Generalised Yield Model

The Generalised Yield Model is a fully flexible fish population projection model for examining the effects of different harvest strategies on stocks while taking account of uncertainties in input parameters (Constable & de la Mare, 1996). It was designed to provide a generalised version of the methods for estimating fishery yield based on the precautionary approach developed in the Commission for the Conservation of Antarctic Marine Living Resources. That precautionary approach was based on the work of Beddington & Cooke (1983) and developed to assess the precautionary yield of Antarctic krill. The assessment of krill yield was undertaken using the "Krill Yield Model" (Butterworth et al.,1992, 1994).

The Generalised Yield Model was first developed in 1995 for use in WG-FSA to provide an assessment tool based on the KYM but allowing for its general application to other stocks, such as Patagonian toothfish, and to provide for other forms for assessing the impacts of different catch scenarios, including trajectories based on a series of catches (mass) or fishing mortalities.

The primary part of the model is a flexible method for assessing the influence of different patterns of growth, natural mortality, spawning and fishing on estimates of yield and yield per recruit. It can also be used to evaluate stochastic stock trajectories under a specified catch regime. The model uses an adaptive Runge-Kutta algorithm to calculate stock trajectories and catch rates over a specified simulation period. The procedure numerically integrates a set of differential equations which incorporate functions that specify growth, mortality, age dependent selectivity and seasonal patterns in fishing mortality.

The outputs of the population model can then be used to determine catch limits according to CCAMLR decision rules (Constable et al. 2000) or for exploring the consequences of different types of harvest setting rules.

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Relevant documents

Constable, A.J., de la Mare, W.K. 1996. A generalised model for evaluating yield and the long-term status of fish stocks under conditions of uncertainty. CCAMLR Science3:31–54.

Constable, A. J., W. de la Mare, D. J. Agnew, I. Everson, and D.Miller. 2000. Managing fisheries to conserve the Antarctic marine ecosystem: practical implementation of the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR). ICES Journal of Marine Science 57: 778–791.