Kate Siegfried & Bruno SansÃ³

12/31/2006 09:00 AM

Applied Mathematics & Statistics

The von Bertalanffy growth equation is commonly used in ecology and fisheries management to model individual growth of an organism. Generally, a nonlinear regression is used with length-at-age data to recover key life history parameters: L∞ (asymptotic size), k (the growth coefficient), and t0 (a time used to calculate size at age 0). However, age data are often unavailable for many species of interest, which makes the regression impossible. To confront this problem, we have developed a Bayesian model to find L∞ using only length data. We use length-at-age data for female blue shark, Prionace glauca, to test our hypothesis. Preliminary comparisons of the model output and the results of a nonlinear regression using the von Bertalanffy growth equation show similar estimates of L∞.

We also developed a full Bayesian model that fits the von Bertalanffy growth equation to the same data used in the classical regression and the length-based Bayesian model. Classical regression methods are highly sensitive to missing data points, and our analysis shows that fitting the von Bertalanffy growth equation in a Bayesian framework is more robust. We investigate the assumptions made with the traditional curve fitting methods, and argue that either the full Bayesian or the length-based Bayesian models are preferable to classical nonlinear regressions. These methods clarify and address assumptions made in classical regressions using von Bertalanffy growth and facilitate more detailed stock assessments of species for which data are sparse.