Effects of age-based movement on the estimation of growth assuming random-at-age or random-at-length data

Simulation methods were used to generate paired data from a simulated population that included the age-based process of movement and the length-based process of gear selection. The age-based process caused bias in the estimates of growth parameters assuming random at length, even when relatively few age classes were affected. Methods that assumed random at age were biased by the subsequent inclusion of the length-based process of gear selection. Additional knowledge of the age structure of the sampled area is needed to ensure an unbiased estimate of the growth parameters when using the length-conditional approach in the presence of age-based movement. Estimates of the variability in the length-at-age relationship were better estimated with the length-conditional than the traditional method even when the assumptions of random at length were violated. Inclusion of paired observations of length and associated age inside the population dynamics model may be the most appropriate way of estimating growth.