A Proposed, Tested, and Applied Adjustment to Account for Bias in Growth Parameter Estimates Due to Selectivity

Growth information informs spawning stock biomass and therefore stock status in assessments.  Data for estimating growth are often only available from fishery-dependent sampling, which can lead to biased estimates of true underlying population growth parameters because of selectivity.  The objectives of our study were to develop and test a method to estimate unbiased growth parameters and to apply the method to fit unbiased population growth parameters for Gulf and Atlantic menhaden.  The proposed method was tested on simulated data and applied to Gulf and Atlantic menhaden data.  Use of the adjustments was robust and resulted in reduced bias in growth parameter estimates with accuracy being affected by both sample size and variability in mean length at age.  Parameters estimated for the unadjusted Gulf menhaden growth curve were L_∞=240.8, k=0.38, t₀=-1.14, and CV of length at age=0.06 (N=366,710; 1977-2011) and L_∞=350.9, k=0.32, t₀=-0.83, and CV of length at age=0.12 (N=480,668; 1955-2011) for Atlantic menhaden.  Adjustment for maximum length of capture had a large impact on the overall growth parameters for both species, while adjustment for minimum length of capture had less impact.  Bias in the growth curve parameter estimates can be reduced by using the method outlined to account for selectivity.