Growth rates of bonnetheads (Sphyrna tiburo) estimated from tag-recapture data

Abstract

Results from published age and growth models for bonnetheads (Sphyrna tiburo) indicate significant differences in life history between populations in the eastern Gulf of Mexico (GOM) and those in estuarine waters of the Atlantic coast of the southeastern United States (hereafter referred to as the Atlantic region). An age-independent model, GROTAG, was used with region-specific tag-recapture data to generate estimates of von Bertalanffy growth parameters and growth rates for sharks in each of these regions, and these estimates were compared with age-based life history characteristics. Results from the GROTAG model indicate that female bonnetheads in the GOM initially grew faster and attained a smaller maximum size than females in the Atlantic region. The final GROTAG model for females in the Atlantic region produced estimates of von Bertalanffy parameters and growth rates similar to those produced by the age-based growth model. For the population in the GOM, GROTAG model results indicate that growth rates were slower and aver-age maximum size and longevity were greater than those from age-based models. Although models for males were generated with tag-recapture data, large 95% confidence intervals hindered comparisons. For both sexes and regions, calculated maximum longevity and age at 50% maturity are larger than published estimates, indicating that age underestimation may have occurred in both age and growth studies, with significant differences in estimates of life history characteristics for bonnetheads in the GOM.