Estimating sea turtle population sizes

A study recently published in Proceedings of the Royal Society B demonstrates how numbers of nesting turtles may be overestimated by a factor of two. Does this mean that there may only be half as many turtles as previously thought?

Sea turtle population size estimates have traditionally depended on walking kilometres of beach to record turtle sightings, tracks and nests, night after night. Marking turtles with small metal flipper tags helps to identify individuals and determine how many times the average female lays eggs. However, since it is impossible to flipper tag and intercept every turtle every time it nests, there is a tendency to underestimate the number of egg clutches that a female lays. Based on existing data, scientists have assumed that green turtles lay on average 3.5 clutches in a nesting season. This means that if, for example, 210 egg clutches were recorded on one beach, then the local nesting population would consist of 60 individual females. Currently, most population size estimates around the world work with this assumption.

In this new study, researchers used satellite tags to track individual female green turtles in the Indian Ocean to assess how many times they nested during the breeding season. The high-accuracy GPS location data revealed that individual turtles laid on average six clutches of eggs – almost twice as many as previously thought. On the basis of these data, a recording of 210 egg clutches would result in a much smaller nesting population of 35 females.



Fastloc-GPS Argos SPLASH tags (a) were attached to nesting green sea turtles (b) to record how many clutches females lay in a breeding season. (Source: Proceedings of the Royal Society B)

This research confirms similar conclusions of studies on green turtles nesting in Ascension Island and loggerhead turtles nesting in Florida. This suggests that scientists and conservationists need to re-examine their assumptions about sea turtle nesting frequency and take into account the possibility that many sea turtle nesting population numbers are being over-estimated. Dr Jeanne A Mortimer, an author of the study, comments: “We are not saying that all sea turtle populations have been overestimated by a factor of two. But we demonstrate how easy it is to do so inadvertently.” So while the absolute number of sea turtles in the oceans has not changed, our understanding of their biology and our estimates of their population sizes have improved. The authors hope that this new research “will encourage more people to use satellite tracking technology to help solve the many remaining mysteries about sea turtles that are so important to enabling us to effectively assess and manage their populations.”

How numbers of nesting sea turtles can be overestimated by nearly a factor of two” was published by Proceedings of the Royal Society B (2017). Authors: Nicole Esteban, Jeanne A. Mortimer and Graeme C. Hays.



Drones and sea turtle research

Unmanned Aerial Vehicles (UAVs – drones) are increasingly employed to monitor and protect wildlife. The new technology has proven to be particularly useful to survey species and habitats that are difficult to access. In sea turtle research drones are being used for various purposes. In Australia conservationists use them to count turtles at Raine Island, the world’s largest green sea turtle nesting site, and map the island’s topography. In Suriname the World Wide Fund for Nature (WWF) uses drones to survey beaches and collect evidence of the illegal poaching of turtle eggs.

Drones can also be used to answer questions about turtles’ lives in the oceans. A new study published recently in Herpetological Review  deployed UAVs to observe the behaviour of green sea turtles off the coast of Mexico. One of the benefits of using this technology is that there is little risk of disturbing the animals that are being observed. The study revealed interesting footage of the courtship and mating behaviours of turtles at sea. The authors of the study conclude that “UAV technology is useful for not only enhancing our understanding of sea turtle behaviors in the natural environment, but also in identifying the location of critical habitat for important life-history events, such as courtship and mating.” One of the other benefits is that drones will show a unique bird’s-eye view of turtles returning to the water after nesting!

Video courtesy of The Leatherback Trust

The importance of beach vegetation

Female-biased sea turtle populations are reported at important sea turtle rookeries globally. This heightens concerns for the conservation of sea turtles in the long-term. For this reason researchers are measuring temperatures at nesting beaches around the world to better understand the male-to-female ratio that these rookeries are producing.

I recently collaborated on a research project looking at the incubation temperatures of turtle nests in the Chagos Archipelago (Western Indian Ocean) where both hawksbill and green turtles breed. In this study, we recorded sand temperatures on Diego Garcia, the largest island of the archipelago. Temperature loggers were placed at nest depths in the different areas where the hawksbill and green turtles nest. The results showed relatively cool temperatures. The beaches of Diego Garcia have several characteristics that make for these relatively cool nest temperatures. Firstly, the island, which is in the world’s largest marine protected area, has intact natural vegetation that provides heavy shade where some turtles nest. Together with heavy rainfall and narrow beach platforms, which require sea turtles to nest close to the sea, this provides for cool sand temperatures. Consequently, we expect that hatchling sex ratios at this site are currently fairly balanced, producing 53% and 63% male hatchlings for hawksbill and green turtles respectively. The results of this study were published in Scientific Reports this week.

Dr Jeanne A Mortimer, one of the authors of the article, has studied Western Indian Ocean sea turtles since 1981. She states that “our study helps us to better understand why different species of sea turtles choose the nesting sites that they do. Our results demonstrate that in order to produce offspring with a relatively balanced sex ratio, these hawksbill turtles need to lay their eggs amongst vegetation on the upper beach crest.  Hawksbills are relatively small sea turtles with an average nest depth of only about 30-50 cm (compared to the larger green turtles whose nest depths average some 70-85 cm). Green turtles often lay their eggs on the open beach platform. Our results tell us, however, that hawksbill nests constructed in an area of open sand are more likely to produce female-biased offspring, and in some cases might even be too warm to produce viable offspring.  This highlights the importance for habitat managers to maintain the natural vegetation on the beach crest in order to provide optimal nesting habitat for hawksbill turtles – a species currently listed by IUCN as a Critically Endangered species. Our research also helps to answer the very basic question asked by just about anyone who has watched hawksbill turtles nest: ‘How come they always seem to go up into the bushes to lay their eggs?’“

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(photos courtesy of Daniel Barker, Nicole Esteban, Kip Evans and Graeme Hays)

Male hatchling production in sea turtles from one of the world’s largest marine protected areas, the Chagos Archipelago” was published by Scientific Reports (2016). Authors: Nicole Esteban, Jacques-Olivier Laloë, Jeanne A. Mortimer, Antenor Guzman and Graeme C. Hays.

Cold-stunned turtles

Banana boxes? Check. Air support? Check. Volunteers? Check. After several years’ experience, conservation groups on Cape Cod know exactly what they need to rescue sea turtles that wash up on their shores every winter. But how do these turtles end up stranded in Massachusetts, USA, a place that seems so far from their usual tropical waters?

Sea turtles come and feed in Cape Cod bay during the warm summer months. Come the colder months, the turtles migrate back to warmer waters in the South. However, due to the geography of the Cape, some turtles cannot navigate their way out of the bay and get trapped. As the water cools these cold-blooded animals suffer from hypothermia, loose their ability to move, and wash up on the shores of the Cape.

Fortunately volunteers patrol the beaches to help the turtles. The cold-stunned turtles are taken to an animal care centre, such as the New England Aquarium, where they receive medical treatment. The turtles’ body temperatures are gradually raised every day and their progress is carefully monitored. Full recovery can take anywhere between a few days and several months, after which the turtles are placed in specially prepared banana boxes and are either flown or driven back South and released in warmer waters.

An improvised turtle centre. (photo courtesy of Liz Maloney)

The cold-stunned turtles are cared for at rescue centres
(photo courtesy of Liz Maloney)

This year, for the first time since the start of the rescue activities, volunteers are recovering turtles from the beach as late as January. This may be due to rising temperatures, which causes the turtles to migrate south later in the season. Already over 500 cold-stunned turtles were reported this season, amongst which are Kemp’s ridley, loggerhead, and green turtles. In 2014 a record of 1200 turtles were recovered from the beaches around the Cape. Thanks to the efforts of the volunteers and rescue centres, the majority of these turtles made it back home safely.

a turtle in a box (photo courtesy of Liz Maloney)

Getting ready for the trip home!
(photo courtesy of Liz Maloney)

A diet of jellyfish

If you mention jellyfish to a sea turtle biologist, the first thing he will think of is a leatherback turtle as the diet of a leatherback consists exclusively of jellyfish. The digestive track of a leatherback is uniquely designed to efficiently process jellyfish prey. The green turtle, on the other hand, has a diet that consists mainly of algae and seagrasses. The anatomy of the green turtle is perfect for this diet: its finely serrated beak is ideal for scraping algae off rocks and chomping on seagrasses. However, scientist might have been wrong about this ‘vegetarian turtle’: there are more and more reports of green turtles feeding on jellyfish.

Video courtesy of Sundive Byron Bay

How is it possible that it is only discovered is recent years that green turtles eat jellyfish? Could it be that green turtles only recently developped a taste for jellyfish? More likely is the explanation that green turtles have always opportunistically eaten jellyfish but that is was underreported in the past. With more and more people using cameras to film nature, rare or unusual animal behaviours are sometimes uncovered!

Geomagnetic navigation

Sea turtles carry out long-distance migrations between their feeding grounds and their breeding grounds. For example, some green sea turtles feeding off the coast of Brazil migrate over 2000 kilometers to Ascension Island in the middle of the Atlantic. But how do the turtles navigate the oceans? One hypothesis is that they use Earth’s magnetic field for navigation. This was once tested by attaching powerful static magnets to turtles and seeing if it would impede the turtles’ navigational skills. Results from such studies were the first field evidence for the involvement of geomagnetic cues in sea-turtle navigation.

In a new research published recently in the scientific journal Current Biology, researchers from the University of North Carolina were interested in knowing if marine turtles could imprint on the geomagnetic signature of their natal beach and use that information to return to it year after year. Their starting hypothesis was that if turtles imprint on their natal beach using a geomagnetic signature, the turtles would nest at slightly different locations year after year since Earth’s magnetic field shifts slightly over time. To test this they mapped out 19 years of loggerhead turtle nesting data along the Atlantic coast of Florida. They observed that nesting locations would vary from year to year with some beaches being more popular than others depending on the year. They then examined how Earth’s magnetic field changed at their study site over the same period of time and found that there was indeed a strong association between the spatial distribution of turtle nests and subtle changes in Earth’s magnetic field. So it would seem that turtles are able to imprint on their natal beach’s unique geomagnetic signature and use this information to return there to nest!

James Gould, a professor in Ecology and Evolutionary Biology at Princeton University, reports in Current Biology that this is an “extremely clever analysis” in which the authors of the study took “initially unpromising data” and looked at it in an “entirely new way.” But what is the reason for a turtle to return to the beach where she was born in order to nest? J. Roger Brothers, one of the authors of the paper, says: “The only way a female turtle can be sure that she is nesting in a place favourable for egg development is to nest on the same beach where she hatched. The logic of sea turtles seems to be that ‘If it worked for me, it should work for my offspring.'”

Terrestrial basking and climate change

Terrestrial basking is a rare behavior observed in populations of green sea turtles in Hawai’i, Western Australia, and the Galápagos Archipelago. Being cold-blooded, the main reason why turtles bask on land is probably to regulate their body temperature but scientists speculate that terrestrial basking may also aid immune function, predator avoidance, and may even prevent unwanted courtship.

New research published in Biology Letters examined the relationship between terrestrial basking and climate. Having counted the number of turtles that bask on one Hawaiian beach every day for six years, the researchers found that terrestrial basking peaks in the year when the sea surface temperatures are lowest. Terrestrial basking generally happens when sea surface temperatures fall below 23°C. This suggests that terrestrial basking is a response to seasonally cool ocean temperatures.

Picture 2

Basking varies seasonally in concert with cool SST. Green circles are standardized anomalies of the number of turtles observed basking weekly at Laniakea, Oahu. Blue circles are weekly AVHRR SST data for this location. Thick dark lines are the Fourier series for each timeseries. (Source: Biology Letters)

However, since the sea surface temperatures at the sites where turtles bask on land is warming on average 0.04°C per year, the researchers predict that in the future the waters will be warm enough that the turtles will no longer come on land for warmth. The researchers estimate terrestrial basking may cease in Hawaii by 2039, in Australia by 2086, and in the Galápagos by 2102. Since other populations of marine turtles are successful without having to resort to terrestrial basking, this will probably not have drastic negative impacts on these green turtle population, but this does mean that beach goers of the future will not have the privilege of sharing the beach with napping turtles.