We have constructed a statistical model to forecast, with uncertainty, the stock of Norwegian farmed Atlantic salmon ( Salmo salar ). The model provided good predictions of future biomass of Norwegian farmed salmon and can also be used to perform “what-if” analysis exploring the impact of varying scenarios for stocking and slaughtering. The model is based on the number of fish in each mass class (0–1, 1–2, …, 10+ kg) and their average mass. The model, which is related to standard size-structured models, computes the number of fish growing into the next mass class the next month and the number of fish remaining in the same mass class. In addition, the number of new fish stocked, fish lost, slaughtered, and wasted, as well as the sea temperature related to the growth, were modelled. All the model parameters were estimated based on monthly data from 2002 to 2007, and the model was validated statistically. Any animal production involving cycles may benefit from this forecasting tool.