Biology of three hammerhead sharks (Eusphyra blochii, Sphyrna mokarran and S. lewini) from Northern Australia

Abstract

The hammerhead sharks Eusphyra blochii, Sphyrna mokarran and S. lewini form part of the incidental catch of a commercial gill-net fishery off northern Australia. Of the specimens sampled between June 1980 and December 1986, 46% of S. mokarran, 41% of E. blochii and 31% of S. lewini were females. Few adult female S. lewini were caught and it is suggested that these occur offshore of the study area. In northern Australia, the usual size at maturity of male E. blochii, S. lewini and S. mokarran is 108, 150 and 225 cm total length (TL), ,and of females is 120, 200 and 210 cm TL, respectively. S. mokarran and E. blochii gave birth in January and February/March, respectively, after a gestation period of 10-11 months. S. Iewini appears to have a more extended seasonal cycle: the young are born between October and January after 9-10 months gestation. Size at birth is about 45-50 cm TL in E. blochii and S. lewini, and 65 cm TL in S. mokarran. Mean litter size is 12 in E. blochii, 15 in S. mokarran and 17 in S. lewini. Individual E. blochii females breed every year, whereas S. mokarran females probably breed every other year. Fish are an important component of the diet of all three species as, to a lesser extent, are cephalopods for S. lewini and crustaceans for S. mokarran and E. blochii.