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Tuesday, February 7 • 6:00pm - 9:00pm
Poster Display. Determining Potential Bias by Chaoborus During Hydroacoustic Surveys of Prey Fish Biomass in Ohio Reservoirs

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AUTHORS: Rebecca A. Dillon, The Ohio State University; Joseph D. Conroy, Ohio Division of Wildlife; Stuart A. Ludsin, The Ohio State University

ABSTRACT: Hydroacoustic surveys provide lake-wide estimates of prey fish distribution, density, and biomass. These surveys, however, do not exclusively detect the target of interest. For example, the aquatic larval stage of the dipteran Chaoborus, a macroinvertebrate commonly found in inland lakes and reservoirs, has two air bladders which make it resonate well at a frequency (200 kHz) used for hydroacoustic surveys of prey fish. Although Chaoborus occupies deeper water during the day, it migrates into the water column at night and may contribute greatly to total acoustic backscatter, biasing hydroacoustic surveys. The Ohio Division of Wildlife conducts annual hydroacoustic assessments of prey fish biomass using 200-kHz transducers but has not accounted previously for potential bias due to the presence of Chaoborus. To determine this potential bias, we combined multi-frequency (70- and 200-kHz) hydroacoustic surveys with discrete-depth (n = 3 depths) pump and vertical net (153-micrometer mesh) tows to provide zooplankton and Chaoborus density estimates and horizontal, and discrete-depth (n = 2 depths) paired ichthyoplankton net (500-micrometer mesh) trawls to provide fish density estimates. Surveys were completed monthly during April–August 2016 in Alum Creek Lake. Combining multiple sampling approaches identified all acoustic scatters and provided in situ density estimates. We found Chaoborus on all dates sampled, with greater densities in samples collected at night. Hydroacoustic data indicated lower prey fish biomass from the 70-kHz transducer data compared to the 200-kHz transducer data, indicating bias from Chaoborus. Additional analyses will more fully quantify the seasonal contribution by Chaoborus to total acoustic backscatter and its effect on hydroacoustic estimates of forage fish biomass.

Tuesday February 7, 2017 6:00pm - 9:00pm
Lancaster Ballroom

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