Robust models relating climate change to fish production require an adequate description of planktonic intermediaries between phytoplankton and fish in end-to-end models. In turn this requires and justifies a proper testing of zooplankton models. Fundamental issues regarding inclusion of zooplankton in these end-to-end models are discussed. It is argued that the complexity of the zooplankton component requires careful consideration and should not be simplified arbitrarily relative to higher and lower trophic levels. Future modelling studies are needed to rigorously examine the effects of increasing complexity within the zooplankton component on ecosystem dynamics. Acquisition of data from targeted field and laboratory studies, including mesocosms, is needed for testing mechanistic end-to-end models and optimizing the balance between fidelity and simplicity in the zooplankton component.