A length-structured partial ecosystem model for cod in the North Sea

In this paper we present a general modelling approach for coupled length-structured fish populations. The model includes a target set of fish species which are modelled with full length structure from egg to adult, together with a highly simplified size-spectrum representation of the rest of the ecosystem (zooplankton, benthos and other fish). For the structured fish species the food requirements for growth, maintenance and reproduction, together with a length-based prey preferences, are used to calculate predation mortalities on the prey. Model output can be assessed against a wide variety of independent observations. As an example we develop a model for cod (Gadus morhua) and nine of its most important fish predators and prey in the North Sea. The model is confronted with time series of stock biomass, recruitment, and landings, as well as survey data on length distributions, and diet data. The results suggest that herring (Clupea harengus) predation on early life history stages of cod is dynamically important, and that high herring abundance may play a role in the decline of stocks even during periods of declining fishing pressure. We show that the maximum sustainable yield of cod is strongly dependent on herring abundance, and that current levels of cod exploitation may become unsustainable if herring recruitment returns to historical high levels.