A nutrient to fish model for the example of the Baltic Sea

The paper describes an application of a NPZDF-model, which couples interactively a biogeochemical and a fish production model. The Baltic Sea is chosen as an example system, where the fish community is dominated by two prey species (sprat and herring) and one predator (cod). The linkage of the model components is established through feeding of prey fish on zooplankton and recycling of fish biomass to nutrients and detritus. The fish dynamics is driven by size dependent predator-prey interactions, reproduction and mortality. A challenge is the quantitative description of the multitude of processes involved. The model formulation is constrained by strict mass conservation. Examples of experimental runs over 40 years are discussed: A baseline simulation gives an approximate hindcast scenario of stock variations and catches over the time period of the years from 1963 to 2003 with increasing and declining cod catches. As known from observations, the stock size of sprat shows a reverse trend, while the herring stock appears to be relatively stable. The modeled results display several observed features, e.g. stock sizes and magnitude of changes, but the response times and phases of the variations are not well reproduced. Moreover, scenarios are simulated to indicate how moderate adjustments of fishing rates of cod over different time periods would change the stock and catches. (C) 2009 Elsevier B.V. All rights reserved.