A generalised model for evaluating yield and the long term status of fish stocks under conditions of uncertainty

This paper presents a flexible structure for a general yield model, which provides flexibility for assessing the influence of different patterns of growth, natural mortality, spawning and fishing on the estimates of yield per recruit. Also, it can be used to evaluate the performance of a stock under a specified catch regime. The model uses an adaptive Runge-Kutta procedure to evaluate stock trajectories and catch rates over a simulation period. In particular, this procedure calculates catches over each year by integrating a set of differential equations which incorporate functions that specify the rate of change or magnitude of parameters, such as growth, mortality, age dependent selectivity and seasonal patterns in fishing mortality, at specified time intervals during the year. This general approach also includes an option for a stock to experience a known catch history before the constant catch regime is introduced.