A simple method for comparing age–length keys reveals significant regional differences within a single stock of haddock (Melanogrammus aeglefinus)

A multinomial logistic model is presented as a tool for comparing two or more age–length keys. The model provides an objective way to fill in missing values and can be used for estimating uncertainty and visualizing age–length keys (ALKs). An example of haddock (Melanogrammus aeglefinus) in ICES Division VIa (West of Scotland) is used to illustrate significant regional differences in the proportions of age-at-length. These differences are caused by regional variation in both length-at-age and relative abundance at age. As the length-at-age data are normally not weighted by the local catch rate (abundance), the ALK of the combined age data can result in strongly biased estimates of numbers-at-age. In the present case, the use of unweighted age data would have resulted in an overestimate of recruitment of nearly 200%, and an underestimate of spawning-stock biomass of 15%. Comparing ALKs using this method has several practical applications.