Our two ecosystems of interests are the Georges Bank cod population and the Northeast Arctic cod stocks. They are locate at the latitudinal extremes of the North Atlantic, and are used here as examples of two systems that can be studied in order of understanding how the governing processes required to ensure high growth and survival are structured in each system. To understand this we rely on the use of mechanistic individual based models that consists of the major physical and biological processes that we know govern growth of larval cod