Abstract The effect of target strength oscillations, generated by surface or bottom-induced sound... more Abstract The effect of target strength oscillations, generated by surface or bottom-induced sound multi-pathing, can cause serious errors in estimates of fish biomass in horizontal acoustic surveys of extremely shallow inland waters (depth 1.7–2 m). In this study, the oscillations were shown to occur in both windy and calm weather conditions, and depended on the target’s depth and range from a transducer. In calm weather the oscillations were more pronounced and targets that were located close to the water surface and further from the transducer were most affected. Errors in target strength reached up to an 8 dB difference from the theoretical value. Fish abundance and size structure estimates are likely to be affected even more than biomass estimates. To avoid boundary-induced errors in target strength, simulations of fish biomass surveys demonstrated that the data-processing range should be limited to 10 m from the transducer for surveys conducted in extremely shallow waters.
Chaoborus larvae inhabit frequently the water column of lakes, when they can be mistaken for smal... more 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 Římov 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 (<25 mm in length) overlapped the...
ABSTRACT This study addresses a fish species discrimination method based on normalised elliptic F... more ABSTRACT This study addresses a fish species discrimination method based on normalised elliptic Fourier descriptors applied to acoustic shadows derived by Dual-frequency Identification Sonar (DIDSON). Acoustic shadows of templates (20, 30, 40 and 50-ácm) and live fish of four species [bream, Abramis brama (L.); barbel, Barbus barbus (L.); chub, Leuciscus cephalus (L.); and trout, Salmo trutta (L.)] were projected on a plate in an experimental set-up and tested on suitability for species discrimination. Twenty-centimetre templates were correctly classified in 97.5% of the cases, indicating a size threshold. The larger templates reached values of 100% correct classification based on cross-validated discriminant function analysis. It was also possible to classify moving fish based on screenshots of their acoustic shadows with a certainty of 83.9%. Extended field tests are required to evaluate the method for use in practical monitoring applications in multispecies river environments
Traditionally fish counting based on signals from split-beam sonars have been performed by passin... more Traditionally fish counting based on signals from split-beam sonars have been performed by passing the data trough a single echo detector (SED) and a tracking algorithm. This methods works fine in situations with high signal to noise ratio. However, in riverrine environment that can be characterised by low signal to noise ratio in combination with high reflective boundaries the method
We have experienced difficulties with fish detection in sonar-data from horizontally aligned tran... more We have experienced difficulties with fish detection in sonar-data from horizontally aligned transducers in shallow water. Recorded material revealed that echoes from fish often took on high ping-to-ping variations in variables like pulse length, intensity, shape and phase. Single echo detection methods based on a set of echo criteria tended to overlook echoes from fish and to accept echoes from
Target detection in data from the DIDSON (dual-frequency identification sonar) is a challenge. Du... more Target detection in data from the DIDSON (dual-frequency identification sonar) is a challenge. Due to high frequency and many narrow beams, the system can take series of acoustic pictures of passing fish. The entire body can be seen, but are often blurred and unclear in individual pictures. It is only when series of pictures are played like a film one
The Journal of the Acoustical Society of America, 2005
Acoustic target detection is commonly carried out with parametric single echo detectors. These de... more Acoustic target detection is commonly carried out with parametric single echo detectors. These detectors test one ping at a time and look for echoes fulfilling a set of criteria such as echo duration and shape. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the parametric detector to produce fractionated
Abstract The effect of target strength oscillations, generated by surface or bottom-induced sound... more Abstract The effect of target strength oscillations, generated by surface or bottom-induced sound multi-pathing, can cause serious errors in estimates of fish biomass in horizontal acoustic surveys of extremely shallow inland waters (depth 1.7–2 m). In this study, the oscillations were shown to occur in both windy and calm weather conditions, and depended on the target’s depth and range from a transducer. In calm weather the oscillations were more pronounced and targets that were located close to the water surface and further from the transducer were most affected. Errors in target strength reached up to an 8 dB difference from the theoretical value. Fish abundance and size structure estimates are likely to be affected even more than biomass estimates. To avoid boundary-induced errors in target strength, simulations of fish biomass surveys demonstrated that the data-processing range should be limited to 10 m from the transducer for surveys conducted in extremely shallow waters.
Chaoborus larvae inhabit frequently the water column of lakes, when they can be mistaken for smal... more 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 Římov 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 (<25 mm in length) overlapped the...
ABSTRACT This study addresses a fish species discrimination method based on normalised elliptic F... more ABSTRACT This study addresses a fish species discrimination method based on normalised elliptic Fourier descriptors applied to acoustic shadows derived by Dual-frequency Identification Sonar (DIDSON). Acoustic shadows of templates (20, 30, 40 and 50-ácm) and live fish of four species [bream, Abramis brama (L.); barbel, Barbus barbus (L.); chub, Leuciscus cephalus (L.); and trout, Salmo trutta (L.)] were projected on a plate in an experimental set-up and tested on suitability for species discrimination. Twenty-centimetre templates were correctly classified in 97.5% of the cases, indicating a size threshold. The larger templates reached values of 100% correct classification based on cross-validated discriminant function analysis. It was also possible to classify moving fish based on screenshots of their acoustic shadows with a certainty of 83.9%. Extended field tests are required to evaluate the method for use in practical monitoring applications in multispecies river environments
Traditionally fish counting based on signals from split-beam sonars have been performed by passin... more Traditionally fish counting based on signals from split-beam sonars have been performed by passing the data trough a single echo detector (SED) and a tracking algorithm. This methods works fine in situations with high signal to noise ratio. However, in riverrine environment that can be characterised by low signal to noise ratio in combination with high reflective boundaries the method
We have experienced difficulties with fish detection in sonar-data from horizontally aligned tran... more We have experienced difficulties with fish detection in sonar-data from horizontally aligned transducers in shallow water. Recorded material revealed that echoes from fish often took on high ping-to-ping variations in variables like pulse length, intensity, shape and phase. Single echo detection methods based on a set of echo criteria tended to overlook echoes from fish and to accept echoes from
Target detection in data from the DIDSON (dual-frequency identification sonar) is a challenge. Du... more Target detection in data from the DIDSON (dual-frequency identification sonar) is a challenge. Due to high frequency and many narrow beams, the system can take series of acoustic pictures of passing fish. The entire body can be seen, but are often blurred and unclear in individual pictures. It is only when series of pictures are played like a film one
The Journal of the Acoustical Society of America, 2005
Acoustic target detection is commonly carried out with parametric single echo detectors. These de... more Acoustic target detection is commonly carried out with parametric single echo detectors. These detectors test one ping at a time and look for echoes fulfilling a set of criteria such as echo duration and shape. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the parametric detector to produce fractionated
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