Beverton and Holt Reloaded – Incorporating Variable Growth into a Yield Per Recruit Model

Abstract

Animals to be fattened must be fed with nearly the maximum feeding rate to achieve maximum yields – food supply and food consumption must coincide. This we know for centuries from agriculture. Yields decline immediately when the food supply is limited. Are there parallels between agriculture and fisheries? The only way to control the food consumption of fish stocks is by influencing the population size by fishing. In standard yield per recruit analyses fixed growth parameters are assumed. The effect of food supply on growth is not considered and thus, the implications of changes in growth for yield- and spawner-per-recruit curves due to variations in the availability of food are neglected. Under stock recovery density dependent changes in growth may severely bias biological reference points derived from such curves. Here, we present a revised Beverton and Holt yield per recruit model including a revised von Bertalanffy Growth model that is triggered by available food for assimilation and demonstrate the implications for fisheries reference points such as Fmax. The traditional Beverton and Holt yield-model should be replaced by our model; for overfished stocks it delivers identical output values but, if dietary deficiency is detected in overstocked populations then the results correspond to the measured response of the ecosystem.