Credits: © Rainer von Brandis
Credits: © Rainer von Brandis
Feeding head down helps the turtle to probe tight crevices with its beak and break off stubborn pieces of the reef. These medium-sized sea turtles attain about one metre in length and 80 kilogrammes in weight. They are Critically Endangered (IUCN Red List). As in other sea turtle species, their late age at sexual maturity and relatively low reproductive rate make them highly susceptible to overexploitation. Relatively new threats such as pollution, habitat destruction, hybridisation and incidental capture by fishing vessels further reduce the outlook for this species
Timid and difficult to locate underwater, hawksbill sea turtles in their marine habitat have long managed to evade the efforts of researchers. But Rainer von Brandis struck it lucky when he found himself swimming among hawksbills in the Amirante Islands of Seychelles, and his subsequent study revealed that these graceful reptiles are vital to the maintenance of coral reef biodiversity. Rainer von Brandis struck it lucky when he found himself swimming among hawksbills in the Amirante Islands of Seychelles, and his subsequent study revealed that these graceful reptiles are vital to the maintenance of coral reef biodiversity.
Over the four-year period, he clocked up a total of 312 hours underwater with the turtles and identified 15 resident individuals. Among other things, he was able to determine their prey preferences, the quantity of food they consumed, and their diving and activity patterns, social interactions and habitat requirements. Most significantly, however, he established that the hawksbills play a vital role in maintaining the biodiversity of the coral reef he was working on. Because hawksbill foraging pressure is high on this small reef, the sponges they eat are restricted to well-hidden locations inside the reef substrate. The turtles therefore have to dig them out by using either their flippers to rip open the substrate or their beaks to pry the reef apart and lift out loose pieces that may be sheltering their prey. In doing so, they not only shape the reef topographically, but also expose food for fishes and create sheltered micro-habitats for other reef-dwellers such as moray eels, brittle stars, shrimps and a range of invertebrates. Moreover, since sponges usually out-compete hard corals for space, the consumption of a quarter of a tonne of sponge by a single turtle each year enables hard corals to become established. This is especially important because higher than usual sea-water temperatures in 1998 resulted in the death of approximately 90 per cent of hard-coral communities in the region. If hawksbills had not been present, the reef would probably resemble a featureless expanse of mainly sponges and support a much lower diversity of reef organisms. In all likelihood, many coral reefs were more spectacular in the days preceding the mass slaughter of hawksbills for tortoiseshell. It is hoped that the results of this research can be used to improve the hawksbill's conservation status and strengthen resolve against a potential renewal of the tortoiseshell trade.
Japan continues to lobby in support of reopening the trade so as to keep its tortoiseshell-manufacturing industry alive, and recent reports indicate that clandestine dealings in some Asian and Central American countries are on the increase again. In addition, the findings provide important information about hawksbill biology and habitat requirements, which is vital for the effective management of their populations and foraging habitats.
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