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Transboundary Resources Assessment Committee : Assessment of Eastern Georges Bank Atlantic Cod for 2018
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2018
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[PDF-4.25 MB]
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Description:The combined 2017 Canada/USA Atlantic Cod catches were 526 mt with a quota of 730 mt. Catches in all three research surveys decreased since the 2017 assessment. Both fishery and survey catches showed truncated age structure in recent years. The VPA “M 0.8” model from the 2013 benchmark assessment was used to provide catch advice in conjunction with a consequence analysis of the uncertainties in the VPA “M 0.8” and ASAP model results and consideration of an empirical approach. The VPA “M 0.8” model assumes a natural mortality (M) of 0.2 on all ages, except ages 6+ which are assumed to have an M=0.8 since 1994; the ASAP model assumes an M=0.2 for all ages and years. While management measures have resulted in a decreased in relative F since 1995, total mortality has remained high and adult biomass has fluctuated at a low level. The adult population biomass at the beginning of 2018 was estimated at 9,502 mt, which was about 20% of the adult biomass in 1978, and F was estimated to be 0.04 in 2017. There is a retrospective pattern that creates uncertainty around the magnitude of recent estimates of biomass and F, so a Mohn’s rho adjustment was applied to the estimates to help quantify the uncertainty. The rho adjusted values of 2018 biomass and 2017 F were 6,402 mt and 0.053 respectively. High total mortality, low weights at age in the population and poor recruitment have contributed to the lack of rebuilding. In 2019, a 50% probability of not exceeding fishing reference point F=0.11 corresponds to catches of 860 mt. Due to the 2013 year class progressing through the ages with high natural mortality, there is a >90% risk that adult biomass will decrease from 2019 to 2020, even with no fishing. In 2020, a catch of 668 mt corresponds to a 50% probability of not exceeding F=0.11; however, even with no fishing in 2020 there is a >90% risk of a decrease in age 3+ biomass from 2020 to 2021 due to the 2013 year class entering the ages of high natural mortality. Given the extremely low spawning stock biomass (SSB), management measures should aim to rebuild SSB. A consequence analysis to understand the risks associated with assumptions of the VPA “M 0.8” and ASAP “M 0.2” model was examined in the projection and risk analysis. The consequence analysis reflects the uncertainties in the assessment model assumptions. In 2016, an empirical method was developed as one of the approaches for providing quota advice for Eastern Georges Bank Cod. This method adjusts recent quotas by recent population biomass trends from three research surveys. The CV weighted average biomass decreased 75% from 2017 to 2018. Application of the empirical method in 2018 produced a median bias corrected slope of 0.96. Scaling recent average quota by the 25th-50th-75th percentiles of bias corrected slope produces quota advice of 600-644-697 mt, respectively. Considering the low productivity of Cod, despite model uncertainties, all assessment results indicate that low catches are needed to promote rebuilding.
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Rights Information:Public Domain
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