Population Assessment of Pelagic Fishes Using Hydroacoustics

Led By

Bill Pate at Colorado State University, Andrew Treble, and Lake and Reservoir Researchers

Study Area

Select coldwater reservoirs

Project Status

Ongoing

Research Objectives

• Monitor the depth-distribution, density, size structure, and abundance of pelagic fish species in key coldwater reservoirs.
• Monitor zooplankton species assemblage and density.
• Monitor invasive Mysis shrimp densities where present.

Project Description

​Hydroacoustic surveys enable rapid estimation of the abundance, density, size structure, and depth-distribution of pelagic (or offshore/open water) fish species such as kokanee and lake trout. Mobile hydroacoustics is a quantitative method for sampling the water column of a large lake or reservoir with sound that allows for greater spatial coverage than gill netting or midwater trawling. Survey transects are completed overnight during the dark of the moon to allow for greater spatial distribution of the fish.

Hydroacoustic surveys are generally most effective when few pelagic species are present, and Colorado has many coldwater reservoirs containing relatively simple pelagic fish assemblages. Kokanee waters, such as Lake Granby and Blue Mesa Reservoir, provide upwards of $30 million in economic benefit to the state annually and are vital broodstocks that supply eggs to support statewide stocking efforts. Hydroacoustic surveys conducted on these and other waters are necessary for monitoring the health of these valuable fisheries and identifying when additional management interventions or research might be required to ensure stable populations.

In addition, monitoring zooplankton species assemblages and Mysis shrimp densities are an important aspect of this assessment project. Kokanee are an obligate planktivorous species that feed on zooplankton like Daphnia, and lake trout rely upon Mysis shrimp when young before turning to kokanee or other fishes as a forage base. In addition, detrimental invasive species of zooplankton such as Daphnia lumholtzi can be detected as early as possible through this monitoring program. Better monitoring practices allow for more informed management decisions for a given water body.

Associated Publications

Lepak, J.M., W.M. Pate, P. Cadmus, A.G. Hansen, K.D. Gallaher, and D. Silver. 2024. Response of an invasive aquatic crustacean to the fish toxicant rotenone. Lake and Reservoir Management 40:330-337.