Aspects of the reproductive biology of dusky, spinner and sandbar sharks (Family Carcharhinidae) from the Tasman Sea

Abstract

Increased harvest pressure exerted on sharks worldwide has created a necessity for more information concerning the basic biology of targeted species. This study marks the first dedicated assessment of the reproductive biology of Carcharhinus obscurus, C. brevipinna and C. plumbeus in eastern Australian waters, where these species support a demersal longline fishery. We found all three to be late maturing species of low reproductive output, suggestive of an overarching susceptibility to stock depletion in the region. Length- (L50, cm LT) and age-at-maturity (A50, years), for females and males were respectively 281.1 and 15.5, and 271.9 and 14.6 for C. obscurus; 224.9 and 10.1, and 208.9 and 8.5 for C. brevipinna; and, 174.8 and 9.5, and 164.5 and 7.0 for C. plumbeus. Uterine fecundity (i.e. litter size) was observed at 5–12 (=9.6), 5–14 (=10.6) and 3–12 (=7.8) for the same three species respectively, and increased significantly with maternal length in C. brevipinna. Length-at-birth (L0, cm LT) ranged from 92 to 96 for C. obscurus, 79–82 for C. brevipinna and 66–76 for C. plumbeus, and all three species exhibited lengthy gestation periods, overall embryonic sex ratios of 1:1 and synchronous parturition in autumn. However, given limitations with respect to sample size and temporal distribution inherent in this study, the reproductive parameters defined herein are necessarily preliminary. Nevertheless, this research challenges a range of findings emanating from other parts of the world and, in doing so, raises pertinent questions relating to the resilience to targeted fishing activities of these species in New South Wales waters compared to other oceanic regions. Moreover, it reinforces the importance of locally derived demographic parameters for population modelling and stock assessment.