For over 3,000 years, green sea turtles have returned to the same seagrass meadows to eat, according to a study published in PNAS. The Dutch team combined modern data and archaeological findings to find that sea turtles migrate between specific breeding and eating places and have been doing so for generations. According to the authors, this only highlights the importance of protecting seagrass meadows along the coast of North Africa.
By the time young sea turtles hatch, their parents are long gone. The young offspring make their way to the water but stay in the area for years until they’re capable of doing long migrations. During this time, they follow an omnivore’s diet, but when they’re about five years old, they swim to the same area where their parents went and start eating a strict herbivore diet of seagrass.
There are already some measures to protect the young sea turtles and the place where they hatch, but not the seagrass meadows. “We currently spend a lot of effort protecting the babies but not the place where they spend most of their time: the seagrass meadows,” said Willemien de Kock, a historical ecologist at the University of Groningen in The Netherlands.The problem is these seagrass meadows are suffering from the effects of the climate crisis.
It all started at the Groningen Institute of Archaeology at the University of Groningen, where De Kock could access remains of sea turtles collected in archaeological sites in the Mediterranean Sea area. With these bones, De Kock was able to distinguish two species: the green sea turtle and the loggerhead turtle.
The researcher was also able to determine what these sea turtles had been eating. By inspecting the collagen present in the bones, de Kock could detect what kind of plants the sea turtles ingested. “For instance,” De Kock explained, “one plant might contain more of the lighter carbon-12 than another plant, which contains more of the heavier carbon-13. Because carbon does not change when it is digested, we can detect what ratio of carbon is present in the bones and infer the diet from that.”
The second part of the study involved modern satellite tracking data from the University of Exeter. This provided information about current travelling patterns of today’s sea turtles. Combining old and new data, de Kock connected the diets of sea turtles alive thousands of years ago with those alive today. The results show that for about 3,000 years, generations of sea turtles feed in the same sea grass meadows along the coasts of Egypt and West Libya.
According to the authors, this is a crucial way to avoid the shifting baseline syndrome and compare data going back thousands of years. “Even long-term data goes back only about 100 years,” said De Kock. “But tracing back further in time using archaeological data allows us to better see human-induced effects on the environment. And it allows us to predict a bit.” Recent models show a high risk of widespread loss of seagrass in exactly the spots where green sea turtles have been going to feed for millennia. The authors warn this could be extremely detrimental to green sea turtles because they seem to go back to the same places repeatedly.
de Kock W, Mackie M, Ramsøe M, Allentoft M, Broderick A, Haywood J, Godley B, Snape R, Bradshaw P, Genz H, Tersch M, Dee M, Palsbøll P, Alexander M, Taurozzi M, Çakırlar C (2023) Threatened North African seagrass meadows have supported green turtle populations for millennia. PNAS, https://doi.org/10.1073/pnas.2220747120