Correcting density-dependent effects in abundance estimates from bottom-trawl surveys

Indices of abundance are important for estimating population trends in stock assessment and ideally should be based on fishery-independent surveys to avoid problems associated with the hyperstability of the commercial catch per unit effort (cpue) data. However, recent studies indicate that the efficiency of the survey bottom trawl (BT) for some species can be density-dependent, which could affect the reliability of survey-derived indices of abundance. A function q(e)similar to f(u), where q(e) is the BT efficiency and u the catch rate, was derived using experimentally derived acoustic dead-zone correction and BT efficiency parameters obtained from combining a subset of BT catch data with synchronously collected acoustic data from walleye pollock (Theragra chalcogramma) in the eastern Bering Sea (EBS). We found that q(e) decreased with increasing BT catches resulting in hyperstability of the index of abundance derived from BT survey. Density-dependent q(e) resulted in spatially and temporarily variable bias in survey cpue and biased population age structure derived from survey data. We used the relationship q(e)similar to f(u) to correct the EBS trawl survey index of abundance for density-dependence. We also obtained a variance-covariance matrix for a new index that accounted for sampling variability and the uncertainty associated with the q(e). We found that incorporating estimates of the new index of abundance changed outputs from the walleye pollock stock assessment model. Although changes were minor, we advocate incorporating estimates of density-dependent q(e) into the walleye pollock stock assessment as a precautionary measure that should be undertaken to avoid negative consequences of the density-dependent q(e).