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

Abstract

Age and growth rates of bull shark Carcharhinus leucas[n = 255; 555–2230 mm fork length (LF)] 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 t0 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 LF, a growth coefficient (K) of 0·042 year−1 and a theoretical age at zero length (t0) of –6·844 years. The modified VBGM with a fixed size‐at‐birth intercept of 565 mm LF predicted an L∞ of 2289·2 mm LF 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 t0. 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.