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Fish body orientation significantly influences the size obtained with hydroacoustic signals, and thus the estimate of fish size and biomass. For this reason, each characteristic of a target’s echo can be advantageous for developing algorithms to refine acoustic fish estimates. We measured pulse-compressed broadband acoustic signals from tethered fish (common bream Abramis brama) in different dorsal positions. Based on generalized additive mixed models (GAMM), we initially tested the influence of the fish dorsal aspect on the amplitude echo envelope and amplitude echo descriptors (amplitude maximum and amplitude echo length at seven different levels below the maximum) by altering the fish dorsal orientation. Our study confirmed that the dorsal aspect influenced the shapes of the amplitude echo envelopes in both fast- and slow-tapered pulses. Furthermore, we found that echo lengths approximately 15 dB below the amplitude maximum, especially for fast-tapered signals, could provide good characteristics of the echo-envelope shape for determining the fish dorsal aspect and facilitating thus the conversion between acoustic target strength and true fish length.

Monitoring fish populations in large, deep water bodies by conventional capture methodologies requires intensive fishing effort and often causes mass mortality of fish. Thus, it can be difficult to collect sufficient data using capture methods for understanding fine scale community dynamics associated with issues such as climate change or species invasion. Hydroacoustic monitoring is an alternative, less invasive technology that can collect higher resolution data over large temporal and spatial scales. Monitoring multiple species with hydroacoustics, however, usually requires conventional sampling to provide species level information. The ability to identify the species identity of similar-sized individuals using only hydroacoustic data would greatly expand monitoring capabilities and further reduce the need for conventional sampling. In this study, wideband hydroacoustic technology was used in a mesocosm experiment to differentiate between free swimming, similar-sized individuals of two swim-bladdered species: whitefish (Coregonus wartmanni) and stickleback (Gasterosteus aculeatus). Individual targets were identified in echograms and variation in wideband acoustic responses among individuals, across different orientations, and between species was quantified and visually examined. Random forest classification was then used to classify individual targets of known species identity, and had an accuracy of 73.4% for the testing dataset. The results show that species can be identified with reasonable accuracy using wideband hydroacoustics. It is expected that further mesocosm and field studies will help determine capabilities and limitations for classifying additional species and monitoring fish communities. Hydroacoustic species differentiation may offer novel possibilities for fisheries managers and scientists, marking the next crucial step in non-invasive fish monitoring.

Simultaneous observations of fish and cyanobacteria were conducted in the shallow Sulejow Reservoir (Poland) during the occurrence of Microcystis bloom. A Simrad EY60 split beam echosounder with a 200 kHz transducer beaming horizontally was applied to assess fish and cyanobacteria spatial distribution. Additionally, fish size distribution and species composition were evaluated with gillnets, and cyanobacterial biomass was determined by using an online phycocyanin fluorescence probe. Physico-chemical parameters and water samples for biological analyses were collected at 14 fixed stations situated along the acoustic transects. We found cyanobacteria represented by the genus Microcystis, with their toxigenic genotypes in all analyzed samples. The hydroacoustic results provided direct evidence for fish night avoidance of the bloom. The biomass of fish and cyanobacteria demonstrated opposing trends and their peak values spatially mismatched. The number of fish caught in gillnets within the bloom area was about half that caught outside the bloom area. In spite of the presence of intracellular microcystins (hepatotoxin) at all stations, no extracellular microcystins were identified in water samples and in fish tissues.

Horizontally-aligned, fixed and mobile, transducers are routinely deployed at depths of 0.5–0.75 m to survey the surface layer of waterbodies for fish. However, simulations and measurements demonstrate that a smooth surface can cause serious errors in the target strength (TS) and split-beam angular position estimates. Errors in TS up to 10 dB and depth position up to 0.5 m were observed. Simulations suggested that multi-path signal propagation interfered with the direct path of that signal. Furthermore, when a standard target (calibration copper sphere) was moved away from the transducer at fixed depths, the estimated TS and depth of the target started to oscillate as a function of range. The amplitude increased with increasing range. The frequency decreased from the oscillation start until a certain range where the oscillation stopped. The region of oscillation depended on both the transducer and target depth. Horizontal observations of known fish echoes behaved similarly. Experiments in a lake showed that the influence from the surface disappeared when the surface became rippled due to wind.

International audience; Hydroacoustics has become a requisite method to assess fish populations and allows to describe the relationships of fish with other elements of the aquatic ecosystem. This nonintrusive method is currently an integral part of the sampling procedures recommended for fish stock assessment by the Water Framework Directive and has been standardized by the European Committee for Standardization [CEN (2014) CSN EN 15910 - Water quality - Guidance on the estimation of fish abundance with mobile hydroacoustic methods, Category: 7577 Water quality. Biological.]. In Europe, hydroacoustic surveys are performed in freshwater using different frequencies. Consequently, there is a need to evaluate if survey results can be compared. This study aimed to carry out in situ comparisons at the 38 kHz frequency (noted f) with two other commonly used frequencies, 70 and 200 kHz. The 38 kHz frequency has seldom been compared with other frequencies in freshwater although it is widely used worldwide, especially in the Great Lakes of North America and in Sweden. In 2016, hydroacoustic data were acquired in Lakes Annecy and Bourget using methods validated in previous studies that compared the frequencies 70, 120 and 200 kHz. This study showed similar density and biomass estimations as a function of frequency, density(f) and biomass(f), between the frequencies studied for low to moderate fish densities. For higher fish densities, the results were more variable and need to be verified. Fish density(f) and biomass(f) estimations sometimes exhibit differences between frequencies, which is not fully in agreement with theoretical calculations. The aim of this study was to evaluate frequency comparisons in practise. However, if the differences on acoustic metrics, density(f) or biomass(f) between frequencies were occasionally statistically significant, the differences were small enough to be considered negligible for fish population management. These analyses led to better knowledge of the responses from fish in temperate lakes for the studied frequencies. Our findings should be considered when revising the CEN standard.

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.

This study addresses fish behaviour at the mouth of a midwater trawl in two temperate reservoirs. Fish distribution and behaviour were monitored using a SIMRAD EK60 (38 kHz) split-beam echosounder with the transducer deployed at the water surface, attached to the surface trawl headrope. We were able to describe day and night patterns of vertical distribution of fish in the reservoirs, describe fish behaviour near the footrope (i.e. vertical swimming velocity, fish tortuosity, vertical direction of swimming), and find the proportion of fish performing avoidance reactions. Significant differences were found between day and night distributions and behaviours. In total, only 35 of the 1514 acoustically recorded fish performed apparent avoidance behaviour. Twenty-seven avoidance reactions were observed in the day (6.3-% of total fish) while eight were observed at night (0.7-% of total fish). Fish were more active in the daytime, while tending to stay still in the water column with no observable reactions at night. We found significant dependence between the size of fish directly avoiding the trawl and their swimming velocities. The greatest echogram slope values belonged to those fish with a strong avoidance reaction. When the footrope was perceived, the fish reacted from approximately 1 m vertical distances and escaped up to 5 m below the footrope (usually 1–2.5 m). Light, water transparency and temperature stratification were the dominant factors influencing vertical distribution and fish avoidance behaviour.

The DIDSON multi-beam sonar has become a popular tool for monitoring fish. The aim of this study was to determine how well the DIDSON sonar can detect and measure fish during stationary applications in lakes. Cyprinid fish of known sizes (10–60 cm long) and with horizontal body orientations were deployed in known positions at two ranges (6.3 and 9.5 m) within a DIDSON high-frequency array of beams. A new method for estimating fish length from multi-beam sonars was thus developed and applied. At both ranges, all the deployed fish were invariably detected when they were aligned with their sides perpendicular to the beam. Increasing fish body angle, however, reduced the ability of the system to detect fish. Only the largest fish were detectable when the fish were parallel to the beam (head or tail aspect). The estimated fish length agreed well with the actual length when the fish were positioned perpendicular to the center of the multi-beam array. The lengths were underestimated for fish that were at the edges of the array, further away from the transducer or that had an increased body aspect, especially small-sized fish. Additionally, we observed that the wide girth of large fish can shadow the rest of the body, resulting in a decreased estimated length. We showed that determining the actual length of fish is challenging and is not a trivial task, and we raise the question of where to define fish length along the echo intensity of fish. We conclude that including the error functions for length estimates allows the DIDSON to obtain more reliable and accurate biological information.

For ethical reasons and animal welfare, it is becoming increasingly more important to carry out ecological surveys with a non-invasive approach. Information about fish distribution and abundance in the upper water column is often fundamental. However, this information is extremely hard to obtain using classical hydroacoustic methods. We developed a rigid frame system for pushing upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0–3 m and 3–6 m depth layers, which are utilized by almost all juvenile fish in summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods. The comparison of abundance estimates gathered by the two frequencies were also not significantly different. The predicted mean lengths from acoustic sampling and the trawl catches differed by less than 10 mm in all comparisons. Results suggest that mobile hydroacoustic upward-looking systems can fill the methodological gap in non-invasive surveying of surface fishes.

Observing and quantifying fish behaviour towards active sampling gears like trawls is challenging. A high-frequency imaging sonar (DIDSON) was applied to directly observe and record fish avoidance behaviour in front of the mouth of an active pelagic surface trawl in the meso- to eutrophic Czech reservoir Želivka in August 2009. In this multi-species environment the fish displayed a variety of behavioural patterns. We classified 11 different categories of avoidance ( n = 1051) based on fish reaction and swimming behaviour towards the active trawl using Sonar5-Pro software. Classification of these categories was based on eight continuous variables: (1) total fish length, (2) distance to the trawl, (3) tortuosity, (4) mean speed of total track, (5) mean speed of first avoidance track, (6) mean speed of total avoidance track, (7) angle between initial and first avoidance track and (8) angle between initial and total avoidance track. The acoustic analysis showed that 76% of the fish in trawling area was captured and 24% swam out of it. A higher proportion of the fish showed an avoidance reaction during the day (44%) than during the night (6%), indicating that ambient light conditions trigger this behaviour during the day. A two-step cluster analysis of the tracks revealed three distinct clusters that corresponded to the most abundant fish species in the reservoir: ‘bleak-cluster’, ‘bream-cluster’ and ‘silver carp-cluster’.

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 cm) 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.

The in situ avoidance reaction of vendace, Coregonus albula L., in relation to continuous artificial light in terms of a scaring device was tested by split-beam echosounding. The tests were carried out in the mesotrophic Bigge Reservoir (Germany) in November 2007 and 2008 using a Simrad EK 60 echosounder (120 kHz). An underwater lamp with light turned on and off was lowered to 30 m water depth at four sampling sites. No reaction of vendace was observed for the sampled references (light off), but strong avoidance behaviour by swimming mainly downwards was observed when the light was turned on. The light avoidance reaction was confirmed by the volume backscattering strength (Sv) and calculated fish densities (fish ha−1). The mean fish densities for the samples carried out with light turned on were lower (268 fish ha−1, ±409 SD, n = 32) than for the references with light turned off (5028 fish ha−1, ±2317 SD, n = 56). The related median values (33 and 3726 fish ha−1, respectively, n = 88) differed significantly (Mann–Whitney test, P =

We measured acoustic backscattering from Mysis relicta , a common invertebrate in northern lakes, using five frequencies (38, 120, 200, 430, and 710 kHz). Acoustic backscattering from mysids was highest at 430 kHz and lowest at 38 kHz (19 dB lower). Maximum difference between the four other frequencies was 5.2 dB. Mysid target strength (TS) ranged from –80.1 dB at 430 kHz to –99.4 dB at 38 kHz (12 mm average length, range 5–21 mm). A theoretical scattering model (Stanton’s fluid-like, bent-cylinder model) predicted TS within 0.3–1.9 dB of observed TS for the different frequencies. The detection range was lowest at 38 and 710 kHz and greatest at 120 and 200 kHz. Fish were common above the mysid layer and produced higher acoustic backscattering at 38 kHz than at the other frequencies. A combination of 38 kHz and 120 or 200 kHz provides a strong contrast between mysid and fish acoustic backscattering that would help separate these groups using acoustic data.

A high variability in fish's aspect, causing strong target-strength (TS) fluctuations, was observed in nase Chondrostoma nasus (L.) during upstream spawning migration through a horizontal, bottom-aligned beam in the Fischa River, a tributary of the Danube. Size estimates based on traditional mean target strength underestimated the total length of nase. Applying maximum TS from side-aspect detections overestimated size due to signal amplifications of up to 10 dB in the single-echo detections of tracked fish. Based on Lilja's simulations, the strong signal amplifications probably originated from flash-effects. Inhomogeneous sound-propagation due to cylindrical spreading and the maturity state of spawning nase probably contributed to signal amplification of up to 6 dB. In order to find better size estimators than the mean and max size estimator, we tested percentile estimators, and we developed a new statistical method—the Maximum Excess Test and Maximum Isolation Test (MET–MIT). This method links information from catch data with the TS-distribution from tracked fish in order to exclude outliers. The results show that both the MET–MIT and the percentiles gave reasonably similar results and that both were better size estimators than the traditional mean-target-strength estimator.

SUMMARY 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.

Hydroacoustic split-beam techniques have been applied to enumerate salmon migrating in the river Tana (northern Norway) during the summers 1998 and 1999. Analysing data by single echo detection and tracking was difficult. Missing echoes in tracks from fish, combined with noise in the output from the single echo detector was seen as reasons for this. An improved counting method is presented. Contours from moving targets are detected by image analysis. Then, detected single echoes within these contours are combined into tracks. This procedure reduces problems related to noise, and to tracking fish with few accepted single echoes.

Several decades of research have led to the acceptance of hydroacoustics as a reliable measurement method to monitor fish population in lakes, but full standardisation and intercalibration are still lacking. The aim of this study was to investigate the effect of sound frequency on acoustic parameters, such as volume backscattering strength, target strength, and the estimation of fish abundance. Data were recorded in situ using 3 frequencies (70, 120, 200 kHz) simultaneously in 2 different lakes. The results among the frequencies were compared and statistically tested. Data from the 70 and 120 kHz frequencies yielded similar results, but the 200 kHz echosounder estimates in temperate lakes seemed different, especially in cases of high fish abundance, which is typical of eutrophic ecosystems. This work indicates that the abundance estimates of fish populations in temperate lakes based on 200 kHz frequency may differ from results obtained using lower frequencies, and that further study is needed.

Since two large specimens of Chinese paddlefish Psephurus gladius were found unexpectedly in December 2002 and January 2003, a series of measures have been undertaken to attempt to save this critical endangered species. As a part of the rescue plan, a total of eight hydroacoustic and capture surveys were conducted in the upper Yangtze River between 2006 and 2013. Preliminary results from the first four surveys (2006−2008) were reported in 2009. The later four surveys were conducted in the same area and consisted of 578 setlines, 8 anchored setlines and 2003 drift net catches. A total of 1982 fish belonging to 38 species were captured. For the hydroacoustic surveys, a BioSonics echo sounder with a vertically down looking 199 kHz split-beam transducer was applied, both for data recording, and for guiding the capture trials by drift nets and setlines. Post processing applying automatic tracking identified 7217 single targets from the eight acoustic surveys. Body size was estimated from TS values of single targets. No P. gladius were captured. The acoustic data, however, revealed at least a few observations that are difficult to interpret as something other than large P. gladius . Although not conclusive, this gives hope that this species may still exist in the river.

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 tracks from fish surrounded by numerous noise detections. Parametric detectors utilize only small portions of the information available in a split beam echogram. By including information from more than one ping and from the background reverberation, a more robust fish detector has been designed. This detector, called the Cross Filter Detector (CFD), has now been further improved by applying the variance in the angle measurements.

Freshwater hydroacoustic surveys are frequently performed with small research boats. Thus, the transducer, usually fixed on a pole on the side of the boat, could be misaligned with the horizontal axis (i.e., the lake surface). Given that fish acoustic responses are strongly directional, the transducer angle could induce attenuation of their backscattering strengths. To test the influence of a small shift of the transducer orientation, we compared hydroacoustics estimates from two sampling nights on the Lake Annecy: the first one being a transducer angle of $$1.5^{\circ }$$ with the horizontal and the second one being strictly horizontal. We showed that the effect of such an angle is negligible over the sampling variability on the TS distribution of the fish population and the biomass assessment. We conclude that a small shift in the transducer horizontality ( $$\le 1.5^{\circ }$$ ) will not degrade the quality of the acoustic data.