Use of parameters within Euphausia superba Grym simulations

The SC-CAMLR last estimated a precautionary harvest rate (gamma) used for management advice in the Area 48 krill fisheries in 2007 (SC-CAMLR, 2007 Annex 4 paras. 2.38-2.39). Subsequently, a range of various methods have been tried, with limited success, to produce both input parameters and stock assessments which can be used for management advice. Following the work plan outlined by SC-CAMLR (2019, Annex 4 paras. 2.60 – 2.64) endorsed by the Commission (CCAMLR, 2019, para. 5.53) the Generalised Yield Model (GYM) software was reimplemented in R as the Grym package, with the intention of updating the previous krill assessment. However, the methods used to estimate some input parameters to the previous assessment were not documented in detail, and there remains considerable ambiguity as to how to compute these inputs, and how these inputs are used within the assessment simulations. Here we aim to clarify what each parameter within the Grym models are used for, and where possible provide examples as to how they have been calculated.