A new method is introduced and applied to the British Columbia halibut fishery to analyze changes in productivity of firms harvesting a natural capital stock. The index-number technique decomposes the contributions of output prices, variable input prices, fixed inputs and productivity to firm profits, adjusted for changes in the natural capital stock. An application of the method is given using micro-level data from a common-pool resource. The indexes provide a ready-made comparison of all firms to the most profitable firm per unit of resource stock. Benchmarking with the decompositions also allows firms and regulators to improve overall industry performance by allowing them to analyze what components are contributing most to (relative) economic profits.