Biology of two commercially important carcharhinid sharks from northern Australia

Abstract

Sharks represent 78% of the total catch by weight of a Taiwanese surface gill-net fishery off northern Australia. Two carcharhinids, Carcharhrnus tilstoni (previously described as C. limbatus) and C. sorrah, together comprise 83% of this shark catch by number. C. tilstoni is distinguished from C. limbatus by differences in enzyme systems, vertebral counts, size data and pelvic fin coloration. Of the specimens of C. tilstoni and C. sorrah caught in the Arafura and Timor Seas from 1981 to 1983, 43% and 47%, respectively, were female; at birth these proportions were 46% and 50%, respectively. In both species, females tended to be relatively more abundant in catches of mature fish, except around March, when males predominated. In northern Australia, the usual size at maturity for C. tilstoni is 110 cm for males and 115 cm for females; for C. sorrah, it is 90 cm and 95 cm, respectively. Both species exhibit placental viviparity and have almost identical restricted reproductive cycles. Mating occurs in February-March, ovulation in March-April and the main parturition period is in January. The gestation period is 10 months and individual fish breed each year. The average litter size for both species is three. The size at birth is about 60 cm for C. tilstoni and 50 cm for C. sorrah. Stomach contents indicate that teleost fish are an important component of the diet of both species and there is some indication of a change in feeding depth with shark size.