Modelling the spatial dynamics and fisheries of North Sea plaice (Pleuronectes platessa L.) based on tagging data

This paper presents a simulation model of the spatial and temporal dynamics and the fishery of North Sea plaice that provides a powerful tool for exploring the effects of fishing and growth on the spatial dynamics, and the effects of technical measures such as closed areas. The model simulates the basic biological processes of growth, recruitment, migration, and mortality, employing a spatial resolution of 30 miles and a time step of 1 week or less. Six size-classes are distinguished: two pre-recruitment size-classes (5–14 cm; 15–26 cm) and four commercial size classes (27–33, 34–37, 38–40, ≥41 cm). Parameterisation was based on empirical observations from the literature except for the transport coefficients which were estimated from migration vectors. Such vectors, that describe the rate and direction of the migration for each rectangle in each calendar month, were estimated from first order periodic functions fitted through the average observed x and y coordinates of the monthly recaptures of tagged fish. These x and y coordinates were calculated taking account of the heterogeneity in recapture probability due to differences in spatial distribution of fishing effort. Migration rate increased almost linearly with fish size. Sensitivity of the model was explored for variations in growth and migration. Comparison of the spatial distribution patterns of specific age groups and discard percentages with observed distributions showed that model performance was promising. Simulations showed that exploitation substantially affected the spatial distribution patterns of age groups. Due to size–dependent migration, exploitation was shown to be size–selective, leading to a lower perceived growth of the surviving population as compared with the true growth of the simulated, unexploited population. Possibilities of application and future improvements of the model are discussed.