Review of the assessment framework for Atlantic Cod in NAFO 3Pn4RS : treatment of catch and individual weights, and other assessment model considerations

Abstract

In 2021, Fisheries and Oceans Canada’s Science Branch in the Quebec region initiated a review of the assessment framework for the stock of Atlantic Cod (Gadus morhua) in the northern Gulf of St. Lawrence (nGSL; NAFO Subdivision 3Pn and Divisions 4RS). The review was divided into two parts. The first part, which took place in the spring of 2021, reviewed key inputs to the stock assessment. The second part, which took place in May 2022, reviewed proposed analytical models for the nGSL cod stock and additional inputs to those models. This document presents some model inputs which were not reviewed in 2021, and the results of analyses that motivated some important considerations incorporated into the assessment model. First, we review modifications made to the fishery catch-at-age series and proposals for the definition and use of catch bounds in censored catch modelling incorporated into the revised assessment model. Second, we review and revise annual values for the beginning-of-year stock weights also used in the assessment model. Third, we review approaches incorporated into the assessment model to address changes in survey coverage which occurred in the past in two fishery-independent surveys. Fourth, we present evidence that particular calibration factors used to account for a change in vessel and gear in the research vessel survey series may be inadequate for young cod. This evidence motivated explicit estimation of relative catchability in the assessment model for these ages. Fifth, we present evidence for somewhat different trends displayed by groups of abundance indices included in the assessment model, and briefly discuss the possible causes and how these differences could be accounted for in assessment modelling. Finally, we briefly discuss published research findings which had not previously been incorporated into the assessment, but provide useful support in the modelling.