Estimating catch-at-age by combining data from different sources

Estimating the catch-at-age of commercial fish species is an important part of the quota-setting process for many different species and almost all countries with a fishing fleet. Current procedures are usually very time-consuming and somewhat ad hoc, and the estimates have no measure of uncertainty. We previously developed a method for catch-at-age of Norwegian Atlantic cod (Gadus morhua), but this only considered aged fish sampled randomly from random hauls. In most countries, the sampling scheme is not so simple. There are usually a very large number of length-only samples from which the age must be estimated using an age-length relationship, and often some or all of the age samples are collected from data that are first stratified by length. This adds considerably to the difficulties in the estimation. In this paper, we model the three different kinds of data simultaneously using a development of our earlier Bayesian hierarchical model. This enables us to obtain estimates of the catch-at-age with appropriate uncertainty and also to provide advice on how best to sample data in the future. The data types are random samples of age, length, and weight; age and weight stratified by length; and length only.