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GRYM — stock assessment & precautionary yield
Module 2 is a faithful wrapper over the CCAMLR GRYM (Generalised Yield Model) Base Case — the same framework CCAMLR uses to set the krill precautionary catch limit. It runs a Monte Carlo projection of the stock over a grid of harvest rates and applies the two CCAMLR decision rules:
- Depletion rule (γ₁) — the highest harvest rate that keeps the chance of the spawning stock dropping below 20% of its unfished level at or under 10%.
- Escapement rule (γ₂) — the highest harvest rate that leaves a median spawning-stock escapement of at least 75%.
- Selected γ is the smaller of the two, and the precautionary yield is that γ times the estimated biomass.
The biomass and krill length frequency can come straight from a Module 1 run, or you can supply your own. Results include the two gammas, the selected gamma, the yield in tonnes, and a spawning-stock depletion fan chart.
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Full GRYM Module
Open the assessment dashboard. It starts from the published baseline — Table 1 of WG-FSA-2022 Appendix G — and puts every parameter on a slider that steps in 1% of its baseline value, so it works as a sensitivity instrument: move one value, re-run, and watch what the two decision rules do. Results cover the summary statistics, the spawning-stock status chart, both gamma rules, individual run trajectories against published survey data, the juvenile/adult split, and how GRYM's recruitment compares with the Ricker and Beverton-Holt curves.
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