Worldwide there are seven species of marine turtles and five occur off the coast of Tanzania (green, hawksbill, loggerhead, leatherback and olive ridley), although only green and hawksbill turtles nest in Tanzania.
Sea turtles can grow quickly but take decades to reach sexual maturity. As immature turtles, they drift on ocean currents for many years. At between 20 – 50 years old, females migrate long distances (thousands of kilometres in some cases!) from their feeding ground to a nesting beach on which they themselves were born.
Two months or so after mating, a female turtle crawls out of the sea and uses her front flippers to drag herself up the beach to a nest site. She digs out a body pit with her front flippers and excavates a vertical egg chamber. She then spends up to an hour laying the clutch of leathery-shelled eggs. After laying, the turtle fills the egg chamber with sand using her hind flippers and then fills the body pit using all four flippers. Finally she crawls back to the sea about three to four hours after emerging, exhausted.
The temperature of the nest during incubation determines the sex of hatchlings. Warm, dark sand produces mostly females. Eggs laid in cool, white sand result mostly in males and generally take longer to hatch.
The eggs hatch after an average of 55 days. The hatchlings take two or more days to reach the surface of the nest where they emerge, usually at night. To find the sea, hatchlings orient towards the brightest direction and use the topography of the surrounding horizon line. Once in the sea, hatchlings use a combination of cues (wave direction, current, and magnetic fields) to orient themselves to deeper offshore areas. Crossing the beach and swimming away is believed to imprint the hatchlings with the cues necessary to find their way back when they are ready to breed.
Green turtles are the most common species in Tanzania, feeding on extensive sea grass beds found along the Tanzanian coast. Hawksbills feed on sponges and are frequently observed by divers on coral reefs in Mafia Island. Olive ridley, loggerhead and leatherback do not nest in Tanzania but pass through Tanzanian waters on route to feeding and breeding grounds elsewhere in the region.
Sea turtles in Tanzania are under threat as a result of centuries of human exploitation for food, oil, leather and ornaments, as well as mortality associated with incidental capture in the fishing industry, marine and land-based pollution and degradation of feeding habitats. Unregulated coastal development poses a significant threat to nesting beaches.