Comparing Imprecise Abundance Indices with a Symmetric Model

Abstract

Richards and Schnute (1986. Can. J. Fish. Aquat. Sci. 43: 1214–1227) presented an errors-in-variables model for testing the assumption that catch per unit effort (CPUE) is an index of stock abundance. Here, I generalize the model and extend its application to any pair of abundance indices for which the ratio of variances in the two indices can be estimated. I used the model to relate density indices from two years, CPUE values from two years, and the two CPUE–density combinations. Density estimates of nearshore reef fishes from the strait of Georgia, British Columbia, were obtained by SCUBA, and CPUE estimates were obtained by angling at the same sites. Among-site differences in both CPUE and density tended to be consistent between years. For quillback rockfish (Sebastes maliger), the relationship between CPUE and density was one of strict proportionality, as found by Richards and Schute over a different depth range.