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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2 thoughts on “Estimating sea turtle population sizes

  1. I think it is very important for someone not to present this as “bad news”. For example in the linked article the main title is “Sea Turtles Are in Much Worse Shape Than Previously Thought”. No they are not. The population did not magically decrease by a number of two from one day to another. Imagine for example a very healthy, stable population of X individuals with no anthropogenic threats. Then suddenly we realise that we have been overestimating this number and it is only X/2. Does that mean that this specific population is closer to extinction? I don’t think so, keeping in mind, that natural processes have done so, such that this is the equilibrium point for that population.
    Cool blog though! Keep the posts coming!

    Like

    • Hi seaturtlesallthewaydown,

      thank you for your insightful comment. I agree with you that the title of the article I link to sensationalised the findings of the new study. This certainly gives the impression that the results of this new research are “bad news” for sea turtles. In my post I tried to remain objective and present the results of the research without over-interpreting them. I thought that Dr Mortimer’s quote at the end of my post made it clear that their research is not implying that there are all of a sudden half as many turtles as before. I added an extra sentence to make it even clearer now.

      You are absolutely right, the status and health of an animal population is typically based on population trends (e.g. increasing, decreasing, stable). So a population that is doing well is still doing well, even in the light of this new research. However, having fewer individuals in a population can be detrimental for a number of reasons, including reduced genetic diversity and reduced ability to withstand exploitation (i.e. population bottlenecks). So on one hand, yes, a population that is doing well (i.e. stable or growing and with many individuals) is still doing well, but on the other hand a small population that is not doing so well may in fact be doing worse than previously thought. I think this is an important point to be aware of if we are to effectively protect sea turtle populations locally and globally.

      For more information, there is an interesting conversation that touches upon the points that you raise that can be found here:

      https://lists.ufl.edu/cgi-bin/wa?A2=ind1702&L=CTURTLE&T=0&F=&S=&P=44538

      Thank you for your input. I am glad that you found my blog interesting!

      Like

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