Helge Balk

• Tušer, Michal; Brabec, Marek; Balk, Helge; Draštík, Vladislav; Kubečka, Jan & Frouzová, Jaroslava (2023). Feasibility of Time-Dependent Amplitude in Pulse-Compressed Broadband Acoustic Signals for Determining the Dorsal Orientation of Fish. Water. 15(8). doi: 10.3390/w15081596. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Godlewska, Małgorzata; Balk, Helge; Izydorczyk, Katarzyna; Kaczkowski, Zbigniew; Mankiewicz-Boczek, Joanna & Ye, Shaowen (2023). Rapid in situ assessment of high-resolution spatial and temporal distribution of cyanobacterial blooms using fishery echosounder. Science of the Total Environment. ISSN 0048-9697. 857. doi: 10.1016/j.scitotenv.2022.159492. Fulltekst i vitenarkiv
• Gugele, Sarah M.; Widmer, Marcus; Baer, Jan; DeWeber, J. Tyrell; Balk, Helge & Brinker, Alexander (2021). Differentiation of two swim bladdered fish species using next generation wideband hydroacoustics. Scientific Reports. 11(1). doi: 10.1038/s41598-021-89941-7. Fulltekst i vitenarkiv Vis sammendrag

  Abstract 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.

• Mouget, Anne; Goulon, Chloe; Axenrot, Thomas; Balk, Helge; Lebourges-Dhaussy, Anne & Godlewska, Małgorzata [Vis alle 7 forfattere av denne artikkelen] (2019). Including 38 kHz in the standardization protocol for hydroacoustic fish surveys in temperate lakes. Remote Sensing in Ecology and Conservation. 5(4), s. 332–345. doi: 10.1002/rse2.112. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Koliada, Ievgen; Balk, Helge; Tušer, Michal; Ptacek, Ladislav & Kubecka, Jan (2019). Limitations of target detection in horizontal acoustic surveys of extremely shallow water bodies. Fisheries Research. ISSN 0165-7836. 218, s. 94–104. doi: 10.1016/j.fishres.2019.05.005. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Baran, Roman; Tušer, Michal; Balk, Helge; Blabolil, Petr; Martin, CECH & Drastik, Vladislav [Vis alle 13 forfattere av denne artikkelen] (2019). Quantification of Chaoborus and small fish by mobile upward-looking echosounding. Journal of Limnology. ISSN 1129-5767. 78(1), s. 60–70. doi: 10.4081/jlimnol.2018.1837. Fulltekst i vitenarkiv Vis 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 (<25 mm in length) overlapped the TS-distribution of Chaoborus. The number of these smaller juvenile fish was so small compared with the number of Chaoborus that they did not seriously bias acoustic Chaoborus estimate. The correlation between the density of Chaoborus with small contamination of juvenile fish larvae from trawling and acoustic recording made with the 120 kHz echosounder was high (R2= 0.88), but the acoustic densities from trace counting appeared to underestimate Chaoborus abundance when the density was > 1.5 ind.m^3

• Godlewska, Małgorzata; Balk, Helge; Kaczkowski, Zbigniew; Jurczak, Tomasz; Izydorczyk, Katarzyna & Dlugoszewski, Bronislaw [Vis alle 9 forfattere av denne artikkelen] (2018). Night fish avoidance of Microcystis bloom revealed by simultaneous hydroacoustic measurements of both organisms. Fisheries Research. ISSN 0165-7836. 207, s. 74–84. doi: 10.1016/j.fishres.2018.05.025. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Simultaneous observations of fish and cyanobacteria were conducted in the shallow Sulejów 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.

• Drastik, Vladislav; Godlewska, Małgorzata; Balk, Helge; Clabburn, Peter; Kubečka, Jan & Morrissey, Emma [Vis alle 10 forfattere av denne artikkelen] (2017). Fish hydroacoustic survey standardization: A step forward based on comparisons of methods and systems from vertical surveys of a large deep lake. Limnology and Oceanography : Methods. 15(10), s. 836–846. doi: 10.1002/lom3.10202. Fulltekst i vitenarkiv
• Baran, Roman; Juza, Tomas; Tušer, Michal; Balk, Helge; Blabolil, Petr & Cech, Martin [Vis alle 16 forfattere av denne artikkelen] (2017). A novel upward-looking hydroacoustic method for improving pelagic fish surveys. Scientific Reports. 7(1). doi: 10.1038/s41598-017-04953-6. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Balk, Helge; Søvegjarto, Bendik Sørensen; Tušer, Michal; Froutzova, Jaroslava; Muška, Milan & Drastik, Vladislav [Vis alle 8 forfattere av denne artikkelen] (2016). Surface-induced errors in target strength and position estimates during horizontal acoustic surveys. Fisheries Research. ISSN 0165-7836. 188, s. 149–156. doi: 10.1016/j.fishres.2016.12.017. Fulltekst i vitenarkiv
• Zhang, Hui; Balk, Helge; Wang, Chengyou; Wu, Jinming; Du, Hao & Shen, Li [Vis alle 8 forfattere av denne artikkelen] (2016). Search for Chinese paddlefish (Psephurus gladius) in the upper Yangtze River during 2009-2013 including reevaluation of data from 2006 to 2008. Aquatic Living Resources. ISSN 0990-7440. 29(1). doi: 10.1051/alr/2016008. Fulltekst i vitenarkiv
• Sajdlová', Zuzana; Draštík, Vladislav; Jůza, Tomáš; Říha, Milan; Frouzová, Jaroslava & Čech, Martin [Vis alle 15 forfattere av denne artikkelen] (2015). Fish behaviour in response to a midwater trawl footrope in temperate reservoirs. Fisheries Research. ISSN 0165-7836. 172, s. 105–113. doi: 10.1016/j.fishres.2015.06.025. Fulltekst i vitenarkiv
• Guillard, Jean; Lebourges-Daussy, Anne; Balk, Helge; Colon, Michel; Godlewska, Małgorzata & Jóźwik, Adam (2014). Comparing hydroacoustic fish stock estimates in the pelagic zone of temperate deep lakes using three sound frequencies (70, 120, 200 kHz). INLAND WATERS. ISSN 2044-2041. 4, s. 435–444. doi: 10.5268/IW-4.4.733. Fulltekst i vitenarkiv
• Tuser, Michal; Frouzova, Jaroslava; Balk, Helge; Muska, Milan; Mrkvička, Tomáš & Kubecka, Jan (2014). Evaluation of potential bias in observing fish with a DIDSON acoustic camera. Fisheries Research. ISSN 0165-7836. 155, s. 114–121. doi: 10.1016/j.fishres.2014.02.031. Fulltekst i vitenarkiv
• Rakowitz, Georg; Tuser, Michal; Riha, Milan; Juza, Tomas; Balk, Helge & Kubecka, Jan (2012). Use of high-frequency imaging sonar (DIDSON) to observe fish behaviour towards a surface trawl. Fisheries Research. ISSN 0165-7836. 123, s. 37–48. doi: 10.1016/j.fishres.2011.11.018. Fulltekst i vitenarkiv
• Langkau, M C; Balk, Helge; Schmidt, MB & Borcherding, J (2012). Can acoustic shadows identify fish species? A novel application of imaging sonar data. Fisheries Management and Ecology. ISSN 0969-997X. 19(4), s. 313–322. doi: 10.1111/j.1365-2400.2011.00843.x. Fulltekst i vitenarkiv
• Tuser, Michael; Balk, Helge; Mrkvička, Tomáš; Frouzova, Jaroslava; Čech, Martin & Muska, Milan [Vis alle 7 forfattere av denne artikkelen] (2011). Validation of current acoustic dead-zone estimation methods in lakes with strongly sloped bottoms. Limnology and Oceanography : Methods. 9, s. 507–514. doi: 10.4319/lom.2011.9.507. Fulltekst i vitenarkiv
• Schmidt, MB; Balk, Helge & Gassner, H (2009). Testing in situ avoidance reaction of vendace, Coregonus albula, in relation to continuous artificial light from stationary vertical split-beam echosounding. Fisheries Management and Ecology. ISSN 0969-997X. 16(5), s. 376–385. doi: 10.1111/j.1365-2400.2009.00689.x. Fulltekst i vitenarkiv
• Hohausova, E; Kubecka, J; Frouzova, J; Husak, S & Balk, Helge (2008). Experimental biomass assessment of three species of freshwater aquatic plants by horizontal acoustics. Journal of aquatic plant management. ISSN 0146-6623. 46. Fulltekst i vitenarkiv
• Rakowitz, G; Herold, W; Fesl, C; Keckeis, H; Kubecka, J & Balk, Helge (2008). Two methods to improve the accuracy of target-strength estimates for horizontal beaming. Fisheries Research. ISSN 0165-7836. 93(3), s. 324–331. doi: 10.1016/j.fishres.2008.06.005. Fulltekst i vitenarkiv
• Rudstam, Lars G.; Knudsen, Frank R.; Balk, Helge; Gal, Gideon; Boscarino, Brent T. & Axenrot, Thomas (2008). Acoustic characterization of Mysis relicta at multiple frequencies. Canadian Journal of Fisheries and Aquatic Sciences. ISSN 0706-652X. 65(12), s. 2769–2779. doi: 10.1139/Fo8-179. Fulltekst i vitenarkiv Vis sammendrag

  Abstract: 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.

• Jurvelius, Juha; Knudsen, Frank R.; Balk, Helge; Marjomäki, Timo J.; Peltonen, Heikki & Taskinen, Jouni [Vis alle 8 forfattere av denne artikkelen] (2008). Echo-sounding can discriminate between fish and macroinvertebrates in fresh water. Freshwater Biology. ISSN 0046-5070. 53. doi: 10.1111/j.1365-2427.2007.01944.x. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Peltonen, H & Balk, Helge (2005). The acoustic target strength of herring (Clupea harengus L.) in the northern Baltic Sea. ICES Journal of Marine Science. ISSN 1054-3139. 62, s. 803–808. Fulltekst i vitenarkiv
• Frouzova, J; Kubecka, J; Balk, Helge & Frouz, J (2005). Target strength of some European fish species and its dependence on fish body parameters. Fisheries Research. ISSN 0165-7836. 75, s. 86–96. Fulltekst i vitenarkiv
• Balk, Helge & Lindem, Torfinn (2000). Improved fish detection in data from split-beam sonar. Aquatic Living Resources. ISSN 0990-7440. 13, s. 297–303. Fulltekst i vitenarkiv Vis sammendrag

  Hydroacostic split-beam techniques have been applied to enumerate salmon migrating in the river Tana (northern Norway) during summer 1998 and 1999. Analysing data by single echo detection and tracking was difficult. Missing echoes in tracks from fish, combined with noise from the single echo detector was seen as a reason 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 fishing with few accepted single echoes.

Se alle arbeider i NVA

• Balk, Helge & Kubecka, Jan (2022). Methodology of monitoring fish communities in reservoirs and lakes. Biology Centre CAS. Institute of Hydrobiology. ISBN 9788086668956. 110 s. Fulltekst i vitenarkiv Vis sammendrag

  The book contains the overview and the description of the methods for the monitoring of qualitative and quantitative composition of the fish stock of lakes and reservoirs. It gives thorough description of the fieldwork and primary processing of information collected. Template field protocols to accompany every sample are attached. Use of multimesh gillnets, hydroacoustics, beach seining, long lines and electorshocking boat is described for the basic monitoring within the scope of the EU Water Framework Directive (2000/60/EC) monitoring of ecological quality or potential of lakes and reservoirs.

• Balk, Helge (2005). Lineær kretselektronikk. Helge Balk. 305 s. Fulltekst i vitenarkiv Vis sammendrag

  Boken omhandler Fourier analyse, Laplacetransformasjoner, AC-analyse og bodeplot,Operasjonsforsterkere, reguleringsteknikk, filtere og digital signal behandling. Boken er skrevet som lærebok til kurset FYS3220 og benyttet ved Fysisk institutt fra og med høsten 2005.

Se alle arbeider i NVA

• Balk, Helge (2022). Processing Acoustic Camera data with Sonar5-Pro. Fulltekst i vitenarkiv
• Balk, Helge (2022). Bredbånd verktøy i Sonar5-Pro. Fulltekst i vitenarkiv
• Kubecka, Jan; Balk, Helge; Roman, Baran; Boswell, Kevin; Froutsova, Jaroslava & Rudstam, Lars G. [Vis alle 7 forfattere av denne artikkelen] (2015). Advances in Shallow Water Hydroacoustics. Fulltekst i vitenarkiv
• Rakowitz, Georg; Tuser, Michal; Riha, Milan; Balk, Helge & Kubecka, Jan (2010). Use of high-frequency imaging sonar to observe fish behaviour with respect to an active surface trawl. Fulltekst i vitenarkiv
• Balk, Helge; Kubecka, Jan & Lindem, Torfinn (2010). Detection, tracking and sizing of fish in data from DIDSON multibeam sonars. Fulltekst i vitenarkiv
• Drastik, Vladislav; Hohausova, Eva; Kubecka, Jan; Balk, Helge & Froutsova, Jaroslava (2009). Classification of fish and plant echoes in acoustic surveys. Fulltekst i vitenarkiv Vis sammendrag

  Echoes from submerged plants could cause serious problems during acoustic estimates of fish in lakes and reservoirs. Defined experiments with known fish and submerged plants in front of horizontally oriented split beam echosounder SIMRAD EK 60, 120 kHz were carried out in calibration pond. Three widespread species of aquatic plants with different “stem/stalk” morphology: Myriophyllum spicatum, Polygonum amphibium and Potamgeton pectinatus were installed in front of the transducer. Single plants as well as patches of different sizes of each species were recorded. Several acoustic parameters were used to qualify the differences of fish and plant echoes. The echo duration of plant patches exceeded any values detected with fish while single plants were able to produce well defined single echoes. Moreover, larger fish could produce wider echoes than theoretical point scatterers. The overlap of pulse durations gives little hope for distinguishing between single stems and fish. Fish echoes had much less variable phase readings compared to even single plants. Study of fish and plant echoes allowed distinguishing of submerged plants from fish and provides more precise fish stock assessment.

• Balk, Helge & Lindem, Torfinn (2009). Target detection in data from splitbeam echosounders and multi beam sonar systems. Fulltekst i vitenarkiv
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2009). New Cubic Cross filter detector for multi beam data recorded with DIDSON acoustic camera. Fulltekst i vitenarkiv Vis sammendrag

  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 can see that the target actually is a swimming fish. Hence, a good target detector should not be based on individual frames, but on using series of pictures. The Crossfilter detector was originally designed for single, dual and split beam systems, to detect targets under very low signal to noise ratio conditions. In order to operate on multi beam data, we have added an extra dimension to the detecting filters. This resulted in a detector that takes into account both the time, range, and width dimensions in the data during the detection process. The talk will focus on the principles and construction of the detector, and discuss experience and test results.

• Kubecka, Jan; Frouzova, Jaroslava; Balk, Helge; Čech, Martin; Draštík, Vladislav & Prchalová, Marie (2009). REGRESSIONS FOR CONVERSION BETWEEN TARGET STRENGTH AND FISH LENGTH IN HORIZONTAL ACOUSTIC SURVEYS. Fulltekst i vitenarkiv Vis sammendrag

  Abstract: Both ways conversions between fish target strength (TS) and length (L) are extremely important for the accuracy of acoustic fisheries surveys. At present, no universal method is available for aspect corrections during mobile horizontal surveys. It is highly recommended to check common assumption of random fish aspect distribution by the series of fixed-location observations. For random aspect situations, the deconvolution of fish aspect in the beam can be used converting all TS to side aspect whose relationship to length is well known. For converting catch data to TS, the regression of TS of average horizontal backscattering strength (TSAS) on length can be applied successfully. The average regressions for common European species are being suggested for routine acoustic surveys in combination with direct pelagic fishing for the assessment of ecological status of lakes and reservoirs for EU Water framework directive 2000/60/EC. This approach reduces the resolution towards local fish size variations but simplifies the processing and provides much better definition of fish abundance.

• Froutzova, Jaroslava; Kubecka, Jan & Balk, Helge (2008). Fat echoes of oblique aspects : consequences for fish single echo detection. Fulltekst i vitenarkiv Vis sammendrag

  Echo duration is one of basic definitions of fish single target. Many of wild fish echoes fail to satisfy theoretical requirements based on pulse duration, bandwith and signal-to-noise ratio. The relationship between the echo duration and fish aspect was studied using rotating carousel by horizontally oriented split-beam echosounder, 120 kHz. While at side aspect most fish performed like point scatterer, the echo duration appear to increase nonlinearly at more diagonal aspects especially with large fish. Head and tail of larger fish may consist of several very close peaks, which may correspond to different body structures. Regressions between fish size, aspect and pulse durations are presented for common European freshwater species. The consequences for the settings of single echo detectors are discussed.

• Gjelland, Karl Øystein; Knudsen, Frank & Balk, Helge (2008). Bias in swimming speed estimates from splitbeam acoustics should be corrected for with smoothing. Fulltekst i vitenarkiv Vis sammendrag

  Swimming speed estimates based directly on splitbeam acoustic recordings are likely to be biased due to instability in single echo detections. Although this is well known, experiments addressing the problem have been lacking. This study quantifies the bias and evaluates the effect of smoothing fish tracks to obtain accurate swimming speed estimates. Field recordings of fish and controlled experiments with calibrations spheres were used to derive target speed detected with stationary split-beam echo sounding. The fish tracks were used to determine an appropriate smoothing window that conserved the shape of the swimming trajectories, while removing detection jitter. Speeds estimated from smoothed and nonsmoothed trajectories were then compared to investigate the effect of smoothing. The results from controlled experiments and field data showed that swimming speed estimated from non-smoothed fish tracks are likely to be heavily biased. The bias increased with range and was inversely related to the actual swimming speed. Smoothing reduced the variance of the estimates, and removed any effect of range and speed on the coefficient of variation. We conclude that fish tracks recorded with splitbeam echo sounders should be smoothed to yield reliable and precise swimming speed estimates.

• George, Rakowitz; Keckeis, Wolfgang; Herold, Wolfgang; Kubecka, Jan & Balk, Helge (2008). Evaluation of different fish size estimators in horizontal acoustic applications. Fulltekst i vitenarkiv Vis sammendrag

  The commonly applied mean TS from tracked fish obtained by horizontal echo sounding in shallow rivers is unreliable for fish sizing due to high within track TS variations and the way echoes are formed. In order to find a better size-estimator than the existing we tested mean-TS, 13° pan corrected mean-TS, 75th, 80th, 85th, 90th 95th TSpercentiles, the MET-MIT-statistic size estimation method and echo length with regard to real size data. Total-length estimates from the mean TS as well as from the 13° pan corrected mean TS underestimated the size of the fish. The TS-percentiles of the upper quartile make use of the side aspect detections within a fish-track, represented by the strongest signals. We found that the 95% TSpercentile was the best estimator for fish sizing.

• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2008). Split beam versus multi beam for monitoring migrating fish in rivers and shallow lakes. Fulltekst i vitenarkiv
• Balk, Helge & Lindem, Torfinn (2008). Analysis and visualization of data from DIDSON Acoustic camera. Fulltekst i vitenarkiv
• Kubecka, Jan; Balk, Helge; Drastic, Vladislav; Frouzova, Jaroslava; Gassner, Hubert & Herzig, Alois [Vis alle 10 forfattere av denne artikkelen] (2008). Large-scale acoustic surveys of an extremely shallow lake: Neusiedler See, Austria. Fulltekst i vitenarkiv Vis sammendrag

  Neusiedler See covers an area of 321 km2, the average and maximum depth is 1.2 m and 2 m respectively. Thermal stratification is absent at night. The lake was surveyed for 10 years by extensive mobile night surveys using 120 kHz split-beam echosounders with horizontally oriented elliptical transducer. Some surveys were accompanied by direct fish capture by 15 kW DC electroshocker, specially constructed wheeled trawl, push net for sampling larval and early juvenile fish and Nordic multimesh gillnets. The paper addresses the question of maximizing target detection in data characterized with low signal-to-noise ratio (cross filters), threshold strategies, avoidane of spherical spreading, multipathing, distinguishing macrophytes and fish and sizing of horizontally recorded fish. In calm conditions it was possible to detect fish down to young-of-the-year and the length frequency distribution was similar to the catch of direct methods combined. The fish presence detected by all methods was the highest in protected areas of the lake. CPUE of the gear show good correlation with acoustically detected densities showing that acoustic surveying can be useful even in extremely shallow habitats.

• Tuser, Michael; Balk, Helge; Froutsova, Jaroslava; Kubecka, Jan & Muska, Milan (2008). Suitability of DIDSON acoustic camera for quantitative fish stock assessment by mobile surveys. Fulltekst i vitenarkiv Vis sammendrag

  Open water of lakes and rivers could be surveyed by DIDSON (dual-frequency identification sonar) beaming sideways from a moving survey boat. Surface-oriented fish is then onsequently recorded in all DIDSON beams. The detection is considered successful if fish is seen and its size can be read reliably. For mobile horizontal surveying by DIDSON, vertical opening of the sampling volume is essential. The standard acoustic experiment with a “fish rotating carousel” was conducted with DIDSON in the Rimov reservoir (Czech Republic). Cyprinid fish (bream Abramis brama, roach Rutilus rutilus and carp Cyprinus carpio) of known size and aspect angle were deployed in known positions of high-frequency DIDSON fan of beams. Fish were stunned and measured before being mounted in a rotatable frame of the carousel. Both fixed and simulated mobile records were obtained in two ranges (approx. 6 and 9.5 meters). The following aspects of fish were sampled: 1) side, head and tail aspects in different off-axis positions achieved by yawing and then tilting of DIDSON transducer, 2) all aspects in the horizontal plane by turning fish at a constant speed, and 3) random aspects by turning fish in the horizontal plane together with tilting of DIDSON transducer. Vertical dimension of the sampling volume, where the fish presence, size and aspect could be recorded varied slightly with aspect and length of fish. This means that every target has own definition of sampling volume. New echocounting philosophy for mobile surveys by DIDSON will be presented.

• Rudstam, Lars G.; Gal, Gideon; Balk, Helge; Knudsen, Frank R.; Axenrot, Thomas & Boscarino, Brent (2007). Multiple acoustic frequencies increase our ability to discriminate between fish, mysids and zooplankton. Fulltekst i vitenarkiv Vis sammendrag

  We analyzed data from five acoustic frequencies (38, 120, 200, 430 and 715 kHz) and two manufacturers (Simrad and Biosonics) collected in Cayuga Lake, one of New York’s Finger Lakes with similar species composition as Lake Ontario, Michigan and Huron. Alewife dominated in gill net catches and Mysis relicta in vertical net hauls. The frequency response was dramatically different for fish and mysids indicating that frequency differencing will be useful for separating fish from mysids in the Great Lakes. We also applied filters to remove fish echoes and calculated mysid and fish densities separately using the filtered data. Empirical measurements of mysid target strength at different frequencies was compared with a published theoretical scattering model previously applied to mysid at 420 kHz by Gal et al. Our results suggest that addition of one additional frequency (38 kHz) would help separation of fish and mysids, and that higher frequencies may be useful for smaller zooplankton.

• Balk, Helge & Lindem, Torfinn (2007). Analysis of data from high frequency identification sonars combined with data from conventional echo-sounders. Fulltekst i vitenarkiv Vis sammendrag

  High frequent identification sonars have short range and produce a waste amount of data similar to videos. Analysis of these data is commonly done by watching the recorded videos. For surveys lasting for days or weeks this method is too time consuming. By generating traditional echograms from the sonar data and by combing the sonar data with data from long range split beam echo-sounders, we can improve the systems range and decrease the analysis time.

• Balk, Helge; Lindem, Torfinn; Kubecka, Jan & Frank, Knudsen (2007). Recent advances in solving signal-to-noise problems in fisheries acoustics. Fulltekst i vitenarkiv Vis sammendrag

  Signal is what we want while noise is disturbance that prevents us from getting it. Especially for horizontal application, surface, bottom, temp profile and unknown fish aspect can cause breakdown in the sonar equations and serious fish stock estimation errors. Noise may blur, hide or misshape the signal so that the echoes cannot be recognised by traditional detectors. Noise may also appear as disguised target echoes and bias the estimates. We will never be able to conquer the noise. However, new detectors and target noise separators, aspect de-convolution, multi beam and multi frequency will be discussed as ways to push the limits. Principle, later improvements and experience with the cross filter detector will be presented. We have measured and modelled sound propagation in lakes and rivers and will show how temperature gradients influence on the sound propagation and how this break down the assumptions behind the sonar equations and cause errors in the TS and Sv estimates. Experiences with multi beam and multi frequency are relatively new in fresh water and we will be shared as well.

• Balk, Helge (2006). Introduction, presentation and theory for multifrequency hydroacoustics. Fulltekst i vitenarkiv
• Balk, Helge (2006). Sound waves and fish abundance. Fulltekst i vitenarkiv
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2006). Enhenced Crossfilter detector for target detection in data from echosounders. Fulltekst i vitenarkiv Vis sammendrag

  Acoustic target detection is commonly carried out with single echo detectors based on echo length. These detectors test one ping at a time and look for echoes with an assumed correct echo length. Obtained echoes are filtered with a set of criteria such as phase stability and position in the beam before they are accepted. In shallow water, noise phenomena can distort echoes from fish and false fish echoes can be generated. This causes the echo length detector to produce fractionated tracks from fish surrounded by numerous noise detections. Echo length 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 been further improved by extracting detection power from the variance in the echogram

• Balk, Helge (2006). Sound waves and fish abundance. Fulltekst i vitenarkiv Vis sammendrag

  Our knowledge about how sound can be applied to see fish, has developed since Leonardo DaVinci discovered that sound waves could propagated through water. To day we have split beam, multi beam, multi-frequency echo sounders. Wide band echo sounders are about to be introduced. The talk will give a brief overview of the history, equipment and consideration about the future. What is possible and what is not possible? Split beam echo sounding is the most common method today and will be a main topic in the talk. Experience, analysis methods, and experimental results from rivers, lakes and ponds in Scandinavia and Europe will be presented.

• Balk, Helge (2005). New methods for fish and bottom detection in data from split beam echosounders. Fulltekst i vitenarkiv
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2005). New single echo detection methods for shallow water fishery acoustics. Fulltekst i vitenarkiv Vis sammendrag

  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 split beam echo sounders phase measurements.

• Balk, Helge (2005). A Ph. D. Class on Marine Bioacoustics: Hydroacoustics. Fulltekst i vitenarkiv
• Balk, Helge; Lindem, Torfinn & Kubecka, Jan (2005). New single echo detection methods for shallow water fisheries acoustics. Fulltekst i vitenarkiv Vis sammendrag

  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 split beam echo sounders phase measurements

• Balk, Helge (2004). Presentation of research results. Advanced analysis methods for fisheries acoustics. Detection methods for data characterised with low signal to noise ration. Fulltekst i vitenarkiv
• Balk, Helge (2004). Reflection and diffraction of sound waves in shallow water and how this influences the sonar equations. Fulltekst i vitenarkiv
• Balk, Helge & Lindem, Torfinn (2003). A new method for single target detection. Fulltekst i vitenarkiv Vis sammendrag

  We have experienced that fish detection in sonar-data from horizontally aligned transducers in shallow water can be difficult. Studying recorded material revealed that echoes from fish often took on high ping to ping variations in variables like pulse length, intensity, shape and phase. Common single-echo detection methods based on a set of echo criteria tended to overlook echoes from fish. At the same time echoes from unwanted targets and fluctuations in the background reverberation was accepted. Hence, tracking and fish counting became difficult. To overcome these problems, we have developed a detection method based on 2-dimensional low-pass filters. By adjusting the cut-off frequency in the time and range domain, noise can be removed, ping to ping variation in signals from fish reduced, and local background reverberation level can be found. By subtracting the echo intensity in the filtered echograms, traces from fish can be detected.

• Balk, Helge & Lindem, Torfinn (2003). Fish detection based on spectral differences in the echogram's range and temporal domain. Fulltekst i vitenarkiv
• Lindem, Torfinn; Balk, Helge; Vibeke, J. & Kjølerbakken, K. M. (2002). Measurements of acoustic sound fields in a shallow river. Fulltekst i vitenarkiv Vis sammendrag

  From earlier experiments, we have seen indications that the sound field from a horizontally aligned transducer in shallow water can differ from the expected field. If this is the case, this phenomenon can have an impact on the measured target strength from fish and on sample volume. Hence, it has been important to verify this by controlled experiments. We have measured the sound field produced by a Simrad ES120-4, 120kHz, 4x10 deg. split beam transducer in a shallow river. These measurements were made with a Reason hydrophone. Reflections from the bottom and surface made modifications to the transmitted pulse. In some positions they could even form separate pulses. These pulses were sufficiently separated in time to produce phantom echoes. Results from this experiment will be presented.

• Balk, Helge & Lindem, Torfinn (2002). Fish detection in rivers with split-beam sonars. Fulltekst i vitenarkiv Vis sammendrag

  Split beam echo sounders can monitor fish in shallow rivers. Positioned horizontally and normal to the river current at a suited place, the conical sound beam can cover major parts of the river cross-section and register passing fish. However, the nearby bottom and surface, air bubbles and debris makes the analysis of the echo sounder data difficult. A summary of our experiences from the last six years with data collection and analysis is presented. Methods such as single echo detections, tracking, cross filter detection and classification are discussed.

• Lindem, Torfinn; Balk, Helge & Knudsen, Frank R. (1999). Sound propagation through shallow water can make unexpected modification to fish echoes recorded with a horizontally looking split beam system. Fulltekst i vitenarkiv Vis sammendrag

  In several experiments with a 120kHz split beam system we have noticed that shallow water in some cases can make dramatic changes to the echo signal. Especially angular measurements in the vertical plane can be modified by surface and bottom boundaries. Erroneous measurements of position and target strength can be the result if beam mapping is not done in a proper way before recording of actual fish echoes. The paper will demonstrate some of these problems.

• Balk, Helge & Lindem, Torfinn (1999). Fish tracking in shallow water by image processing. Fulltekst i vitenarkiv Vis sammendrag

  Automatic fish-counting in data from Split-beam sonar is traditionally performed by a single echo detection (SED) process followed by some sort of neighborhood tracking. The SED process uses parameters such as echo pulse duration and phase deviation to remove echoes with non-valid angular measurements. The neighborhood tracking algorithm then combines echoes close in time and position. This method works fine on split-beam data from vertically mounted sonar in open water. However, with horizontally mounted sonars in shallow water rivers this method tends to fail. In order to use hydroacoustics in shallow river fish stock assessment, improvements on the counting method has to be done with methods from image processing.

• Lindem, Torfinn & Balk, Helge (1999). Problems related to Time Varied Gain (TVG) and phase measurements with elliptic split beam transducers in shallow water, - some observations. Fulltekst i vitenarkiv Vis sammendrag

  For several years split beam echo sounders have been used by biologists for counting fish in shallow water. Special elliptic transducers have been developed by manufactures like SIMRAD and HTI for use in horizontal applications in shallow lakes and rivers. For some time users have reported ¿strange behaviour¿ of their systems in some locations. During our ongoing project for new counting methods for fish in rivers we have experienced these effects and can demonstrate how boundary effects from surface and bottom can give unexpected modifications to echo signals received with split beam systems.

• Balk, Helge & Lindem, Torfinn (1998). Hydroacoustic fish counting in rivers and shallow waters, with focus on problems related to tracking in horizontal scanning sonars. Fulltekst i vitenarkiv
• Järnegren, Johanna; Balk, Helge; Uglem, Ingebrigt & Forseth, Torbjørn (2011). Telling av oppvandrende fisk i Mandalselva ved bruk av DIDSON. En pilotstudie. Norsk institutt for naturforskning (NINA). ISSN 9788242622150. Fulltekst i vitenarkiv Vis sammendrag

  Moderne forvaltning av norske laksebestander basert på gytebestandsmål forutsetter god og pålitelig kunnskap om bestandsstørrelse og/eller beskatningsrater. Antall oppvandrende laks kan estimeres enten indirekte gjennom fangststatistikk og beskatningsrater eller direkte ved bruk av ulike fisketellesystemer. ”Dual frequency identification sonar”- systemet (DIDSON) representerer en ny type identifikasjonssonarer som under optimale forhold kan generere undervannsbilder med tilnærmet videokvalitet. Sammenliknet med videosystemer gir DIDSON fordeler ved at den ikke er avhengig av eksternt lys som dagslys eller kunstig lys, samt at den også fungerer i grumset vann. Optimal bruk av DIDSON er avhenging av observasjonslokaliteten. Hensikten med dette prosjektet var å teste hvorvidt DIDSON kan brukes til fisketelling i Mandalselva og i andre elver av tilsvarende størrelse. DIDSON kan brukes til å måle antall og størrelse på oppvandrende laksefisk i elver hvor denne typen informasjon tidligere har vært vanskelig eller umulig å fremskaffe. DIDSON er brukt for å telle oppvandrende laks blant annet i Canada og England, men systemet er hittil ikke testet under norske forhold. Hvorvidt DIDSON-systemet er anvendbart til rutinemessig telling og måling av oppvandrende laks i norske elver er i hovedsakelig avhengig av to forhold; for det første at systemet kan identifisere oppvandrende fisk på en pålitelig måte og deretter at analysene er kostnadseffektive. Dette er igjen avhengig av optimale observasjonslokaliteter og i hvilken grad analysene kan automatiseres. Lokaliteten som ble vurdert som best egnet til denne studien ligger nedenfor Nødingfossen og data ble samlet inn mellom 11. juni og 20. oktober 2009. En DIDSON-LR enhet (Long Range) med en pan/tilt rotor ble satt ut i elven slik at avstanden fra sonarens front til nærmeste elvebredde var ca 5 meter. Sonaren ble plassert slik at man fikk det beste bildet på skjermen med maksimal rekkevidde. Alle data ble analysert med programvaren Sonar5-Pro (S5), som ble modifisert for å kunne håndtere de store datamengdene som ble samlet inn. Automatisk telling ble vurdert og mange metoder ble testet og forkastet som ikke tilstrekkelig nøyaktige. Metodene detekterte for mange ikke-ønskede mål og overså for mange fisk. Den eneste metoden som fungerte for de innsamlede dataene var manuell klikktelling. Årsaken til dette var mye støy forårsaket av ekko fra bunn, overflate, luftbobler, turbulens og ikke minst den lange avstanden over elven. Med manuell analyse ble ett døgns opptak opparbeidet i løpet av ca 60 min effektiv analysetid. Forholden var ikke gode nok til å måle størrelse på fisk. Totalt ble 4471 fisk talt mellom 11. juni og 20. oktober. Kompensert for tapt opptakstid, areal med manglende dekning samt fisk som vandret forbi sonaren flere ganger estimerte vi en minimum oppvandring på 6451 fisk. Ut fra registrerte fangster gir dette et estimat for maksimal beskatning på 35 %. Konklusjonen er at DIDSON kan brukes for rutinemessig telling. Automatisk telling av oppvandrende fisk forutsetter at lokaliteten tilpasses slik at tilstrekkelig gode data kan samles inn.

• Balk, Helge (2001). Development of hydroacoustic methods for fish detection in shallow water. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Management of salmon stocks from various regions is of great importance. Optimal management can only be achieved with knowledge of the size and the changes in the stocks. Hence, monitoring methods are important. In marine fisheries, split-beam echo sounders are frequently applied to monitor fish. In recent years this hydroacoustic fish detection method has been applied in many rivers. Aligning one or a few transducers horizontally provides a simple way to monitor major parts of a river's cross section. However, the horizontal shallow water application introduces new problems related to the acquisition, analysis, and interpretation of the sonar data. Fieldwork has been carried out in lakes and rivers, and a systematic analysis of these problems has been made. From the experience and the studies of the recorded material, new methods have been developed. Signal processing based on image analysis and classification theory is introduced, and promising results have been achieved.

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