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Biological oceanography

Virtual Sea Turtles: Predicting the Movement of Hatchlings at Sea

A young sea turtle emerges from its nest and races toward the sea and several others are close behind as they dig their way out of the sand. You can probably think of what it’s like for a sea turtle hatchling at the very start of its life as it tries to make it to the safety of the surf, but have you ever wondered about what happens to that little turtle once it reaches the ocean? After all where does the young turtle go until it becomes an adult and returns to the shore it once emerged from?

A loggerhead sea turtle hatchling crawling in the sand towards the water.

Loggerhead sea turtles are named after their large heads and strong jaws which they use to crush their food like fish, shrimp, and mussels. These turtles can migrate long distances and prefer coastal areas with warm waters. Picture Credit: https://commons.wikimedia.org/wiki/File:Loggerhead_Sea_Turtle_Hatchling.jpg

The patterns of adult sea turtle traveling throughout the ocean have been well documented, but this is not the case for hatchlings. Where these young sea turtles go during their juvenile years and how they act out at sea is still a mystery in some areas of the world. The authors of this paper decided to investigate how sea turtle hatchling behavior impacted their dispersal and chances of survival out in the ocean off the coast of Africa. However, instead of tracking hatchlings via satellite, which is a common practice for adult sea turtles since the satellite can pick up the tracker’s signal once the sea turtle comes to the surface of the water for air, the authors used a computer simulation to model how these sea turtles may be dispersed once they enter the ocean. It is usually more difficult to apply a satellite tracker onto a hatchling’s carapace due to their small size. The computer simulation method allows for predicting the movements of hatchlings due to sea water currents moving them around without the potential negative impacts a tracker could have on the survival of the hatchling.

A leatherback sea turtle hatchling.

The leatherback sea turtle is the largest of all the sea turtles and has a unique shell which is not a shell at all, but smooth leathery skin covering flexible bone. This modification of a shell allows these turtles to dive to deep depths (up to 4,000 feet). Jellyfish make up most of their diet, but they will also eat fish and crustaceans. Picture Credit: https://commons.wikimedia.org/wiki/File:Look_ma,_I_can_fly!_(183138247).jpg

Virtual Hatchlings

All the hatchlings in this paper are “virtual”, meaning they are simulated in the model. The authors did collect data on the swimming speeds of hatchlings from nest sites at Bhanga Nek (which has high rates of loggerhead nesting) and Manzengwenya (which has high rates of leatherback nesting). These real-world swimming speeds were put into the model to replicate how fast these hatchlings would be moving once they enter the ocean from the nest site until they reached the main ocean current, which in this case is the Agulhas Current.

The Agulhas Current is located off the east coast of Africa and is one of the major surface currents of the world’s oceans. As the current moves, some of the warm water from the Agulhas gets trapped in rings that tear away from the main current and move into the South Atlantic Ocean. The blue diamond indicates the area where the hatchling sites are in relation to the Agulhas Current. Picture Credit: https://commons.wikimedia.org/wiki/File:Benguela_and_Agulhas_Currents_2.jpg

Where Do the Hatchlings Go?

The model revealed that, after a simulation corresponding to the length of a year, most of the virtual hatchlings were transported to three distinct locations: the Agulhas Return (the portion of the Agulhas current that moves into the Indian Ocean off the southern coast of Africa), the Southeast Atlantic, and the Southern Ocean. However, the dispersal location varied based on when and where the hatchlings emerged from their nests. This means that more research on the ecology and behavior of these hatchlings must be taken into account for the model to be truly accurate, but this is a great start to gathering information for the conservation of these sea turtles as they disperse from their nests into the sea. While more information needs to be added to the simulation model to make it more effective, this method can assist in the conservation of threatened sea turtles to help aid in actions to increase sea turtle population sizes in the long-term to  allow for these animals to persist into the future through the protection of their young.

Paper: Le Gouvello, D. Z., Hart-Davis, M. G., Backeberg, B. C., & Nel, R. (2020). Effects of swimming behaviour and oceanography on sea turtle hatchling dispersal at the intersection of two ocean current systems. Ecological Modelling431, 109130.

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