Life-history and ecology of sharks at Aldabra Atoll, Indian Ocean

Abstract

Ten species of shark belonging to three families were recorded from Aldabra Atoll. Carcharhinus melanopterus and acutidens were the most abundant species in the lagoon, while Carcharhinus albimarginatus was the most common shark outside the reef. Twelve hundred sharks of six species were tagged and individual recapture rates varied from 15 to 34% . Some specimens of C. melanopterus were caught up to seven times. All five species for which recapture data were available are restricted in their movements at Aldabra. C. melanopterus in particular is very localized, normally remaining in an area of a few square kilometres. Length increment data obtained from tagging demonstrated a slow growth rate for C. melanopterus , averaging 3.5 cm a -1 , with no detectable difference between the growth rates of small and large individuals. Limited data for juvenile Negaprion and C. albimarginatus indicated average growth rates of 12.5 and 8.8 cm a -1 respectively. Population densities calculated for several areas in the lagoon varied from 19 to 198 C. melanopterus per square kilometre. It is suggested that C. melanopterus may be foodlimited at Aldabra owing to the intensity of intra- and inter-species competition. C. melanopterus and Negaprion have restricted and almost identical reproductive cycles at Aldabra. C. melanopterus females mature at 110 cm total length and breed every second year giving birth to about four pups after a 10 -11 month gestation period. Stomach contents of the more abundant species indicate that fish are the most important item in the diet, except for Nebrius concolor which feeds principally on octopus.