and fisheries bycatch,” said Weber.

The data also showed that the first nesting emergence date for loggerhead and green turtles shifted significantly earlier in the season over time.

In the early 1980s, the first loggerhead nest was generally laid sometime in the middle of May, while in recent years it occurred towards the end of April.

This trend has been observed on other nesting beaches in Florida as well, and researchers theorize it could be a result of warmer spring sea surface temperatures, which could signal to turtles that it is time to migrate to the breeding grounds and thus yield an earlier onset of nesting season¹.

The earlier shift in nesting could also be due to the nesting population increasing over time, thus lengthening the season.

“Although this data suggests that Sanibel’s sea turtle nesting population has increased compared to those early years when LeBuff first started patrolling the beaches, our data suggests we still have a long way to go to conserve sea turtles,” she added.

When looking at the average hatch success from inventoried loggerhead nests — excluding nests that were washed out or completely depredated by coyotes — from 1998 to 2023, the team found that there was a significant decrease in the loggerhead hatch success over time.

 This could be a result of a growing number of threats such as potential increased brevetoxin (red tide) exposure, or changes to the beach incubation environment, such as increasing sand temperatures.

“These are questions that continue to guide our research initiatives as we look ahead into the future,” said Weber.

This research was presented by Weber at the 42^nd International Sea Turtle Society Symposium in March 2024. View her poster here.

¹ Weishampel, J.F., Bagley, D.A., Ehrhart, L.M., 2004. Earlier nesting by loggerhead sea turtles following sea surface warming. Global Change Biology 10, 1424–1427.