Validated age and growth of the sandbar shark, Carcharhinus plumbeus (Nardo 1827) in the waters off Western Australia

Abstract

Sandbar sharks, Carcharhinus plumbeus, collected from commercial shark fisheries in Western Australia were aged by examination of sectioned vertebrae and analysis of tag-recapture data. Growth curves were derived from consensus counts of growth bands from the vertebrae of 238 individuals ranging in size between 47 and 154 cm fork length (FL). The annual periodicity of growth band formation was validated using vertebrae from tagged sharks, which were injected with oxytetracycline (n = 9) and calcein (n = 23) and were at liberty for up to 8.1 years. The oldest female was estimated to be 25 years of age and the oldest male was 19 years. The ages at which 50% of female and male sharks were mature were estimated to be 16.2 and 13.8 years, respectively. Growth increment data from 104 tagged C. plumbeus, which were at liberty for up to 7.4 years, were used to construct growth curves for comparison with those derived from vertebral analysis. The two methods yielded noticeably different results. Based on a known size at birth of 42.5 cm FL, von Bertalanffy parameters estimated using length at age data from vertebral analysis were: K = 0.039 year−1 and L ∞ = 245.8 cm; K = 0.044 year−1 and L ∞ = 226.3 cm; and K = 0.040 year− and L∞ = 239.6 cm for females, males and both sexes combined. The von Bertalanffy parameters derived from tag-recapture data were: K = 0.153 year−1 and L ∞ = 142.0 cm for combined sexes. However, as sharks longer than 142.0 cm were commonly encountered during sampling, these estimates appear to be biologically unrealistic. Also, given the high variability in growth rates of tagged sharks, compared to those derived from the larger vertebral analysis dataset, vertebral ageing was concluded to provide a better description of age and growth in this study. These results confirm that C. plumbeus is a slow-growing and late maturing species and thus recovery times from periods of overexploitation would be considerable.