Biology of the silky shark Carcharhinus falciformis (Carcharhinidae) in the eastern Indian Ocean, including an approach to estimating age when timing of parturition is not well defined

Biological data were recorded for 1265 silky sharks Carcharhinus falciformis collected from fish landing sites in eastern Indonesia. These represented catches taken in most calendar months by gillnetting and longlining in the eastern Indian Ocean and contained individuals ranging from embryos to fully mature adults. The growth zones in centra, which were shown to form annually, were counted in the vertebrae in a sub-sample of 200 fish for ageing purposes. The embryo lengths in the 5 months for which there were such data, and the presence of neonates in virtually all months, however, indicated that birth occurs throughout the year and thus there was no well-defined birth date for ageing individual fish. The approximate birth date of each individual was thus estimated from a combination of the total length (L_T) at capture and backcalculated L_T at the formation of the birth zone and at the first and last growth zones in the vertebral centra, together with the period that had elapsed between the formation of those last two growth zones. The number of eggs or embryos in uteri ranged from two to 14, with a mean of 7·2. The estimated mean L_T at birth of females (811 mm, range: 799–823 mm) and males (812 mm, range: 794–830 mm), derived from the backcalculations corresponding to the birth zones in the centra, were not significantly different (P > 0·05). The L_T ranges in the catches of post-natal females (570–2592 mm) and males (553–2289 mm) taken by gillnetting were wider than those of the females (1177–2623 mm) and males (1184–2409 mm) taken by longlining. The oldest female and male were 19 and 20 years-old, respectively. The von Bertalanffy growth curves for the two sexes did not differ significantly. The growth coefficient, k, and the asymptotic length, L_T∞. were 0·066 year⁻¹ and 2994 mm for the curve fitted to the combined data for females and males. The lengths L_T50 and ages A₅₀ at which C. falciformis attained maturity were 2156 mm and 15 years for females and 2076 mm and 13 years for males. The very high proportion of C. falciformis with lengths _T50 in current catches places sustainability of the stock at risk.