Estimating Maturity-at-Age from Length Stratified Sampling

Much of the research sampling for age composition in commercial fish species is stratified on the basis of length, with a fixed number of samples taken from each length class. For most fish species, a given age can straddle several length classes. Further, the probability of being mature at a given age is influenced by length at that age. Often, the proportion mature at a given age is estimated from the observed fish at that age without taking into account the size frequency distribution from which the specimens are drawn. This can result in biases of unpredictable magnitude and sign in the estimation of proportion mature-at-age. This study presents a method to estimate proportion mature-at-age from a length stratified sampling scheme. The method accounts for the size distribution at age by weighting observations by the abundances of the size groups. The method is applied to cod (Gadus morhua) and American plaice (Hippoglossoides platessoides) in the Northwest Atlantic and the results compared to those obtained from the unweighted (traditional) method. In general, the differences between the weighted and unweighted estimates are small, but some comparisons show substantial differences.