National Park Service biologist Shelby Moneysmith at a loggerhead turtle nest in Biscayne National Park, Florida. (NPS)
Tallying turtle nests
Female sea turtles typically nest several times in a year. They may leave all of their eggs at one specific beach or nest at several beaches to spread out their reproductive investment. They typically return to the same stretch of coast year after year.
To monitor population trends, scientists count the number of nests made on a beach during an entire nesting season. They estimate how many times an individual female turtle nests during one nesting season, and use simple arithmetic to calculate the estimated number of females that nested that year.
We also walk nesting beaches to find individual turtles, collect data and biological samples from them and attach tags to their flippers. If researchers re-encounter a tagged turtle during a subsequent nesting season, they will record her return and revise their estimate of how many offspring she produces. Sea turtles typically nest every two, three or four years, so biologists need long-term data over multiple decades to track population trends.
On a few beaches, olive ridley sea turtles (Lepidochelys olivacea) emerge synchronously and en masse to nest in enormous groups of hundreds to thousands, known as arribadas (Spanish for “arrival”). When this happens there are so many turtles nesting at one time that a person could walk from shell to shell across the beach without stepping on the sand. It is impossible to count most of these turtles, and finding a tagged individual from among the throngs is like looking for a needle in a haystack.
Witnessing an arribada is the most thrilling wonder of nature I have experienced. The sight, smell and sound of thousands of turtles on a beach digging holes in the sand and laying eggs, choreographed to music only they can hear and understand, is indescribable.
An incomplete picture
Although researchers have used these methods for decades, they do not give us a full enough picture to assess how well global conservation efforts are working.
One challenge is that there are too many turtles and not enough funding to record every nest at most beaches. Many nesting sites are remote, hard to access and logistically challenging places to live and work for months at a time. There are tens of thousands of miles of coastline where no one counts sea turtle nests regularly and systematically.
Second, turtles don’t always produce the same number of young from one season to another. Like all animals, they invest their energy into metabolism, growth, survival and reproduction. When food is limited, they often lay fewer eggs.
Third, and perhaps most importantly, breeding females are not the only important sea turtle demographic group. Biologists want to develop population models they can use to interpret population changes, identify threats in marine habitats, predict risk, evaluate the impacts of management activities and assess sea turtle status and trends. To do this, we also need other demographic information, such as age-specific and sex-specific survival rates and age at sexual maturity. Researchers are trying to collect these kinds of data, but it is logistically challenging when we are dealing with turtles at sea.