Kristen Hart has been surveying a number of nesting loggerhead sea turtle populations for years. Like most turtle ecologists, she knows that the females don’t typically start nesting unless they are at least 87 centimeters long from head to tail.
But she was surprised to find that many of the females in three distinct populations—two in Florida and one in Alabama—were significantly smaller when it came time for them to dig holes and lay eggs.
She wondered what ramifications this small size could have for the population and wildlife management.
Their findings suggest that “the conservation relevance is pretty high,” said Hart, a research ecologist with the U.S. Geological Survey, who is the author of a study on the small female loggerhead sea turtles (Caretta caretta) published recently in Conservation Science and Practice.
Hart and her colleagues, including Allison Benscoter, an ecologist at the U.S. Geological Survey and lead author of the study, survey beaches several times during loggerhead nesting seasons at Dry Tortugas and Everglades national parks in Florida and Gulf Shores in Alabama. Their measurements are added to a long-term dataset USGS keeps on nesting turtles, including their size measurements.
While some of the largest females they found nesting still measured more than 100 centimeters long, many turtles measured only 74 centimeters—almost 15% smaller than the 87 centimeters believed to be the minimum nesting size for loggerheads, Benscoter said.
At Everglades alone, nearly 24% of the nesting females were smaller than the size typically considered reproductive.
The researchers had fitted about 110 turtles from these three distinct populations with satellite tags. Analysis revealed that the smaller sea turtles were traveling less than the larger ones.
“The smaller ones migrated shorter distances and in shallower waters compared to the larger nesters,” Benscoter added.
This may have to do with energetics, Hart said, as the smaller turtles may have trouble dealing with the strong currents in the Straits of Florida between the Gulf and the open Atlantic Ocean to the east. “The current there is pretty ripping,” she said.
But choosing shallower coastal waters could be risky for the small turtles, causing more boat risk both from shipping and recreation.
The researchers aren’t sure exactly why these breeding females are smaller, on average. They also don’t know the nesting females’ ages. Benscoter speculated that it could be that the turtles in these regions grow more slowly than loggerheads elsewhere, or that they mature at an earlier age. Warmer waters due to climate change could play a role as well—the two Florida survey areas are highly susceptible to climate change and sea level rise, while nests in the Alabama location often get flooded during storms.
Hart said the study reinforces that the smaller populations of loggerheads found in these parts of the Gulf may not behave the same as larger populations there and elsewhere in the Atlantic. This factor is important in the statistical analysis of populations, as it could affect models and predictions of the numbers of breeding turtles.
For example, turtles that are killed accidentally by boats or even by oil spills, such as the Deepwater Horizon disaster in 2010, are often categorized by reproducing and non-reproducing individuals. These categories help wildlife managers make decisions on the level of impact that something like an oil spill or the incidental take from a year’s worth of ship strikes might have on populations. If 70-centimeter-long turtles killed during these events are considered non-reproducing, decisions made on mitigation or conservation actions may not be sufficient in some cases.
Similar size-related differences may be occurring in other sea turtle populations found in these areas like green sea turtles (Chelonia mydas), but further research would be needed, the team said.
|Joshua Rapp Learn is a science writer at The Wildlife Society. Contact him at firstname.lastname@example.org with any questions or comments about his article.
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