Contribution to the Biology of the Whitetip Reef Shark (Triaenodon obesus)

Abstract

The whitetip reef shark, Triaenodon obesus (Ruppell), is broadly distributed in t4e tropical and subtropical Indian and Pacific oceans from the Red Sea (type locality) and east coast of Africa to the eastern Pacific. Formerly classified in the Triakidae, it is now regarded as a carcharhinid. It is distinctive in its slender body (depth about 7 in precaudal length), obtuse snout, 43 to 50 teeth in each jaw in at least two functional rows, the teeth with one or two small cusps on each side of the large central cusp, and white tips on at least the first dorsal fin and upper lobe of the caudal fin. It is closely associated with coral reefs and is often seen at rest in caves. There is evidence that this shark has a home cave to which it may return after foraging for food. Although reported to attain a total length of 7 feet (213 em) or more, the largest of 231 individuals measured by the author and associates was 168 em total length (135 cm precaudal length). Seven recoveries were made of 124 whitetips tagged at Johnston Island (time at liberty with tags 16 days to 2 years). Two of the tagged sharks were caught in the same area as that in which they were released and the others were taken .3 to 2.9 km from the tagging sites. The growth rate of the tagged sharks, which ranged from 81 to 105 em precaudal length, varied from 2.1 to 4.2 em/year. Two captive sharks, 73 and 88 em precaudal length, grew at the rate of 4 and 2.3 cm, respectively, in the Gulf of Aqaba, Red Sea over a period of 3 years. A whitetip born in captivity in July at a precaudal length of 43.2 cm at Enewetak, Marshall Islands, grew 2 em in 6 weeks. The stomachs of 56 whitetips were opened; 33 were empty; 17 contained the remains of fishes (those identified were all reef fishes such as scarids and acanthurids); four had eaten octopuses, and two contained both fishes and octopuses. One reference described a diet of xanthid crabs and spiny lobsters. A juvenile whitetip was found in the stomach of a larger grouper (Epinephelus lanceolatus); some of the larger sharks such as the tiger and Galapagos sharks are probably more important predators than this grouper. The sex ratio does not deviate significantly from 1 : I except for whitetips caught in traps; 65 of 107 trap-caught sharks were males. The smallest mature female T. obesus of this study measured 101.2 cm precaudal length (but a smaller size might be expected), and the smallest mature male was 82 cm. Litter size ranges from one to five. Compared to other carcharhinids, T. obesus is a relatively placid species which need not be feared by divers unless provoked or attracted by spearfishing (even then aggressive behavior toward man is rare). This shark may cause ciguatera poisoning, especially if the liver is eaten.