Sea turtle nests in the sand on Sanibel Island hatch at nearly twice the rate as clutches on Captiva Island, and a long-term study to find out why was kept from a setback after Florida Gulf Coast University stepped in when Hurricane Ian crossed Cuba and moved toward the sister barrier islands in late September.
Scientists from the Sanibel-Captiva Conservation Foundation have been monitoring myriad environmental conditions in dozens of nests on both islands trying to figure out what’s going on.
The multi-year project is “taking a deeper look into how physical properties of the incubation environment may interact to impact hatch success, such as temperature, moisture, and sand grain size, compaction, color, and bulk density,” said Jacob Wozney, a sea turtle researcher involved in the study. “The sand on Sanibel has some differences from the sand on Captiva, which has added non-native sand to its beaches as part of ongoing beach renourishment projects.”
As Hurricane Ian approached, scientists at The Water School at Florida Gulf Coast University invited the SCCF researchers to move the key elements of the study to the FGCU campus, where they would more likely to be out of harm’s way. This included sea turtle blood samples, which must be kept at minus 112 degrees.
It was a good call. Hurricane Ian’s flood waters destroyed SCCF’s research area, including upending the freezer that had been housing the blood samples.
A species in peril
All species of sea turtles are threatened or endangered and protected by Florida law and the federal Endangered Species Act.
In the 1970s, overharvesting of sea turtles for meat, eggs, leather, and shells caused populations to dwindle. Since then, Nova Southeastern University in Fort Lauderdale has compiled a list of the many things that have contributed to further decline of the species: polluted waters, trash, chemicals, discarded fishing gear, bycatch in fishing nets, development along the shoreline, beachfront lighting that disorients hatchlings and can have them crawl away from the ocean, and boat strikes are among the reasons, especially during mating, nesting, and hatching season.
Even without manmade obstacles, only one in 1,000 hatchlings that make it to the ocean will survive to adulthood. Many baby turtles that emerge from the nest don’t even make it off the beach, as they succumb to dehydration if they don’t hurry or get stuck in a divot in the sand. Crabs, birds, and other predators also pick off hatchlings as part of the natural food chain.
New barriers to bouncing back
As scientists work on restoring the population of sea turtles, research is proving the success of a nest is quite resolute if the clutch is to hatch as it should: with 90 percent success rate and a mix of healthy male and female baby turtles.
The warming planet is now a key barrier to restoring the sea turtle population, as changing temperatures affect nearly all of the factors in sea turtle reproduction except the act itself, and different temps are already ruining the equilibrium that must be maintained.
The National Oceanic and Atmospheric Administration reports that the average temperature in Southwest Florida has risen 1.4 degrees during the last three decades, and sea levels have risen four inches around Fort Myers during that same time frame.
Sea turtle eggs are laid by the female into a hole she has dug in the sand on the beach where she was born, covered back up with sand and left to mature in baby turtles. When the hatchlings emerge about 55 days later, they race over the beach toward the relative safety of the water. A mother turtle may repeat her efforts several times in a nesting season.
Tropical storms feed off warmer ocean waters and, as seen locally from Hurricane Ian, the stronger storms can change the landscape of the beach and confuse pregnant females trying to recognize their birth beach and give up, releasing the eggs into the water with no chance.
When water warms, it expands. Add that fact to water melting from glaciers at the poles and higher water levels can cover more of the beach face and make a wide beach thinner, which doesn’t leave enough room for all the females who want to lay her eggs there.
And perhaps most challenging is that, unlike most species, sea turtle gender is not determined at fertilization. Whether a hatchling crawls out of the nest male or a female after about 55 days is determined by the temperature of the nest.
If the sand creates an incubation temperature below 81.86 degrees the hatchlings will be male, but at 88.8 degrees or higher they will be female. A nest temperature in between will produce brothers and sisters.
In the January 2018 issue of Current Biology, scientists found that baby sea turtles coming out from nests near the Great Barrier Reef were 99 percent female. Data from past studies of nests near the reef found the clutches developed in sand at the “pivotal temperatures” and produced slightly more females than males, but the ratio was within the amount to keep reproduction and life cycle normal.
“With warming global temperatures and most sea turtle populations naturally producing offspring above the ‘pivotal temperature,’ ” wrote lead author Michael P. Jensen, who is with NOAA’s National Marine Fisheries Service. “It is clear that climate change poses a serious threat to the persistence of these populations.”
Rebuilt beaches another threat
Another consequence of sea level rise is increased erosion on shorelines. And if that beachfront happens to be along a row of McMansions or high-end resorts like on Sanibel Island, often the money is found to have the Army Corps of Engineers drop by and dredge-and-fill the shoreline to replace the beach with sand from somewhere else.
The replaced sand rarely matches up on a microscopic level, but that can be huge to the success or failure of a clutch.
The SCCF researchers been using the lab space provided by FGCU’s Hidetoshi Urakawa to continue working on their long-term study investigating the two-fold difference differences in sea turtle hatch success on Captiva versus Sanibel islands.
The scientists collected two sand samples from 60 nests — one from the top layer and one from several layers deep where eggs would be laid — and the SCCF team also installed probes into each nest to record the temperature, moisture, and the groundwater level every 15 minutes throughout incubation.
And now that the field work is completed, Wozny has been using a specialized machine at FGCU to separate the sand samples by particle size and see where the research leads. It is a result that beach towns whose economy relies on beachgoing tourists, and sea turtle lovers who care more that the animals have a chance, both are hoping will come soon.
"Captiva's hatch rate this year was only 31%, which is alarmingly low compared to nearly 49% and 61% on Sanibel east and Sanibel west, respectively," said Kelly Sloan, SCCF’s sea turtle program coordinator. "We're hoping this project will help us learn more about the factors that could be contributing to low productivity on these beaches."
Environmental reporting for WGCU is funded in part by VoLo Foundation, a non-profit with a mission to accelerate change and global impact by supporting science-based climate solutions, enhancing education, and improving health.
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