The state-space assessment model (SAM) is increasingly used to assess fish stocks in International Council for the Exploration of the Sea. One unique feature of the SAM class is that it allows the fishing selectivity to vary over time, and the degree to which it varies is not subjectively assigned, but estimated within the model. Selection may vary over time due to changes in the spatial pattern of the fish stock or fishing fleet, but a direct cause of selectivity change can be changed in fishing technology or other measures that target specific segments of the fish stocks. If the relative catches from fishing fleets which target different age or size classes of a species are changing over time, then the overall selectivity will also change—even if the selectivity within each fleet is fairly constant. A recent extension of the SAM model allows multiple fleets to be defined. It has been applied to two herring stocks to allow more detailed and fleet-specific management options in forecasts. For both stocks, the assessment from the multi-fleet models was consistent with the results from the single-fleet models, which strengthens confidence in the estimated time-varying selectivity for these and other stocks.