For a multispecies fishery managed to achieve a harvest rate target for a single species, an early and accurate forecast of an in-season or the end-of-season harvest rate index can assist a manager with a decision to control effort. We present a model for rapidly forecasting an in-season or end-of-season harvest rate index for one species, the species of interest, in a multispecies fishery using catch and effort data collected for that fishery. These harvest rate predictions can be statistically evaluated against harvest rate indices calculated independently from demographic and biological data obtained for the fishery. The model structure was defined such that an estimate of both measurement error and process error could be obtained. The data required are the historical time series of catches for the species caught in the fishery, fishing effort, and harvest rate indices for the species of interest calculated independently of the catch and effort data. We illustrate the model with chinook salmon Oncorhynchus tshawytscha as the species of interest in the west coast of Vancouver Island troll fishery and the Strait of Georgia sport fishery. Other important species caught in these multispecies fisheries are coho salmon O. kisutch, sockeye salmon O. nerka, and pink salmon O. gorbusha.