Successful regulation of common-pool resources calls for a better understanding of resource exploitation behavior. This paper introduces an approach that can measure fishermen's responsiveness to stock changes more accurately. In order to deal with the challenge of the latent fish stock, I adopt the method proposed by Zhang and Smith (2011) that derives a stock index from a heterogeneous production function. I use the imputed stock proxy in a count data model that describes fishing trip frequency. By these two steps, I can estimate the stock elasticity of fishing mortality. In the empirical study of the reef-fish fishery in the northeastern Gulf of Mexico, I find that the popular method that uses catch rate as a stock proxy significantly underestimates fishermen's responses to stock changes. This result suggests that policy predictions based on the traditional method are overly optimistic. Copyright © 2011 The Berkeley Electronic Press. All rights reserved.