Where Do Baby Sea Turtles Go When They Hatch?

 

Where Do Baby Sea Turtles Go?

By KATE L. MANSFIELD

Given the terrestrial nature of humans, coastal beaches are where we are most likely to encounter sea turtles, their tracks in the sand, or nests they leave behind. It is incredibly labor-intensive, logistically difficult, and expensive to follow or survey turtles, especially little ones, in the middle of the open ocean. As a result, most of what we know about sea turtle biology derives from work conducted on beaches. Very little is known about sea turtles from the time hatchlings depart their nesting beaches and enter oceanic waters until they return to shallower coastal waters years later as larger “teenage” turtles.

In fact, so little was historically known about this period in sea turtles’ lives that it has been dubbed the “lost years.” Nonetheless, the time that sea turtles spend on land equates to but a blink of an eye when compared to their long lives spent at sea. Understanding sea turtle behavior at all their life stages is critical for ensuring the conservation and survival of those threatened and endangered species.

So, where do the baby turtles go after they leave the beach? How do they get there? How do they interact with their environment? Are they passively drifting with ocean currents or actively orienting and swimming to developmental habitats? Where and when are human activities more likely to affect their survival and their health? How long do they spend in oceanic waters before returning to coastal habitats as larger juveniles?

Historically, much of what we once knew about the sea turtle lost years was based on opportunistic sightings offshore or within boating distance of islands or the coast, or knowledge was derived (a) from lab-based studies of young turtles’ sensory capabilities, behavior, and orientation or (b) from short-term tracking studies (spanning a period of hours) of baby turtles from their nesting habitats. But little by little, technology is enabling us to answer some of the great questions about sea turtle biology.

Beginning in the late 2000s, satellite tags became small enough to enable researchers to track little three to nine-month-old loggerheads in the western North Atlantic. Small, 9-gram, solar-powered bird tags (modified for a marine environment) were attached to the young turtles’ shells with a combination of manicure acrylic, neoprene from old wetsuits, toupee glue, and aquarium silicone. The turtles were then released off their natal beaches in southeast Florida, providing the first long-term data about the movements and dispersal of young, oceanic-stage turtles.

The turtles’ tracks, combined with ocean modeling, confirmed that the young turtles were indeed living offshore, remaining mostly at the surface, and traveling within the large ocean currents that make up the North Atlantic Subtropical Gyre (NASG). But, unexpectedly, many of those turtles left the major ocean currents that make up the NASG and traveled to the Sargasso Sea, an area in the interior of the North Atlantic named for the Sargassum that collects in the region.

This travel makes sense; if small turtles are living on a mat of algae, they can easily find food, blend in with the brown algae to hide from predators, and hang out in a nice warm habitat. They bask at the sea surface while conserving energy by floating with the Sargassum— their “mobile home.” For little cold-blooded animals, having this thermal benefit in a safe, food-rich habitat where they can grow and thrive is likely key to their early survival.

Yet not all tracked turtles entered the Sargasso Sea; some turtles remained in the currents as expected, heading to the Azores in under 200–300 days from offshore of their south Florida natal beaches. The Sargasso Sea is emerging as an important developmental habitat for North Atlantic loggerheads and other species of sea turtle. The currents that make up the NASG may act as an enormous playpen for the young turtles, thus keeping them within the confines of the North Atlantic Ocean and the Sargasso Sea.

Follow-up studies in the Gulf of Mexico and the South Atlantic using passive oceanographic drifters—a fancy term for floating buckets with GPS satellite tags on them—demonstrated that young loggerheads are not always passive drifters being pushed around by ocean currents as was historically assumed. In fact, young (3- to 12-month-old) satellite-tracked loggerheads were observed to actively orient and actively swim in very different directions and with more velocities than the passively drifting buckets. Some loggerheads tagged in the Gulf of Mexico off of the coast of Louisiana, U.S.A., left the Gulf, traveled into the western North Atlantic, and connected with the NASG currents, whereby after less than a month and a half they were off the Grand Banks near Nova Scotia, Canada.

The South Atlantic has a similar gyre system to the north, called the South Atlantic Subtropical Gyre (SASG). Unlike in the Gulf Stream in the North Atlantic, the currents making up the SASG seasonally shift in their location off the coast of Brazil, which is home to the South Atlantic’s main loggerhead nesting beaches. Hence, turtles emerging from nests early in the South Atlantic hatching season will experience different currents (and modes of dispersal transport) than will turtles that hatch later in the season.

Young, oceanic-stage turtles that were satellite tracked early in the hatching season traveled to the south, whereas late-season tracked turtles traveled to the north, crossed the Equator, and entered the North Atlantic and Caribbean waters. Similar to loggerheads tracked from the Gulf of Mexico, Brazilian loggerheads connected with other regions and water bodies. However, none of the turtles tracked in the South Atlantic entered the center of the Gyre (like the turtles observed in the North Atlantic that traveled to the Sargasso Sea).

This is an exciting time. Newer, smaller tags are becoming available, allowing us to satellite-track younger turtles for longer distances. As more turtles are tagged in more regions and more oceans, we are finding that we can’t assume that baby turtles in different oceans are behaving in the same way, for example, hawksbill hatchlings in the East Pacific. Where do the baby turtles go? The answer depends on where in the world the question is asked. What we do know is (a) that little sea turtles are surface-dwelling oceanic creatures that actively orient and actively swim and (b) that we have a long way to go until we fully understand the sea turtle lost years.

 
SWOT Report vol. 14Ashleigh BandimereFAQs, Global