Recent changes in environmental and ecological conditions are a major reason that established stockassessment macro-models no longer provide reliable estimates on the stock status of eastern Baltic cod(Gadus morhua). To account for a complex ecological system, an individual-based modelling approach isneeded that turns the attention from the overall population to the individual fish, its metabolic processes,behaviour, and interaction with environmental conditions.The multi-level rule-based modelling language ML-Rules was used to develop the first iteration of anindividual-based model. The different stratification layers of the estuarine environment are modelledas explicit entities with varying properties and are populated by individual cod and its prey. The codmodel focuses on growth and mortality dependent on abiotic factors and behaviour within the stratifiedenvironment of the Bornholm Basin.Despite the complexity of the formalised interactions, the resulting model specification is succinct andcompact. Its structure and declarative nature facilitates reuse and subsequent extension. To allow easyreplication of simulation experiments, we used the simulation experiment specification language SESSLfor calibration and validation. The extendibility of the model and replicability of simulation experiments,as provided by the domain specific languages ML-Rules and SESSL respectively, will be key assets infuture research, as the model provides a first, but essential, step toward studying the impact of specificenvironmental changes and certain behaviour pattern on the cod population in the Bornholm Basin.
