Individual-based models are becoming an important tool in ecosystems modelling. The main disadvantage encountered with this kind of model is the difficulty to understand how the global dynamics emerges from individual behaviours. We first present a sample of the various processes and sub-models that could intervene in a model of freshwater fishes. Then, we introduce the need for understanding the dynamics of an individual-based model. Finally, we present a framework to help the design and the software implementation of an individual-based model of freshwater fishes.