MODELING THE PREDATION, GROWTH AND POPULATION-DYNAMICS OF FISH WITHINA SPATIALLY-RESOLVED SHELF-SEA ECOSYSTEM MODEL

The development and analysis of a fish population module within the Eu ropean Regional Seas Ecosystem Model (ERSEM) is described. Fixed preda tion rates on ERSEM zooplankton and zoobenthos, which may be used in p lace of this dynamic module of fish, are described also. The dynamic m odule is age-structured, with state variables for the number of indivi duals and their weight, the latter partitioned into structural (skelet al, essentially) and remobilizable 'reserve' components. It is capable of prediction of observed patterns of both positive and negative chan ges in individual weight. The fish are modelled as one population for the whole North Sea, superimposed on the ERSEM box structure. Paramete rs have been derived from the literature where possible and, for herri ng, certain parameters have been adjusted to match independent estimat es of growth in the field. Preliminary results show that herring growt h rate depends inversely on biomass, when changes in the latter are ca used by altered fishing mortality. The magnitude of these effects is i n agreement with historical data for herring in the North Sea. The mod el is discussed in relation to previous attempts to model fish populat ion dynamics, particularly in the North Sea.