Objective of the Study
Green Turtles – Distribution and Habitat
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- Movement Patterns
- Understanding Sea Turtle Migration Trends
Green Turtles – Distribution and HabitatGreen Sea Turtles are distributed throughout the world’s oceans between 35 degrees north-south latitude. They are found in the eastern and western hemispheres and nest on beaches throughout the Atlantic, Pacific and Indian Oceans. Green Sea Turtles enjoy warm, tropical and sub-tropical, shallow waters near continental coasts and around islands where the sea is plentiful. Green Turtles occupy three habitat types: High energy oceanic beaches Convergence zones in the pelagic habitat Benthic feeding grounds in relatively shallow, protected waters Females deposit egg clutches on high energy beaches, usually on islands, where a deep nest cavity can be dug above the high water line. Hatchlings leave the beach and apparently move into convergence zones in the open ocean where they spend an undetermined length of time. When the turtles reach a carapace length of approximately 20 to 25 cm, they leave the pelagic habitat and enter benthic feeding grounds. Most commonly, these foraging habitats are pastures of sea grasses and/or algae, but small green turtles can also be found over coral reefs, warm reefs and rocky bottoms. Coral reefs or rocky outcrops near feeding pastures are often used as resting areas, both at night and during the day. Movement PatternsThe navigation feats of the green turtles are well known but poorly understood. We know that hatchlings and adult females on the nesting beaches orient towards the ocean using light cues. For a long time, no one knew what cues were employed in pelagic movements, in movements among foraging grounds, or in migrations between foraging grounds and nesting beaches. Recently conducted research however suggests that the earth’s magnetic field plays a role in these feats. Because green turtles feed in marine pastures in quiet, low-energy areas, but nest on high energy beaches, their feeding and nesting habitats, are, of necessity, located some distance apart. For instance, green turtles that nests on Ascension Island forage along the coast of Brazil, some 1000 km away. It is generally accepted that green turtles return to nest on the beach where they were born. Green turtles do exhibit strong site fidelity in successive nesting seasons and they also exhibit strong site fidelity for their foraging grounds. Understanding Sea Turtle Migration TrendsS 
ea turtles are elusive and secretive animals spending most of their lives in the sea, mostly migrating between their foraging and nesting area. The male sea turtles don’t leave the sea at all, while the female sea turtles only do so when nesting. Yet, all eight species are endangered or threatened. They are killed for meat and leather; their eggs are taken for food and aphrodisiacs. Their nesting sites go for development. They are ground up by dredges, run over by pleasure boats, poisoned by pollution, strangled by trash, and drowned by fish line and net. And we hardly know them. Scientists and researchers have long known that mysteries surrounding the living patterns and behavior of sea turtles cannot be solved unless knowledge is gained about their time spent in the sea that includes identifying their migration patterns to better understand their life cycle. The long distance migrations of sea turtles involve some of the most remarkable feats of orientation and navigation in the animal kingdom. As hatchlings, the turtles that have never before been in the ocean establish unerring courses towards the open sea as soon as they enter the water and then maintain their headings after swimming beyond sight of land. Adult turtles of several species migrate across hundreds or thousands of kilometers of open ocean to nest on their natal beaches, which are often isolated stretches of continental shores or tiny, remote islands. Such impressive feats are all the more astonishing in view of the fact that they are accomplished in an open ocean environment devoid of visual landmarks and by marine animals whose poor eyesight above water precludes the use of star patterns and other celestial clues. The management of populations of migratory species requires knowledge of the distribution of each population, identification of the jurisdictions (range states) through which the population passes, and cooperation among range states for regional management plans so that the geographic range over which management policies must be integrated can be properly defined. These are difficult but at the same time important to achieve for sea turtles with their extensive and largely obscure migration patterns. Research into investigating the sea turtle migration trends has only recently begun. It was only in 1954 that the father of sea turtle research, a visionary herpetologist, the late Archie Carr, set up camp on the beach at Totuguero, Costa Rica, the largest green turtle rookery in the Caribbean. Green turtle populations had plummeted, and Carr wanted to learn how to protect them. Today, one man on the beach has grown into an international army of biologists and volunteers trying to understand the ways of sea turtles and save them from extinction. Though there remains much to be learnt, there are some signs of progress. Studies done in the East coast of Florida on Loggerhead turtles reveal that hatchlings from the East coast of Florida migrate into the Gulf Stream current and the North Atlantic gyre. As the turtles grow, they remain in the gyre for a period of years, eventually returning to the southeastern Unites States coast. Later, as adults, the females migrate back to their natal beaches to nest. The initial offshore migration, in which hatchlings swim from the east coast of Florida to the Gulf Stream current, has provided a convenient starting point for investigating orientation mechanisms in sea turtles. The investigations suggest that sea turtles use features of the earth’s magnetic field in global position finding and perhaps in navigation. It has been found that Loggerhead sea turtles emerge from underground nests in the night, scramble to the sea and begin a trans-oceanic migration by swimming away from their natal beach and into the open ocean. Evidence suggests that hatchlings use three different sets of cues to maintain orientation during their initial migration offshore. While on the beach, the hatchlings find the ocean by crawling towards the lower, brighter seaward horizon and away from the dark, elevated silhouettes of vegetation and dunes. Upon entering the ocean, turtles initially orient seawards by swimming into waves, which can be detected as orbital movements from under water. Laboratory experiments have demonstrated that turtles can transfer a course initiated on the basis of waves or visual cues to a course mediated by a magnetic compass. Thus, by setting a magnetic course on the basis of near shore cues that indicate the seaward direction, hatchlings may continue on offshore headings after entering deep water beyond sight of land. Sea turtles may use earth’s magnetic field not only as a cue for compass orientation but also as a source of world wide positional information. Recent experiments have demonstrated that Loggerheads can detect subtle differences in magnetic field inclination and intensity, two geo-magnetic features that vary across the surface of the earth. Because most of the nesting beaches and oceanic regions are marked by a unique combination of these features, these findings raise the possibility that adult sea turtles navigate using a bio-coordinate magnetic map. Download 394.97 Kb. Share with your friends:
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