A comparison of initial statistical catch-at-age and catch-at-length assessments of Western Atlantic bluefin tuna.

A concern associated with existing Atlantic bluefin tuna age-based assessments using VPA is that the catch-at-age data inputs are obtained by the cohort-slicing method, which is approximate and might introduce appreciable bias into the results. Current custom in such circumstances is rather to fit the assessment model directly to the basic catch-at-length data available, under the assumption of invariance of the distributions of length-at-age of the fish over time, with statistical models used to formulate the likelihoods maximised in the model fitting process. Initial results are presented for a process of comparing the 2012 ICCAT SCRS VPA assessment of the western stock with first a statistical catch-at-age assessment approach which also uses the same cohort-sliced catch-at-age inputs, and then a statistical catch-at-length method which fits instead to catch-at-length distributions.