The Generalised Yield Model version 5: structure, specifications and examples for validation

The Generalised Yield Model (GYM) was first developed in 1995 as a generalised form of the Krill Yield Model, which was based on the method for evaluating yield developed by Beddington & Cooke (1983). The first version incorporated options for assessing long-term annual yield according to catches set by a proportion of an estimate of pre-exploitation biomass (as in krill), a specified catch in the units of biomass and relative to the recruitment parameters (as in toothfish) or according to a constant fishing mortality (F). It also included the capacity to evaluate yield per recruit. The latest version of GYM (Version 5.01b) differs from earlier versions in 2 main ways: (i) improved storage of output, population characteristics and presentation, and (ii) new features to allow specifying the starting biomass and/or age structure of the population obtained from surveys during a year. In addition, S-plus scripts have been developed to help with output diagnostics. The GYM User’s Manual, Specifications and Examples are also vastly improved. These features now provide the flexibility to undertake a wide range of assessments on stocks, not just specific to CCAMLR. In CCAMLR, the latest version of GYM can used on assessments for Antarctic krill, Patagonian toothfish, and mackerel icefish. This paper presents the specifications for the Generalised Yield Model Version 5, detailing the population model used in the projection program, the algorithm for evaluating yields and the requirements for inputting parameters into the model. It also details how different parts of the model can be manipulated to explore alternative functions. Finally, some examples are presented to show how the GYM can be validated by the user. The input and output files for these examples are available. The latest version of the Generalised Yield Model (GYM), Version 5.01b can be downloaded from the Australian Antarctic Division Website, www.aad.gov.au/marine_eco_software.