Diurnal changes of fish behaviour in a lowland river monitored by a dual-beam echosounder

Abstract

The behaviour of fish was monitored in one of the lower reaches of the River Thames (England), using a BioSonics 102 dual-beam echosounder (420 kHz) with two transducers. One transducer was located in littoral (L) depths of the river whilst the other was sited in 3-m depth of water in mid-river and placed on a especially designed lifting device which allowed different 1-m depth strata to be sampled: surface (S), middle (M) and deep (D). These depth strata were sampled hourly over 24 h. The highest fish densities and biomass were recorded in the S and L strata during the night and early morning hours from 2230 to 0700 when the fish density in the M and D strata was much lower. During the night hours, the largest fish tended to move to the surface (S) and towards the littoral (L), resulting in a marked increase in fish biomass. Most of these fish seemed to move to the deeper strata during the daytime so that many of them became undetectable by horizontal sonar as they were too close to the bottom. Swimming upstream and downstream was more marked in the open water (S,M and D strata) than in the littoral (L), where movements were more random.