The only species of wild giant tortoise on Indian Ocean islands survive on Aldabra Atoll. Here a population of Geochelone gigantea recovered to 150,000 individuals, but recently declined following food depletion. To aid conservation and provide a tourist attraction, tortoises were translocated over several years (from 1978-1983) to Curieuse Island (280 ha; 3.4 × 2 km max. dimensions) in the Seychelles archipelago. This paper evaluates the cumulative success of these translocations, based upon a census undertaken in 1990.
Between 1978 and 1983, 251 (possibly 255) tortoises were released: 95 in May 1978; 78 in April 1980, and between 75 and 79 in 1982. Each was marked with a numbered disk. Three adults from nearby Aride (where introduced earlier) were released in 1983. Tortoises were monitored by visiting scientists and the resident warden. Sporadic population censuses provided minimum estimates of numbers.
A census in 1986 suggested a population decline. In 1990, the most detailed survey since translocation was undertaken, and translocation success of tortoises reintroduced to Frégate Island (Seychelles) was briefly appraised.
In 1990, 117 tortoises were found on Curieuse (73 adult males; 38 adult females; and 6 juveniles derived from breeding on the island). Some adults were diseased. Low growth and reproductive rates suggest resource saturation, even at the present low density. Whilst endemic vegetation had not suffered due to tortoise grazing/browsing, tortoises were shown to spread non-native plants (via droppings).
Recruitment to five years of age was very low, probably due predation by non-native predators. Curieuse has a large population of brown rats Rattus rattus. Whilst there was no direct evidence of rats excavating ‘wild’ tortoise nests and predating eggs or hatchlings, captive hatchlings on Curieuse have been killed by rats. Feral cats Felis catus also occur throughout the island (as indicated by scats).
Poaching also occurs. Giant tortoises are regarded by some as a delicacy, and five or more dismembered carapaces were found; at least two had been killed since 1986. In contrast, giant tortoises have reproduced successfully on the cat- and rat-free Seychelles islands of Aride and Frégate.
It seems that the translocations to Curieuse will ultimately fail to establish a viable giant tortoise population.
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