Research Document 2022/049
Review of the assessment framework for Atlantic cod in NAFO 3Pn4RS: Fishery independent surveys
By Benoît, H.P., Ouellette-Plante, J., Yin, Y, and Brassard, C.
Abstract
Fisheries and Oceans Canada in the Quebec region undertook in 2021-2022 a review of the assessment framework for the Northwest Atlantic Fisheries Organization (NAFO) 3Pn4RS Atlantic cod (Gadus morhua) stock in the northern Gulf of St. Lawrence. A review of assessment inputs, including information on reported and non-reported fishery catches, tagging information and fishery independent monitoring results took place in the spring of 2021. The principal objective of the present document was to review the availability, duration and quality of fishery independent survey indices and information for the stock. In doing so, we considered how these sources of information contribute to understanding population dynamics, via trends in abundance, demographic composition and mortality rates. Several advances were made as part of the review and which are expected to improve the quality of the assessment. First, the principal age-disaggregated indices obtained from a bottom-trawl research vessel survey were extended back in time from 1990 to 1984 by analyzing and applying results of a comparative fishing experiment that allowed to calibrate the series for the vessel used in the early years of the survey. This extends the time series to before the period when the stock collapsed. Second, a single coherent set of age-disaggregated indices for a Sentinel bottom trawl survey was obtained by combining two sets of indices covering different periods of years. Third, data from a winter survey that ran in the 1980s and 1990s and which had been disregarded were found to produce useful information on the age-composition for the cod stock over the 1983-1994 period. Fourth, results from historical surveys conducted in the 1950s and 1960s, the period during which trawls were introduced into the fishery and landings increased considerably, were found to provide useful estimates of mortality rates, notably an estimate of an upper bound for the natural morality rate. Fifth, an overhaul of the methods used to estimate age-disaggregated indices of abundance from the Sentinel fixed gear program resulted in the creation of three independent sets of indices, two based on longline catches and one based on gillnet catches. The overhaul was motivated by several issues that had arisen over time with the traditionally used catch-rate standardization approach. Overall, the revised indices provide a coherent picture of stock dynamics, including very similar trends across indices in the estimated rate of total mortality. Furthermore, the extension of information on the stock to the 1950s and abundance indices to the 1980s prior to stock collapse, is likely to provide a more accurate picture of the historical size and productivity of the stock, which in turn should allow for the establishment of more pertinent reference points for its management.
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