Age and growth of Carcharhinus leucas in the northern Gulf of Mexico: incorporating variability in size at birth |
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Authors: | J A Neer† B A Thompson J K Carlson‡ |
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Institution: | Coastal Fisheries Institute, Louisiana State University, Baton Rouge, Louisiana 70803‐7503, U.S.A. and; NOAA, National Marine Fisheries Service, 3500 Delwood Beach Road, Panama City, Florida 32408, U.S.A;. |
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Abstract: | Age and growth rates of bull shark Carcharhinus leucasn = 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. |
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Keywords: | age and growth bull shark Carcharhinus leucas life history parameter estimation Monte‐Carlo simulation |
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