Variable growth band deposition leads to age and growth uncertainty in the western wobbegong shark, Orectolobus hutchinsi

Abstract

Age and growth parameters of Orectolobus hutchinsi were estimated using micro-radiographs of sectioned vertebrae from 182 wild caught individuals. Two fluorochrome marker dyes, calcein and oxytetracycline, were used to validate the timing and periodicity of vertebral band formation in nine individuals held in the laboratory for between 423 and 472 days. Growth bands were difficult to interpret and final counts were obtained from only 98 (53.8%) individuals ranging in total length (TL) from 63 to 146 cm. The timing of growth band formation in the vertebrae of captive animals had no predictable temporal pattern, with formation occurring during all seasons of the year, making age validation difficult. Growth band formation was hypothesised to be influenced by non-periodic changes in centrum or somatic growth rather than seasonal growth, as observed in many other elasmobranch species. Growth rates of nine O. hutchinsi held in captivity varied considerably, ranging from 3.5 cm year–1 to 13.8 cm year–1 in total length (mean = 7.03 cm year–1). Although the periodicity of vertebral band formation in captive animals did not support a synchronous annual pattern, captive growth rates matched those predicted when an annual band pattern was assumed for wild caught individuals. Von Bertalanffy growth parameters estimated from vertebral analysis assuming an annual banding pattern and a mean size of birth of 24.1 cm were: L∞ = 149.45 cm and K = 0.117 year–1 for both sexes combined. These results illustrate the fundamental importance of validating the periodicity of growth band formation in elasmobranch age and growth studies as it has considerable implications for the management of fisheries that exploit shark and ray species that may exhibit asynchronous growth band deposition.