Echo-sounding can discriminate between fish and macroinvertebrates in fresh water |
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Authors: | JUHA JURVELIUS,FRANK REIER KNUDSEN&dagger ,HELGE BALK&Dagger ,TIMO J. MARJOMÄ KI§ ,HEIKKI PELTONEN¶ ,JOUNI TASKINEN,ANTTI TUOMAALA&dagger &dagger , MARKKU VILJANEN |
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Affiliation: | Saimaa Fisheries Research and Aquaculture, Finnish Game and Fisheries Research Institute, Enonkoski, Finland; Simrad As, Kongsberg Maritime, Horten, Norway; Department of Physics, University of Oslo, Oslo, Norway; Department of Biology and Environmental Science, University of Jyväskylä, Jyväskylä, Finland; Finnish Environment Institute, Helsinki, Finland; Faculty of Biosciences, University of Joensuu, Joensuu, Finland; Faculty of Biosciences, Department of Biological &Environmental Science, University of Helsinki, Helsinki, Finland |
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Abstract: | 1. Acoustic scattering from fish and macroinvertebrates was studied in a boreal Finnish lake at three echosounder frequencies (38, 120 and 200 kHz). Split-beam transducers with partly overlapping 7° beams were employed. Acoustic, fish and invertebrate sampling were undertaken simultaneously. Vertical gradients of temperature and oxygen concentration were measured during the exercise. 2. At all frequencies, a narrow scattering layer coincided with the thermocline. At 38 kHz, fish were detected well with practically no reverberation from invertebrates while 200 kHz detected both fish and invertebrates. 3. Minor differences in the magnitude of acoustic scattering from fish were found between frequencies and between depth layers, but scattering at different frequencies was correlated at all depths. Acoustic scattering and fish density indices from trawl catches, consisting mostly of smelt ( Osmerus eperlanus ) (97%) and vendace ( Coregonus albula ) (3%), were significantly correlated. 4. Acoustic scattering from invertebrates increased with sound frequency. Correlation analysis suggested that the invertebrate scattering was mostly induced by Chaoborus flavicans . A low frequency is recommended for estimating fish abundance without bias from reverberation induced by invertebrate scattering. Although fish and invertebrates can also be successfully discriminated at a single frequency by thresholding and cross filtering, the combination of a low and a high frequency is a more robust tool for effective fish-invertebrate discrimination. |
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Keywords: | Chaoborus flavicans multi-frequency smelt trawl vendace |
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