The Mystery Of The Green Sea Turtle’s “Lost Years”

Until recently, it was widely assumed that during this time the turtles drifted passively with the ocean currents, lacking the size or power to consciously determine their own direction.

 

Found in tropical and subtropical waters worldwide, the green sea turtle is one of the most charismatic marine species. It has been the subject of countless research papers, all aiming to increase our understanding of this turtle’s life cycle and behavior. These studies have revealed many things about the green sea turtle, including its diet, its reproductive habits, and the factors that have caused numbers to decline such that it’s now considered endangered. But despite the fact that the green sea turtle is the most studied of all seven sea turtle species, there is one glaringly obvious gap in our knowledge of this animal’s life cycle.

As divers, we often see adult or adolescent green turtles grazing in the shallow waters of tropical dive sites. Some of us may even have seen them as brand-new hatchlings, making their way from their sandy nesting sites into the surf zone for the very first time. And yet, green turtles — like other turtle species — are rarely seen between these two stages. Once they enter the ocean as hatchlings, they simply disappear, only to reappear several years later as young adults in the shallow waters of the continental shelf. This phenomenon has mystified scientists for decades, and in 1952, famous sea-turtle biologist Archie Carr coined the term “lost years” for this apparent disappearance.

In 2007, the Archie Carr Center for Sea Turtle Research announced that it had made significant progress in decoding the mystery of the lost years. Three of the center’s scientists, Kimberly Reich, Alan Bolten and Karen Bjorndal, published a paper that suggested that green sea turtles spend the first three to five years of their lives in open ocean, where they rely upon a carnivorous diet of mainly jellyfish. This is in direct contrast to the habitat and diet of adult green turtles, which remain predominantly in shallow coastal water and are herbivorous grazers. Reich, Bjorndal and Bolten’s hypothesis, although revolutionary, was supported by chemical elements, or isotopes, ingrained in the turtles’ shells found through analysis.

The research team used non-lethal methods to analyze the chemical elements present in the shells of 44 different green turtles, all of which were captured in shallow water off the Bahamas. Of these, 28 were tagged turtles that had previously been established as long-term residents of the area. Sixteen of the turtles were untagged, having only recently arrived after emerging from their lost years. For each turtle, the scientists compared samples of old shell tissue with samples of newly grown shell tissue, establishing in each the levels of carbon and nitrogen isotopes. Carbon and nitrogen isotope levels differ depending on an animal’s habitat, i.e. whether it lives in open-ocean or in the shallows, and its diet, i.e. whether it is carnivorous or herbivorous.

The results for the 16 new arrivals showed distinct differences between the isotope signatures of old and new shell. The old shell showed a signature similar to that of ocean-going, jellyfish-eating loggerhead turtles, while the new shell showed a signature similar to that of the resident, adult green turtles. It was these results that led the researchers to conclude that the green sea turtle’s lost years are spent in the open ocean.

Until recently, it was widely assumed that during this time the turtles drifted passively with the ocean currents, lacking the size or power to consciously determine their own direction.

However, in April, researchers from the National Oceanic and Atmospheric Administration (NOAA) published the findings of a new study, which proves that juvenile green turtles are in fact exceptionally active swimmers. To do this, Dr. Kate Mansfield from the University of Central Florida’s Marine Turtle Research Group attached solar-powered tags to 24 wild-caught green turtle ‘toddlers.’ Upon releasing each turtle back into the ocean, Mansfield simultaneously deployed a surface buoy carefully weighted to mimic a passively drifting baby turtle. The buoys and the turtles were then tracked for 2 to 3 months, after which the tags harmlessly shed from the turtles’ shells.

The results of the experiment showed that the movements of the turtles and the buoys differed dramatically, contradicting the existing theory that hatchling turtles spend their early years at the mercy of ocean currents. Instead, Mansfield observed a difference of up to 125 miles in the distance between the turtles and the buoys within the first few days. In most instances, the turtles were actively orienting themselves either to reach or to remain within a favorable ocean habitat. The results also showed that the turtles were persistent and directed in their swimming, with almost all of the green turtles heading east, while similarly tagged Kemp’s ridley turtles consistently swam north.

Although it is not yet known why specific species might swim in different directions, this research study shows that our previous assumptions as to the dispersal of green turtle hatchlings during the lost years may be significantly off base. Discoveries like this one and the one made in 2007 go some way towards revealing the secrets of the turtles’ early lives. Knowing where green sea turtles go during this time is crucial to their conservation, as it enables scientists to determine the risks that they face, and as a result, to recommend protective legislation. As Bjorndal, one of the three scientists involved in the 2007 research project succinctly said, “You can’t protect something if you don’t know where it is.”

Many more questions about the green sea turtle’s lost years have yet to be answered. Today, for example, researchers from the Archie Carr Center for Turtle Research are attempting to determine the turtle hatchlings’ precise geographical location through trace-element analysis. Studies like this are critical, because with every piece of the lost years puzzle found, the future of this remarkable endangered species becomes more secure.