Modeling Target Strength Distributions to Improve Hydroacoustic Estimation of Lake Trout Population Size

Abstract

Many management agencies use hydroacoustic surveys to estimate pelagic prey fish abundance and population trends. It would be desirable to simultaneously assess piscivore population size and predation demand. However, multiple sources of variation in target strength complicate the target strength–fish size relationship, impairing managers' ability to distinguish the echoes of predators and prey. This uncertainty may substantially bias population size estimates, especially for piscivores that are greatly outnumbered by other species. We used an in situ estimate of target strength variance combined with fish length-frequency distributions to estimate the distribution of target strengths for prey-sized kokanee Oncorhynchus nerka and piscivorous lake trout Salvelinus namaycush in Blue Mesa Reservoir, Colorado. Comparison of the resulting lake trout population size estimates with those obtained from an intensive mark–recapture study showed that this approach substantially improved the precision and accuracy of hydroacoustic estimates. This technique may be especially useful in systems having relatively few species or species with discrete size-classes, as is the case for many western U.S. reservoirs.