Galápagos 'Yellow' Sea Turtles Turn Out to Be Homebodies, Tracking Study Finds

Nine sea turtles fitted with satellite tags off four Galápagos islands have upended a basic assumption about how their kind lives. A study published Thursday in the journal Frontiers in Amphibian and Reptile Science found that so-called yellow-morph green sea turtles, one of two genetically distinct forms of the species that share Galápagos waters, largely stayed put in the archipelago for years at a stretch rather than ranging across the Pacific as expected.

The turtles were tracked for as long as 418 days apiece. None left the 198,000-square-kilometer Galápagos Marine Reserve during the three-year study, according to the paper and an accompanying release from the journal's publisher, Frontiers. That contradicts the long-standing picture of green turtles as ocean-spanning migrants that return to their birth region only periodically to breed.

The numbers

Researchers led by John W. Rowe, a biology professor at Alma College in Michigan, caught seven adult males and two adult females in shallow water off Española, Floreana, Isabela and San Cristóbal islands in 2015. Each turtle was fitted with an Argos satellite tag and given a name — Bernardo, Sara, Leandro and Ainoa among them — before being released and monitored until its transmitter fell off or stopped working.

• Tracking lasted 45 to 418 days per turtle, averaging just over nine months.
• Five of the nine turtles stayed near a single island for the full study period.
• The other four moved between two or three islands, but only from April to October, the cooler months in the archipelago.
• Average home range size was 1,498 square kilometers, though this varied enormously between individuals, from under 24 square kilometers to more than 27,000.
• Turtles mostly hugged shallow coastal water, occasionally venturing up to 25 kilometers offshore.

Ainoa, one of the two females, ranged farthest, moving from San Cristóbal to Santa Cruz and then on to Isabela. Even so, she and every other tagged turtle stayed inside reserve boundaries for the duration of the study, which ran from 2015 into 2018.

How the mismatch came to light

Green sea turtles are unusual among the world's seven marine turtle species for eating mostly seagrass and algae as adults, after starting life as omnivores. In the Pacific, they come in two shape- and color-based forms: a yellow morph found across the tropical and subtropical western Pacific, and a black morph that lives in the eastern Pacific off the Americas. The Galápagos is the only place on Earth where the two overlap and feed side by side on the same underwater plants, even though yellow morphs tend to grow faster than black morphs as juveniles — a sign, researchers say, of underlying physiological differences that remain poorly understood.

Genetic work has already established that the divide between the morphs runs deep. A 2021 study in the Royal Society journal Proceedings B used genome-wide sequencing across eight Pacific foraging grounds and concluded that the yellow and black forms are genetically distinct populations, probably kept apart by turtles' tendency to home in on the beaches where they themselves hatched. That homing behavior is also why scientists expected the Galápagos yellow morphs, none of which are known to nest on Galápagos beaches, to eventually swim off toward natal nesting grounds in the western Pacific. The new tracking data show that departure, if it happens at all, does not happen on any predictable annual schedule.

Who has a stake in the answer

The findings matter most immediately to those managing the Galápagos Marine Reserve, the protected waters administered by Ecuador that the study's authors say need consistent habitat protection across every island the turtles use, not just the ones where turtles are seen most often. Ecuadorian and international marine scientists studying the reserve — including researchers affiliated with the Universidad San Francisco de Quito, the University of the Sunshine Coast in Australia, Nanyang Technological University in Singapore, and the conservation groups Grupo Tortuguero de las Californias and Turtle Island Restoration Network — collaborated on the paper. Longer term, the results are also relevant to Ecuador's tourism industry, which promotes turtle sightings as a marine-reserve attraction, and to conservation groups such as the Charles Darwin Foundation, which separately monitors nesting green turtles and vessel-strike risk in the islands.

"Here we show that 'yellow-morph' green sea turtles — one of two genetically distinct forms — occupy individual foraging areas that 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 John W. Rowe, the study's corresponding author.

Reaction and next steps

Co-author Juan Pablo Muñoz Pérez, a professor at the Universidad San Francisco de Quito, said the absence of any yellow-morph females nesting in the Galápagos still points to periodic long-distance travel that the study window may simply have missed. "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," Muñoz Pérez said, according to the Frontiers release, which was also carried by Phys.org. He noted that black-morph females, by contrast, do nest in the Galápagos and can end up as year-round residents, occasional island-hoppers, or travelers as far as feeding grounds off Central and South America.

Rowe was more direct about the practical takeaway for reserve managers. Because individual turtles may depend on more than one island's coastline over the course of a year, he said, "the quality of habitat available for foraging should be kept intact across all islands and not allowed to degrade anywhere."

The authors were careful to frame their results as a snapshot rather than a final word. Three years of tracking, however detailed, cannot rule out that yellow-morph turtles make rarer, multi-year dispersals to breeding grounds that fell outside the study's timeframe. The team's next steps, based on the paper's discussion of open questions, point toward longer-duration tagging efforts and closer genetic comparison of Galápagos yellow morphs with populations elsewhere in the Pacific, in hopes of pinning down when, or whether, these turtles ever leave home to breed.