Sentinel Species Research

Remarkable increase in turtle numbers on uninhabited islands

Remarkable increase in turtle numbers on uninhabited islands

Dr Nicole Esteban and Dr Jeanne A. Mortimer authored a new study that reveals a massive increase in the number of turtles nesting on the Chagos Archipelago in the British Indian Ocean Territory, one of the world’s largest Marine Protected Areas (MPAs).

The British Indian Ocean Territory (BIOT), a British overseas territory that is located in the Indian Ocean about halfway between Tanzania and Indonesia, spans 640,000 km² and tops the list of global marine no-take areas. The Territory has some of the most biodiverse waters on the planet with over 220 species of coral, 855 species of fish and 355 species of molluscs swimming through its water. BIOT also contains the Chagos Archipelago, a group of seven atolls comprising more than 60 individual tropical islands.

Dr Esteban, your study revealed an increase of between 225% and 525% from 1996 to today for the critically endangered hawksbill turtle, and between 465% and 930% for the endangered green turtle. Were you expecting these results?

There haven’t been any detailed studies of the numbers of turtles besides the very baseline figures that we had from 1970, 1996 and 2006. We didn’t actually expect the number of turtles to have increased as much as they did.  We were quite surprised.

What do you think the results mean for the conservation of these turtle species?

It’s fantastic news. In terms of conservation efforts, hawksbills are one of the most endangered species of marine turtles; they’re critically endangered on the IUCN Red List.

The British Indian Ocean Territory is a nesting refuge for a large proportion of the turtles nesting in the Western Indian Ocean – up to 51% of all hawksbills nest there. The fact that there’s such a large proportion nesting on uninhabited islands, well-protected by the MPA, is a really good for their conservation.

Do you think that this success is because of the MPA?

I think we have to be a little bit cautious because the MPA has only been formally in existence for just under ten years, and it takes 30-40 years to be able to document the recovery of sea turtle numbers. However, exploitation of turtles has been prohibited in the Territory since the 1970s so we can certainly attribute the recovery of sea turtle numbers in the British Indian Ocean Territory to long-term sea turtle conservation and protection. Laws were introduced in the early 1970s that protected turtles in the Chagos Archipelago and I would expect that this recovery will continue with the additional protection provided by the MPA.

One of the ways that you documented turtle activity was by counting turtle tracks. But how did you know that a single turtle didn’t make multiple tracks?

In fact, each turtle does make multiple tracks, and one of the assumptions that we made was to estimate how many tracks each turtle makes before laying eggs. Typically a turtle emerges from the ocean multiple times and may dig several trial nests before actually laying a clutch of eggs. This is because she may encounter obstacles on the beach or sand that is too soft or too hard before she is finally able to lay eggs. In our data analysis we’ve made the assumption of an average of 1.8 tracks for every egg clutch laid. This is based largely on data from turtles in Seychelles which are genetically related to those in the Chagos Archipelago and utilise very similar nesting habitats.

The other thing to bear in mind is that it’s really difficult to go to a remote island and count the number of tracks because much of the beach gets washed clean every spring tide. Therefore much of the data collected, especially in the remote outer islands, was based on counts of “body pits” high up on the beach platform. Body pits are the depressions turtles leave on the beach when they dig nests. These body pits can last for weeks or months, so they’re a much more reliable source of information.

How did you get involved with the Bertarelli Foundation, the funders of this study?

The Bertarelli Foundation started a marine science programme which involved a number of projects carried out in the British Indian Ocean Territory.  Our team applied for a grant to fund key research relating to hawksbill and green turtles breeding and foraging across the five islanded atolls of the Chagos Archipelago.

Were there any other unexpected discoveries?

At the time we began our study, seagrass meadows (the favourite food of green turtles) were believed to be relatively rare in the Chagos Archipelago. We anticipated that most of the green turtles that nested in the archipelago would migrate to foraging sites elsewhere in the Western Indian Ocean. Thanks to funding from the Bertarelli Foundation, however, we’ve discovered that after nesting in the MPA some of the satellite-tracked green turtles remain within the archipelago and forage on the Great Chagos Bank, the largest living coral atoll in the world. With the help of these satellite-tracked turtles we were able to discover large areas of deep water and previously unknown seagrass meadows in the MPA.

What comes next?

We still have many things to learn about the ecology of sea turtles in the Chagos Archipelago. We would like to do more surveys of nesting activity in the remote uninhabited islands of the archipelago. We need to try to reduce the assumptions that we’re using to calculate total number of turtles nesting.

We also want to learn more about the survival rates of egg clutches and the hatching turtles they produce, so that’s another area of enquiry. We’ve started looking at the effects of temperature on incubation and how beach litter affects the success of turtle nesting. We’re also starting to test different technologies to enhance the ability to monitor remotely.

There’s lots to find out!

This research was published in Oryx and was supported by the Bertarelli Foundation.

Mortimer, J. A., Esteban, N., Guzman, A. N. and Hays, G. C. (2020) “Estimates of marine turtle nesting populations in the south-west Indian Ocean indicate the importance of the Chagos Archipelago,” Oryx. Cambridge University Press, pp. 1–12. DOI: 10.1017/S0030605319001108.