Scientists expected these Galápagos sea turtles to wander. They stayed put
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Sea turtles are known for extraordinary journeys across the ocean. They can navigate using cues from Earth’s magnetic field, the sun, and ocean currents, traveling long distances to find food before eventually returning toward the places where they were born to reproduce.
That reputation made one population in the Galápagos especially surprising. Rather than roaming across vast stretches of ocean, these turtles appeared to spend much of their time close to home.
“Here we show that ‘yellow-morph’ green sea turtles – one of two genetically distinct forms – occupy individual foraging areas which span one or more islands in the Galápagos. We expected them to migrate to distant beaches in the western Pacific to reproduce, yet interestingly, none of our satellite-tagged turtles even left the Galápagos Marine Reserve over the three years of our study,” said Dr. John W Rowe, a professor at Alma College in the United States and the corresponding author of a new study in Frontiers in Amphibian and Reptile Science.
Two Distinct Types of Green Sea Turtle
Green sea turtles stand apart from the other six living sea turtle species because adults are primarily vegetarian. Young turtles begin life eating both plants and animals, but as they mature, their diet shifts mainly to seagrasses and algae.
Scientists recognize two genetically distinct forms involved in this study. Yellow morph green sea turtles are generally found across tropical and subtropical parts of the western Pacific, while black morphs occur in the eastern Pacific west of the Americas and south of southern California.
The Galápagos is the only known place where both forms share the same waters. There, yellow and black morphs mingle and feed on the same types of marine vegetation.
They are not completely alike, however. Young yellow morphs tend to grow more quickly than young black morphs, suggesting that there may be physiological differences between them that scientists have not yet fully characterized.
Tracking Turtles Across the Galápagos
Rowe and his colleagues wanted to better understand where yellow morphs spend their time, how much territory they use, and whether they travel between islands within the 198,000 sq km Galápagos Marine Reserve.
In 2015, the team carefully captured nine adults in shallow water near four islands. The group included seven males and two females. Researchers gave the turtles nicknames and attached electronic transmitters that sent location signals to the low orbit Argos satellite network.
The turtles were followed for as long as 418 days. Tracking generally ended when a tag detached or stopped sending signals.
The results revealed a strong preference for shallow coastal habitat. Although the turtles occasionally traveled as far as 25km from shore, most remained much closer to the islands.
Five of the nine spent most of the study near a single island. Bernardo and Sara moved between Isabela and San Cristóbal, while Leandro traveled between Isabela and Española.
Ainoa was the most adventurous of the group. After being tagged near San Cristóbal, she later traveled to Santa Cruz and then to Isabela.
The researchers also noticed a seasonal pattern. Movement between islands occurred only during the cooler period from April through October.
On average, each turtle used a home range covering 1,498 sq km, although the size of that territory differed considerably from one animal to another.
Surprisingly Reluctant To Leave
Taken together, the tracking data suggest that yellow morph green sea turtles living around the Galápagos can be remarkably homebound.
The researchers cautioned that their observations capture only a limited period in the turtles’ lives. Longer migrations could occur at other times and therefore may not have appeared during the study.
That possibility is especially important for females.
“Since female yellow-morph green sea turtles are not known to lay eggs on the beaches of the Galápagos, we believe that they will eventually leave the area to return to their natal beaches in the western Pacific. In contrast, some female black-morph green turtles do nest in the Galápagos: depending on the individual, those may be year-round residents in the archipelago, travel to other nearby oceanic islands, or even to feeding grounds off Central and South America,” said coauthor Dr. Juan Pablo Muñoz Pérez, a professor at the Universidad San Francisco de Quito in Ecuador.
Protecting a Network of Turtle Habitat
The findings also carry implications for conservation. Because some turtles move among several islands, protecting only one favored feeding location may not be enough.
Researchers emphasized the importance of preserving the coastal waters and routes between islands that the turtles depend on as they move throughout the archipelago.
“Since individual turtles may rely on waters around multiple islands, the quality of habitat that is available for foraging should be maintained intact across all islands, and not allowed to degrade anywhere,” said Rowe.
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