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Towards more robust hydroacoustic estimates of fish abundance in the presence of pelagic macroinvertebrates
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2020
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Source: Fisheries Research, 230, 105667
[PDF-17.17 MB]
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Journal Title:Fisheries Research
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Description:The inclusion of unwanted targets in hydroacoustic surveys biases estimates of fish abundance. Thus, knowledge of frequency-dependent responses of unwanted targets (e.g., pelagic macroinvertebrates) can help ensure that transducer frequencies are used that minimize this bias. We determined how fish density estimates varied across multiple frequencies when the larval stage of a midge, Chaoborus, was present in the water column. We hypothesized that fish density estimates would increase with increasing transducer frequency, owing to greater backscattering by Chaoborus at higher frequencies than lower ones, which allows it to be included with the backscattering caused by fish. We found that fish density estimates were always greater at higher frequencies (e.g., 120 and 200 kHz) compared to a lower one (70 kHz) in several productive north-temperate reservoirs. Furthermore, pairwise comparisons of total (i.e., fish plus Chaoborus) backscattering showed that significantly more backscattering occurred at higher rather than lower frequencies. We also found that fish density estimates varied between spring and summer, partially owing to inter-seasonal size variation in Chaoborus that influenced its backscattering. Beyond demonstrating why the presence of pelagic macroinvertebrates needs to be considered when estimating fish abundance with hydroacoustics, we provide methods to identify and reduce this bias.
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Source:Fisheries Research, 230, 105667
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ISSN:0165-7836
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Rights Information:Accepted Manuscript
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