Sammendrag
Chaoborus larvae inhabit frequently the water column of lakes, when they can be mistaken for
small fish. Because larvae ascend up to the blind zone of downward-looking echo sounding at
night, quantitative acoustic estimation of them is possible only with upward-looking approach.
For this reason, the mobile hydroacoustic upward-looking system (120 and 38 kHz split-beam
echosounder) in combination with a direct catch method (trawling) was tested to investigate the
night community of invertebrates and juvenile fish in the surface layer of the #ímov reservoir
(Czech Republic). In the target strength range of invertebrates (smaller than -59 dB), the 38
kHz echosounder recorded only a small proportion of targets while the 120 kHz echosounder
recorded distinct peaks corresponding to high densities of Chaoborus (target strength, TS range
-70 to -60 dB, average TS -66 to -64 dB). At 120 kHz frequency, the TS distribution of smaller
cohort of juvenile fish ( 1.5 ind.m^3
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