Does It Fit? Estimating the Growth of Sharks

Growth is a fundamental property of living organisms, and is of particular importance in marine ecosystems where an individual’s size can increase numerous orders of magnitude during their lifetime, and determines the niche they occupy in the ecosystem.  Quantifying growth rates allows the calculation of parameters which are important in demographic modelling and stock assessment. However, for large long-lived species there are often few data with which to estimate growth rates.  In elasmobranchs, the von Bertalanffy growth function (VBGF) is commonly used as it provides biologically meaningful parameters, such as maximum asymptotic size (L_∞) and growth completion rate (k). A variant of the VBGF that has been considered advantageous sets one of the VBGF parameters, the y-intercept or L_o, to equal an empirical estimate of size at birth. Using simulation modelling we demonstrate that this simplification does not improve model fit, even worse it biases the growth parameter. Allowing for variability in L_o also biases results. These findings emphasize the importance of choosing the most appropriate growth model, particularly when used for parameterization of risk estimates and stock assessments of vulnerable species.