Reproduction, embryonic development, and growth of the porbeagle shark, Lamna nasus, in the southwest Pacific Ocean

Porbeagle sharks, Lamna nasus, are caught in large numbers as bycatch in tu na longline fisheries in the southwest Pacific Ocean. Information on reprod uction, embryonic development, and size and sex composition was collected b y scientific observers from New Zealand and Australian waters, and suppleme nted with data from other sources. Most sharks were juveniles less than 150 cm fork length (FL), and length-frequency distributions showed 3-5 modal p eaks that we interpret as age classes. Juveniles grow linearly and rapidly (16-20 cm per year), reaching 110-125 cm FL in three years. Females mature at around 165-180 cm. Litter size is usually four embryos and parturition p robably peaks in June-July (winter). This finding contrasts with data for N orth Atlantic porbeagles which give birth in spring-summer. Embryos grow ab out 7 cm per month, and are born at 58-67 cm FL. The gestation period appea rs to be about 8-9 months, but there is considerable variability in embryo length at any one time, suggesting an extended mating period. Embryos are n ourished by oophagy, and develop a grossly distended abdomen as their "yolk stomach" fills with ova. Small embryos have fang-like functional teeth tha t tear open egg capsules to release the contained ova. The fangs are shed a t 34-38 cm FL. The weight of yolk in the stomach peaks at 30-42 cm FL, and accounts for up to 81% of total body weight. Waste products of yolk digesti on accumulate steadily in the spiral valve throughout gestation, and the li ver reaches its maximum size in near-term embryos as excess energy from yol k digestion is stored for postnatal use.