Sea turtles

Posted: 23 July 2007

There are seven recognized species of marine turtles, including the leatherback (Dermochelys coriacea), the loggerhead (Caretta caretta), the hawksbill (Eretmochelys imbricata), green turtle (Chelonia mydas), flatback turtle (Natator depressus), olive ridley (Lepidochelys olivacea), and the Kemps ridley (Lepidochelys kempii). All are either endangered or threatened. Leatherbacks, hawksbills, and Kemps ridleys are considered critically endangered.

Their conservation requires the preservation of intact habitats ranging from tropical nesting beaches to sub-Arctic foraging grounds. Although this is also true for highly migratory species of fish and for most marine mammals, sea turtles are unique in that they rely not only on ocean habitat but also on terrestrial habitat.

Nesting beaches must remain open and secure for sea turtles to utilize them, access to these beaches must be maintained, and the nesting beach environment must be almost pristine to successfully support sea turtle reproduction. The more disturbance on the beach, the greater the chance that the female will abort her eggs in the water or unsuccessfully attempt to make her nest, known in the turtle lingo as a "false crawl". Light contamination on the nesting beach - a common occurrence on most beautiful wide sandy beaches in the tropics where sea turtles lay their eggs - dooms both adults and young, as light from behind the beach fatally draws turtles away from the sea.

Turtle hatchlings
Turtle hatchlings
Hatchlings rushing to the sea. Photo © US Fish and Wildlife Service
Once the hatchlings leave the safety of their underground nest on the sandy beach, they scurry to the ocean to escape the innumerable dangers of land and find relatively safe nursery habitats in which to grow. Just where these nursery grounds are is still a mystery - in the Atlantic Ocean the Sargasso Sea has been fingered as the most probable place where small turtles can both find food and escape predation by hiding in the drifting sargassum weed.

As sub-adults, sea turtles often congregate in nutrient rich shallow waters, to continue their slow growth to adulthood (most species take a decade or more to mature and will live for several.) These critical areas vary according to the species - for herbivorous green turtles, seagrass meadows and areas of algal-encrusted rock reef are preferred; for the sponge-eating hawksbills, diverse and healthy coral reefs are the only habitat where they can survive; for leatherbacks, cold and rich upwelling areas in temperate zones provide large quantities of the jellyfish they consume with vigour, and for the others that are omnivores, areas that support large populations of benthic fish and crustaceans are the coastal habitats of choice.

Adult turtles may go to different feeding grounds altogether, and when sexually mature will travel to breeding areas to mate. Gravid females come ashore on tropical nesting beaches to lay their eggs - a hundred or more at a time, in nest pits that they painstakingly excavate with their hind flippers.

Green turtle
Green turtle
Green turtle visiting the sacred island of Pailao, Guinea Bissau
The process of finding access to the beach, hauling a huge body built for aquatic life onto gravity-encumbered land, then crawling with flippers made for water across wide swaths of sand, rock, and berms, is extraordinarily difficult. Finding a suitable nesting spot (like all reptiles, turtles do not incubate their eggs but rather let the warm sand of tropical beaches do it for them - the temperature and moisture level must be just so...), digging the nest, laying the eggs, then carefully covering the nest and disguising it takes hours, by which time the mother turtle is spent - and highly vulnerable to a host of predators that include man.

While important nesting beaches can be and often are protected as parks or reserves, in many cases the very existence of the beach is at risk from human activities, sometimes far from the coast. With worldwide use of freshwater for irrigation, consumption, and hydroelectric power, estuaries around the world are showing signs of massive sediment starvation (decreases of freshwater limiting the delivery of sediment to the coast).

Others are formed by sands produced through a combination of coralline animal and coralline algae remains. When coastal development or blast fishing destroys part of the reef system, beach formation can cease and beaches erode away. And increasingly frequent tropical storms and the occasional tsunami can instantly erase nesting beaches from the face of the earth.

Perhaps even more important than habitat protections are international agreements, regulations, and enforcement of laws concerning commercial fishing in areas frequented by turtles - either those resident or those migrating through.

Longline fisheries have decimated leatherback turtle populations, especially in the eastern Pacific. Gillnets are devastating to all marine turtle species. And bottom trawls routinely drown loggerhead, ridley, and other species - since sea turtles commonly feed on the very things we wish to catch, such as shrimp or prawns. Since so many sea turtles are killed incidentally in commercial fishing operations, their protection means restructuring how and where we fish - something that is notoriously difficult to do when highly lucrative fishing interests are at stake.

Source: World Oceans Observatory/Tundi Agardy, PhD