Shrimp predation on 0-group plaice: Contrasts between field data and predictions of an individual-based model

An individual-based model has been used to predict the effects of size-dependent predation by shrimp Crangon crangon on populations of newly settled 0-group plaice Pleuronectes platessa. The model predicted changes in population size and thus mortality rate of plaice during settlement and the following period until the plaice reached 30 mm in length and were invulnerable to further shrimp predation. Population parameters derived from seven years' catch data for 0-group plaice and shrimp from the west coast of Scotland were supplied to the model for direct comparison of predicted and observed mortality rates for each of the seven years. The contrast of predicted and observed mortality was used to test the hypothesis that predation by shrimp is a major factor regulating populations of juvenile plaice. There was no evidence for control of plaice populations by shrimp predation at the study site, since a negative relationship between plaice mortality and shrimp density was observed. Alternative explanations for the divergence of prediction and observations included (i) a general failure of the model to adequately describe predator and prey populations, (ii) limited effects of consumption of plaice by shrimp, and (iii) responses of both species to a third factor such as temperature producing an apparent association of survival and recruitment in the respective species.