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.

 

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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.

 

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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)

Female-biased sea turtle populations

The temperature at which a sea turtle egg incubates determines the sex of the hatchling. Below a pivotal incubation temperature of about 29.0°C the majority of individuals are born male and above that temperature the majority of individuals are born female. This unique process, known as Temperature-Dependent Sex Determination (TSD), is widespread among reptiles.

What will happen to sea turtle populations when air temperatures rise due to climate change? This is a central question of my latest research, the results of which were published in the Journal of Experimental Marine Biology and Ecology this week.

The study was a collaboration between researchers from Swansea University (United Kingdom) and Deakin University (Australia), and conservationists working on the Dutch Caribbean island of Saint Eustatius. We recorded sand temperatures at a turtle nesting beach where leatherbacks, hawksbill and green sea turtles nest. These data were combined  with temperature projections from the Intergovernmental Panel on Climate Change (IPCC) to model how sand temperatures will change in the next 100 years.

The results showed that sand temperatures are relatively high (ranging from 29.1-33.3°C) at this nesting site and that all three species of sea turtles are female-biased: we estimate that for the current populations of hawksbill, leatherback and green turtles the percentage of turtles born male are 36%, 24%, and 16%, respectively. Projecting into the future, it is likely that the female-skew will be intensified due to warming air temperatures. For example, projections indicate that only 2.4% of green turtle hatchlings will be males by 2030, 1.0% by 2060, and 0.4% by 2090.

Nicole Esteban, a Swansea University researcher and former St Eustatius National Parks (STENAPA) manager, who is a lead author of the paper says that “there is a real concern that there will not be enough males born on Saint Eustatius in the future. If there are too few males, the local population is at risk of collapsing. Another concern is that turtle eggs do not develop above a certain temperature. The study highlights the extinction risks of climate change to species whose biology is closely linked to temperature.”

This research underlines that there is real need for effective conservation measures to be put in place to prevent the localised extinction of these turtle populations. Potential conservation strategies include shading turtle nests on the beach or moving nests to a cooler section of a beach such as a protected hatchery.

fdfa http://www.frogfishphotography.com/index.html

A hawksbill turtle swimming near a wreck in Saint Eustatius
(photo courtesy of Frogfish Photography)


Sand temperatures for nesting sea turtles in the Caribbean: implications for hatchling sex ratios in the face of climate change” was published by the Journal of Experimental Marine Biology and Ecology (2016). Authors: Jacques-Olivier Laloë, Nicole Esteban, Jessica Berkel and Graeme C Hays.

Plastic pollution

Plastic pollution in the oceans is a real problem that is often easily set aside because we do not see its effects in our daily lives. Occasionally, however, we are reminded of the dramatic consequences that plastic pollution has on life in the oceans.

A group of marine biologists led by Christine Figgener working in Costa Rica came across a male olive ridley with what appeared to be a parasitic worm inside its nostril. After careful inspection, it turned out that the foreign object was in fact a plastic drinking straw. The researchers, who were a few hours from the nearest vet and x-ray machines, were able to successfully remove the straw with a pair of pliers on the boat.

I communicated about the event with herpetologist and sea turtle expert Prof. Jeanette Wyneken from Florida Atlantic University. She tells me that “anatomically, cheloniid (or hard-shelled) sea turtles have a partial secondary palate between the nostrils and the mouth. […] The straw looks like it was in that space, possibly extending along the lateral part of the nasal sac but it could have gone through the choana into the mouth.” Furthermore Prof. Wyneken explains that “turtles, like many reptiles, wall-off foreign bodies with caseous material and fibrin (sort of a scaffolding of protein). That is likely why the straw was so hard to remove. The bleeding nose was likely due to those fibrin components being torn away from the normal lining.”

Thankfully the turtle is reported to have been in good health and was promptly released. “The bleeding stopped pretty much immediately after the removal of the straw,” reported Ms Figgener. “We disinfected the air passageway with iodine and kept the turtle for observation before releasing him back into the wild.[…] He did very obviously not enjoy the procedure very much, but we hope that he is now able to breathe more freely.”

Let’s hope that this video serves as a reminder to be responsible with the disposal of our waste, especially plastic waste. You just never know where your garbage might end up.

Video courtesy of The Leatherback Trust

Identifying sea turtle species

There are seven species of sea turtles and to the untrained eye they look somewhat similar. In fact, turtles are often misidentified in the press: it is not rare to see the picture of a green turtle next to an article about loggerhead turtles. So how can you correctly identify the species of a turtle?

Leatherback turtles are easy to recognize because of their unique soft leathery back. For the other six species it is a bit trickier. The key to correctly identifying a turtle species is by examining its shell and the pattern of its scales (also known as scutes).

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The name of the different scutes on a sea turtle carapace

By counting the costal scutes on a turtle’s carapace and the prefrontal scales on its head, you can tell each species apart. For example, hawksbill turtles have four costal scutes on each side of their carapace and two pairs of prefrontal scales on their head unlike green turtles that have four costal scutes but only one pair of prefrontal scales. By using the following turtle identification key you can easily identify the seven species of sea turtles. Alternatively, check out this interactive species identification key from the very informative Sea Turtle Conservancy.

Sea turtle identification key (courtesy of seaturtle.org)

Sea turtle identification key
(courtesy of seaturtle.org)

Of course there are other ways to quickly identifying turtle species. For example, loggerheads have a typically large head and a beautiful red-orange colour. With experience you learn how to tell apart one species from the other without resorting to counting scutes, but examining the carapace is always a sure way to confirm your identification!

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Can you identify the species of this juvenile sea turtle?

Titanium turtle

Every year boat strikes contribute to the anthropogenic mortality of several threatened marine species including sea turtles, manatees, dugongs, and whales. A boat strike will often result in severe injury and the animal’s death.

However, for the male loggerhead turtle “Akut-3” the story is different. The turtle, named after the Search & Rescue Association that found him, was taken to the DEKAMER Sea Turtle Research, Rescue and Rehabilitation Centre after he was found badly injured and floating off the shores of Turkey. Akut-3 had visibly had a close encounter with a boat propeller: he suffered from severe facial wounds and a fractured jaw. With over 60% of his jaw missing and unable to feed anymore, this turtle’s fate seemed sealed.

The only hope was total facial reconstruction. Working in collaboration with BTech Innovation A.Ş. the veterinary surgeons designed an implant for the turtle and had it 3D printed using medical-grade titanium. After a successful operation, Akup-3 is still recovering at the DEKAMER Centre.

This is the first time a sea turtle benefits from 3D printing technology. As of yet, the turtle does not show any signs of rejecting the implant but it will take time before it is confirmed that the operation was a complete success. Let’s hope that the turtle can be fully rehabilitated and released back into the wild!

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Photos courtesy of BTech Innovation A.Ş..