Age and growth of Carcharhinus leucas in the northern Gulf of Mexico: incorporating variability in size at birth

Age and growth rates of bull shark Carcharhinus leucas [ n  = 255; 555–2230 mm fork length ( L F )] from the northern Gulf of Mexico were estimated from ring counts on vertebral sections collected from fishery‐dependent and ‐independent surveys. Two growth models were fitted to observed data: the von Bertalanffy growth model (VBGM) with t 0 as the third parameter and a modified version of the VBGM using a fixed size‐at‐birth intercept as the third parameter. To address the variability in size‐at‐birth, a Monte Carlo simulation was incorporated into the size‐at‐birth intercept. The sex‐specific growth models were not significantly different, allowing a sexes combined model to be generated. The traditional VBGM predicted a theoretical maximum size ( L ∞ ) of 3007·1 mm L F , a growth coefficient ( K ) of 0·042 year −1 and a theoretical age at zero length ( t 0 ) of –6·844 years. The modified VBGM with a fixed size‐at‐birth intercept of 565 mm L F predicted an L ∞ of 2289·2 mm L F and a K value of 0·089 year −1 . When comparing model estimates to previously published information, the traditional VBGM predicted a significantly lower theoretical maximum size and a higher growth coefficient than those produced using data collected during the 1980s. Overall, results obtained using the VBGM with a fixed size‐at‐birth produced more biologically realistic parameters than that of the VBGM with t 0 . The Monte‐Carlo simulation incorporating variability in size‐at‐birth produced similar results to the VBGM using a fixed size‐at‐birth. This study provides the first attempt to incorporate variability at size‐at‐birth and provide measurements of variability around the individual parameter estimates for an elasmobranch.