| Abstract: | To assess the reliability of depth and density estimates of vertically migrating fish (Coregonus spp.), repeated hydroacoustic surveys were conducted in a stratified lake during the same day, during subsequent nights within a week, and over four months of a year. The reliability of the hydroacoustic method, determined by the coefficient of variance of population depth of fish, was high within nights, but decreased with increasingly longer time scales. Volumetric fish densities were likewise more variable between surveys at longer temporal distances than at short‐term scales. Night‐time population depths of fish were highly correlated to the vertical position of the thermocline. Over the diel cycle, the depth distribution of the coregonids was also correlated to light intensities at the surface. We conclude that single hydroacoustic surveys reliably reflect the depth distribution and volumetric density of the fish populations in a deep lake at temporal scales of up to a week. At longer scales, physical forcing may change the depth distribution of fish, and population dynamics may alter the density estimates substantially (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) |
