MODELING 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 dyn amics and the fishery of North Sea plaice that provides a powerful too l for exploring the effects of fishing and growth on the spatial dynam ics, and the effects of technical measures such as closed areas. The m odel simulates the basic biological processes of growth, recruitment, migration, and mortality, employing a spatial resolution of 30 miles a nd 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 commerci al size classes (27-33, 34-37, 38-40, greater than or equal to 41 cm). Parameterisation was based on empirical observations from the literat ure except for the transport coefficients which were estimated from mi gration vectors. Such vectors, that describe the rate and direction of the migration for each rectangle in each calendar month, were estimat ed from first order periodic functions fitted through the average obse rved x and y coordinates of the monthly recaptures of tagged fish. The se x and y coordinates were calculated taking account of the heterogen eity in recapture probability due to differences in spatial distributi on of fishing effort Migration rate increased almost linearly with fis h size. Sensitivity of the model was explored for variations in growth and migration. Comparison of the spatial distribution patterns of spe cific age groups and discard percentages with observed distributions s howed that model performance was promising. Simulations showed that ex ploitation substantially affected the spatial distribution patterns of age groups. Due to size-dependent migration, exploitation was shown t o be size-selective, leading to a lower perceived growth of the surviv ing population as compared with the true growth of the simulated, unex ploited population. Possibilities of application and future improvemen ts of the model are discussed. (C) 1995 International Council for the Exploration of the Sea