Search

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.

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 ( 1.5 ind.m-3.

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.

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.

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.

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.