Age and growth of the spinner shark Carcharhinus brevipinna (Muller and Henle, 1839) off the Kwazulu-Natal Coast, South Africa

Age and growth of the spinner shark Carcharhinus brevipinna off the KwaZulu-Natal coast of South Africa was investigated from vertebral growth ring counts of 67 specimens (54-213 cm precaudal length, PCL). Counts were made from sectioned vertebral centra by two readers. There was a statistically significant difference between the growth functions of both sexes and Von Bertalanffy growth functions were L-infinity = 196.3 cm, k = 0.146 year(-1) and t(0) = -2.3 year for males; L-infinity = 232.8 cm, k = 0.1 year(-1) and to = -2.9 year for females. This difference was thought to be attributable to initially faster growth rate of males until maturity, after which their growth rate slowed below that of females. Number of growth rings at maturity was 8-10 for both males (150 cm PCL) and females (160 cm PCL). The oldest female and male in the sample had 17 (213 cm PCL) and 19 growth rings (192 cm PCL) respectively. Annual growth ring deposition could not be validated using marginal increment ratio analysis because of the small sample size. Geographic variation in growth rate and maximum attainable size was evident between the spinner shark populations of the Indian and Atlantic oceans. Although C. brevipinna grow slowly, the protective nets off the KwaZulu-Natal coast do not appear to have any detrimental effect on the population.