The population structure, reproductive biology, age and growth, and diet of shortfin makos caught by pelagic longliners (2005–10) and bather protection nets (1978–2010) in the south-west Indian Ocean were investigated. The mean fork length (FL) of makos measured by observers on longliners targeting tuna, swordfish and sharks was similar, and decreased from east to west, with the smallest individuals occurring near the Agulhas Bank edge, June to November. Nearly all makos caught by longliners were immature, with equal sex ratio. Makos caught by bather protection nets were significantly larger, males were more frequent, and 93% of males and 55% of females were mature. Age was assessed from band counts of sectioned vertebrae, and a von Bertalanffy growth model fitted to sex-pooled length-at-age data predicted a birth size (L0) of 90 cm, maximum FL (L∞) of 285 cm and growth coefficient (k) of 0.113 y–1. Males matured at 190 cm FL, aged 7 years, and females at 250 cm, aged 15 years. Litter sizes ranged from nine to 14 pups, and the presence of gravid females in bather protection nets suggested that some pupping occurred in shelf waters. Teleosts (mainly Trachurus capensis) occurred in 84% of stomachs collected on longliners, whereas elasmobranchs (63.5%) were most common in samples collected from bather protection nets, followed by teleosts (43.1%) and cephalopods (36.5%). Larger prey size may be a factor that attracts large makos to coastal waters.
Population structure and biology of shortfin mako, Isurus oxyrinchus, in the south-west Indian Ocean
