By Stuart and Bronwyn Dunlop, Conservation Managers at Cousine Island, Seychelles…
Hawksbill turtles, Eretmochelys imbricata, are medium sized sea turtles with an average weight range of 65 – 80 kilograms and length of one meter. This species of turtle is characterised by large overlapping scales on its carapace and an elongated, tapered head that, as the common name suggests, ends in a protruding hooked upper beak. An additional identifiable feature are the two claws on the forelimbs of hawksbills.
DISTRIBUTION: Hawksbill turtles are widely distributed throughout the tropics, and to a smaller extent, the subtropical waters of the Atlantic, Pacific and Indian oceans. Although they are known to inhabit coastal waters in more than 108 countries around the world, nesting has only been recorded in 70 countries located in their known geographical range.
HABITAT: Due to their migratory nature, hawksbill turtles have been observed over a wide range of habitats, from open ocean to lagoons, as well as mangrove swamps in estuaries. Hatchlings and young juveniles (carapace length < 20cm) are pelagic and passively migrate with the prevailing ocean currents, while sub-adults and adults are found over a continental shelf habitat amongst the tropical coral reefs.
BIOLOGY: Hawksbills are opportunistic omnivores, feeding on sea sponges in particular, but also many species of invertebrates. Their narrow, sharp beak is an excellent adaptation for foraging among coral crevices. They take between 20 to 40 years to reach sexual maturity depending on their locality. Studies completed in the Caribbean have suggested 25 years to maturity, whereas 35 years has been noted in the Indo-Pacific region.
Once sexually mature, this species is known to undertake breeding migrations between foraging and nesting grounds at intervals of several years. Female hawksbills display strong site fidelity to their natal rookery, returning at two- to five-year intervals throughout their reproductive years. The nesting season varies considerably according to geographical location. Nesting occurs throughout the tropical and subtropical regions on insular and mainland sandy beaches or amongst beach vegetation.
The season typically coincides with the local rainy season, low wind velocities and calmer seas. Although the islands of the Seychelles have reported hawksbill nesting during the day, many other countries have noted this behaviour only at night. This suggests that hawksbills may nest at night in heavily populated areas, but during the day in uninhabited areas.
Hawksbills lay on average four times within a given season in intervals of roughly 15 – 18 days. Clutch size is approximately 140 eggs per nesting attempt, with an incubation period that lasts about 60 days. Interestingly, hatchlings move from the egg chamber to the surface by periodic phases of group thrashing. This scrapes sand from the top and sides of the chamber which builds up the chamber floor until it reaches the surface. This process can take up to six days before hatchlings emerge together and head towards the ocean.
MOVEMENT: Highly migratory species.
BEHAVIOUR: Although critically endangered, hawksbills are a widely spread species with sub-adults and adults regularly being encountered when diving or snorkelling, particularly in areas with abundant coral reefs. Although seemingly unperturbed by the presence of divers, which allows for uninterrupted observations of either feeding, resting or swimming behaviour, hawksbills will move away from a diver or snorkeler should they get too close.
Juveniles (< 20 cm) are very rarely encountered while diving/snorkeling as they tend to be found further offshore around floating debris. Newly hatched hawksbill turtles can be encountered on various beaches where the large females lay their eggs. If you are in an area where females nest, it is best to organize a guided tour to guarantee your chances of this once in a life time experience.
CONSERVATION STATUS: Hawksbill turtles are currently categorised as “critically endangered” on the IUCN Red List of Threatened Species, which means that they face an extremely high risk of extinction in the wild. These reptiles are heavily exploited for their carapace, which is used in the making of jewelry, combs, sunglasses, ornaments as well as other luxury and decorative items.
Juveniles are often also collected and stuffed for sale as tourist curios. Although the trade in hawksbill turtles has been prohibited and the volume of trade diminished, the industry in Japan remains active and intact. It is estimated that Japan imported two million hawksbill turtles between 1950 and 1992.
Besides being over-exploited, all sea turtles are especially susceptible to the effects of consuming marine debris. Sea turtles have downward facing spines in their throats which prevent the possibility of regurgitation. The plastics get trapped in their stomach, which prevents them from properly swallowing food. This subsequently leads to starvation or makes them an easy target for predators.
Other threats to the population include increased development of coastal zones resulting in beachfront lighting and habitat loss, nest predation, and incidental catch by fishermen. The combination of all these threats has severely impacted on the hawksbills and has resulted in the continued decline of the population over time.