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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Long-distance migrations

Most sea turtles come back to nest on the beach where they were born. How they do this, is still a mystery.

In their breeding years turtles will migrate from their feeding grounds to their nesting grounds. Before modern technology was available, scientists could only guess how far or where the turtles would go. Nowadays, GPS tags allow us to study the migrating behaviour of sea turtles.

The most common practice is to tag a nesting female turtle by placing a satellite tag on her shell. Out in the ocean, the GPS calculates the geographic position of the turtle every time she surfaces to breath. This information is relayed to the researcher’s computer via satellite. The turtle can be tracked for as long as the tag works, which can be weeks, months, and sometimes even years.

A green sea turtle returning to the sea with a satellite tag on her carapace (photo couretesy of Nicole Esteban)

A green sea turtle returning to the sea with a satellite tag on her carapace
(photo courtesy of Nicole Esteban)

Thanks to satellite telemetry studies, we now know where nesting turtles have their feeding grounds. For example, we now know that some of the turtles nesting on Ascension Island migrate to their feeding grounds in Brazil. This migration is quite a feat, considering that the trip from Brazil to Ascension Island is over 2000 kilometres and the turtles’ destination is a remote island less than twenty kilometres across.

Science has yet to find out how turtles navigate such vast distances. One hypothesis is that they use Earth’s magnetic field for navigation. In fact, a turtle’s ability to navigate the oceans of the world is one that amazed Charles Darwin himself, as shown in his Letter to Nature in 1873:

“Even if we grant to animals a sense of the points of the compass, of which there is no evidence, how can we account, for instance, for the turtles which formerly congregated in multitudes, only at one season of the year, on the shores of the Isle of Ascension, finding their way to that speck of land in the midst of the great Atlantic Ocean?”

Recently, the longest ever published migration for an adult cheloniid (a hard-shelled marine turtle) was recorded: a green turtle swam almost 4000 kilometres from its breeding grounds in Diego Garcia to its feeding grounds off the coast of Somalia!

The lost years

Sea turtles hatchlings go straight to the ocean after hatching. They will return to coastal waters as juveniles to forage. Not much is known about the years in between these two events, which is why this period has been dubbed the “lost years”. One thing that is known is that when the hatchlings first reach the surf they swim out to open waters for several days on end during what is called the juvenile frenzy. One of the ways this was demonstrated in the 1970s was by having good swimmers follow the hatchlings out at sea. It is thought that after a certain amount of time, the great oceanic currents catch the hatchlings. New research carried out recently in Boa Vista, Cape Verde, confirms this hypothesis. The researchers followed hatchlings with a boat for up to eight hours and up to 15 km out at sea using acoustic nano-tags. Dr Rebecca Scott, the lead author of the study reports that “for years, people have always spoken about hatchlings being swept away in the currents, but this is really the first good, direct evidence for that happening.” The results of this study are valuable to help better understand the biology of sea turtles and to get more information about the “lost years”. This, in turn, will be vital to develop more efficient conservation policies to protect this endangered species.

Video courtesy of the New York Times – http://nyti.ms/1tyjsF1

Naming turtles

One of the really fun things about being a sea turtle researcher is that you get to name sea turtles! When we tag new nesters, in order to refer to the turtle in a different way than just “turtle EES 962” we also name the turtle. It makes it really fun and exciting to then “follow” your turtle and see how regularly she nests.

If you could name a wild sea turtle, which name would you pick?

Lisa the Loggerhead returns to the sea (photo courtesy of Jacquie Cozens)

Lisa the Loggerhead returns to the sea
(photo courtesy of Jacquie Cozens)

 

Tagging turtles

Tagging turtles is a common practice in many sea turtle conservation projects worldwide. When you find a turtle that has never been observed nesting before you tag her flippers with small metal tags so you can identify her if you see her again. This simple practice allows to gather a wide variety of information on the turtle and its population. For example:

  • Do turtles nest every year?
  • Do turtles nest more than once in a season?
  • Do turtles always nest on the same beach?
  • How big is the local turtle population?
  • How far do turtles migrate?

This season I already tagged 4 new turtles. It is great to think that despite tagging turtles since 2008, SOS Tartarugas are still discovering new nesters every year!

Turtle tags

Turtle tags