Estimating the consumption of Calanus finmarchicus by planktivorous fish in the Norwegian Sea using a fully coupled 3D model system

Energy flow and trophic regulation are often well understood in closed freshwater ecosystems. Such regulation is harder to understand in large marine ecosystems, as they extend over vast areas and are influenced by adjacent marine areas. Fully coupled biophysical models have increased in popularity during recent years, but these models are still in their infancy. Here we present a coupled model system of the Norwegian Sea, including an ocean model, a phytoplankton model and individual-based models of the copepod Calanus finmarchicus and planktivorous fish. The fish migration model is driven by survey observations, but the C. finmarchicus densities also affect the fish migrations. This leads to a realistic predation pressure on zooplankton by fish in time and space. The predation pressure of fish on zooplankton is calculated by modelling individual fish growth using bioenergetics models. We estimated that the major pelagic fish stocks in the Norwegian Sea, herring, mackerel and blue whiting, consumed a total of 82 million tonnes of zooplankton, including 35 million tonnes C. finmarchicus, in 1997. Furthermore, the interspecific competition has the greatest effect on blue whiting consumption, while herring consumption is unaffected by the other fish species in this one-year simulation. We also analysed the spatial distribution of C. finmarchicus in June and August and found the highest densities in the northern Norwegian Sea and around Iceland. The results give us a better understanding of the effect from fish predation on the Norwegian Sea ecosystem, and how it affects the C. finmarchicus stock.