Modelling prey consumption and switching by UK grey seals

Abstract

British grey seals (Halichoerus grypus) are adaptable generalist predators whose diet includes commercial fish species such as cod. Consumption by the seals may reduce the abundance of some fish species, and a further concern is that consumption by predators might adversely affect stock recovery programs because predation may trap sparse prey in a ‘predator pit’. The likelihood of these outcomes depends on the way in which consumption (and consequent predation mortality) responds to changes in the availability of prey. We present a model of consumption as a function of prey abundance, which was fitted using data on seal diet and prey availability. Bayesian methodology was employed to account for uncertainties in both dependent and independent variables, improve estimation convergence by the use of informative priors, and allow the estimation of missing prey abundance data. Both Type 1 and Type 2 functional response models were fitted to the data and the Type 2 model was clearly favoured during model selection. The selected model was able to reproduce important contrasts in diet observed in different years and sampling locations.