Where do the turtles rest on Rapa Nui?

by Camila González e Ignacio Petit

ave you ever asked yourself how the animals sleep in the ocean?   It’s not an easy question to answer, especially for those animals which can’t breathe under water and need to regularly return to the surface, such as the dolphins, whales and turtles.  Although we know that marine animals don’t sleep in the same way as humans, and therefore we can’t really call it “sleep”, it is well known that they rest in a programmed manner and require certain specific conditions to do so.

In the case of marine turtles, it is known that they can remain submerged for long periods and that the duration of that time can depend on the amount of energy that they have to expend during that period. Although it has been documented that they can remain submerged for several hours while they rest, this probably will also depend heavily on their geographic location, whether they need to be more hidden to protect themselves from predators (for example, humans and/or sharks), the force of the swells and waves, etc.

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Rapa Nui is a refuge for marine turtles in the middle of the Pacific Ocean. They don’t have predators here (for example, large sharks) and the human don’t eat them as happens in other parts of Polynesia (for example, Tahiti). There is a local population of green turtles (Chelonia mydas) which has been studied by citizen-scientists since 2010.  More than 40 individuals have been documented through photographs of their faces (photo 1) which can be as individual as human finger prints.

After many years of diving with them (hundreds of hours under the water), we began to identify different points on the coral reef where the turtles seem to rest, so we began to call them “beds” (photos 2 and 3), all of them located in front of the Hanga Roa cove. These beds are used by one turtle at a time and are small and semi-circular. Due to the repeated use of them, they can be identified by the erosion caused by the rubbing between the plastron (stomach shell) of the turtle and the reef (photo 4).

After we identified their beds (around 20 of them), our observations became sharper and, through the use of photography, we were able to discover that different turtles occupied the same beds in different periods of time and that some turtles are resting in the same beds for more than 5 years! This tells us that these beds are not chosen at random and that they share similar characteristics.  They are found among the reefs, protected from the waves at a depth of approximately 10 meters (33 feet). These characteristics probably are related to decreased expenditure of energy. At this depth, the movement of water is less than at the surface and, among the reefs protected from the waves, they find enough calm to remain immobile and rest tranquilly while saving energy.

 

Butt then…  when do they use these beds? was the new question that arose. After collecting many photos of turtles resting in their beds, over many consecutive years, and checking the time that each photo was taken, we were surprised to find that all the registries of all the years happened at low tide. So now we knew where they rested and at what moment of the day.

To fill out the research, we wanted to find out what happened at high tide. We suspected that during those hours the turtles spent their time feeding along the coast at a well-known site in Hanga Roa cove. Following a year of observation, together with some of the local divers, we discovered that our suspicions were correct. During high tide, it’s always possible to observe many more turtles than at low tide, probably because during high tide they have better access to their food (green algae) which grows on the intertidal rocks.

Therefore, the green turtles of Rapa Nui feed at high tide in the Hanga Roa cove and rest during low tide in their beds on the reef. This behavior of using resting beds (not for reproduction), and with high levels of assurance that this has not been previously described for marine turtles, is comparable to the behavior of terrestrial vertebrates, such as orangutans or gorillas who select specific branches of trees to build their nests and also reuse them.

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We should note that in a 24-hour cycle (a complete day), there are normally two low tides and two high tides. Our team was only able to do our research during those tides that occurred in the daytime. So, what happens at night?  That is still an unknown aspect that scientists and local divers will have to study.

We also have to note that the turtles, at high tide, are found near the quay at Hanga Roa, where the boats of the diving centers and the fishermen come and go.  It is necessary that they pay more attention to the turtles, especially at this time of day, to avoid accidents that could be fatal for these lovely reptiles (photo 5).

Marine turtles are important species for the ecosystem, iconic and profoundly integrated in the culture of Polynesia and Rapa Nui. This can be seen in the numerous artistic expressions around the entire island, from archaeological drawings in the volcanic rocks to ancestral stories and even in modern art as sculptures, mosaics, tattoos, among many others.  For these reasons, we all must contribute to the conservation of these beautiful and ancient marine reptiles. You can help, too!  If you see turtles feeding in the cove, observe them at a distance so that they can eat tranquilly. Just as well, if you see turtles sitting on the sea floor during a dive, don’t bother them. Remember that they are resting placidly after a good meal on the coast of the Island.

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