Dual-frequency Identification Sonar Research Articles

Environmental Influences on Fish Migration in a Hydropeaking River

Fish have evolved traits and life history characteristics that enable them to survive, exploit and depend on the flow regime of rivers, particularly the timing and predictability of flows for spawning and rearing their young. It is unclear to what degree pulsed flows from hydropower facilities and other environmental variables influence migratory behaviours. We used Dual Frequency Identification Sonar in the Michipicoten River, Canada, to address the relationship between fish migration and environmental factors with a focus on flow magnitude and fluctuation. In both 2007 and 2009, the peak of the Chinook (Oncorhynchus tshawytscha) and pink salmon (Oncorhynchus gorbuscha) migration occurred on 8 September. Mean water temperature on this date was 18 °C and precipitously dropped afterwards. The photoperiod was roughly 12-h long with sunrise at 700 h and sunset at 2000 h. Most fishes moved upstream during the hours of darkness between 2000 and 600 h. The lowest counts of fish occurred from noon to just before sunset, whereas highest counts commonly occurred from 1 to 2 h after sunset. Fish moved upstream during all magnitudes of flow; however, there was a slight preference for larger flows in 2007 but not in 2009. Changes in flow magnitude occurred both during the day and night, with flows typically increasing during the day to meet electrical demand and decreasing at night. Most fishes moved upstream during periods of little to no change in flow. High flows and changing flows may deter salmon from moving up the Michipicoten River but not likely in a significant manner to cause energetic stress or harm. Other adverse effects of pulsed flows, however, must still be considered for spawning, hatching and rearing success. Copyright © 2014 John Wiley & Sons, Ltd.

Gray Seal Behavior in a Fish Weir Observed Using Dual-Frequency Identification Sonar

AbstractMarine mammal depredation can have significant negative effects on the performance of fixed fishing gears. Gray seals (Halichoerus grypus grypus) have been observed feeding on fish weir catches in Nantucket Sound, and remnants of longfin inshore squid (Doryteuthis pealeii) and finfish in the nets are attributable to seal depredation. A dual-frequency identification sonar (DIDSON) deployed in a weir to monitor squid and fish occurrence captured observations of seal presence and behavior. Continuous recordings of 24+ h were subsampled at multiple intervals to optimize the likelihood of seal detection while minimizing processing time. Targets detected and identified to species included squid, bluefish (Pomatomus saltatrix), and gray seals. Seal occurrence in the weir was observed primarily at night. The presence and behavior of seals may negatively affect the efficiency of the weir due to disruption of the passage of schooling squid and finfish into the catch chamber. Behavioral observations collected using DIDSON can aid in the design of fishing gear modifications to reduce seal depredation.

Application of Mobile Dual-frequency Identification Sonar (DIDSON) to Fish in Estuarine Habitats

Dual-frequency identification sonar (DIDSON) offers important advantages over other sampling tools for observing pelagic and benthic fishes in situ. Because it relies on sound, DIDSON can detect fish in a non-destructive and non-intrusive manner. In our unique application, the equipment's small size and low power requirements allow deployment from a kayak for increased maneuverability in complex habitats. Characteristics that typify echograms of different fishes can be extracted using multivariate ordination techniques, such as principal components analysis (PCA), with in situ groundtruthing. Here we present reference images, techniques, and human-observer-error estimates from DIDSON application. Together, these approaches enhance our ability to sample fishes and even observe certain behaviors in complex, turbid environments during a full diel cycle.

Deep sea organisms density estimation using a Dual Frequency Identification Sonar (DIDSON)

Estimating the density of organism living in deep sea scatting layers is of key importance for understanding the biomass in the mesopelagic layers. Scientific echosounders are routinely used for this task; however, new imaging sonar technologies pose the opportunity for estimating density of organism, as well as identification at smaller scales. During the 2013 NOAA KONA Integrated Ecosystem, a Dual Frequency Identification Sonar (DIDSON) (SoundMetrics Inc.) was used to estimate the density, length of organisms in the deep sea scattering layers during nighttime and daytime along the KONA coast of the island of Hawaii. At each station, an EK60 38 kHz echosounder was used to find the depth of the deep sea scattering layers, and the DIDSON was lower into the layer (or layers if two were present) (about 500 and 600 m), and underneath the deeper layer (about 800 m). A total of 4621 organisms were counted and sized. We estimated densities ranging from 6 to 1 organism/m3. Density shows some variation between locations and depth and organism as big as 3 m were sighted.

Multibeam sonar (DIDSON) assessment of American shad (Alosa sapidissima) approaching a hydroelectric dam

We investigated the fish community approaching the Veazie Dam on the Penobscot River, Maine, prior to implementation of a major dam removal and river restoration project. Multibeam sonar (dual-frequency identification sonar, DIDSON) surveys were conducted continuously at the fishway entrance from May to July in 2011. A 5% subsample of DIDSON data contained 43 793 fish targets, the majority of which were of Excellent (15.7%) or Good (73.01%) observation quality. Excellent quality DIDSON targets (n = 6876) were apportioned by species using a Bayesian mixture model based on four known fork length distributions (river herring (alewife, Alosa psuedoharengus, and blueback herring, Alosa aestivalis), American shad, Alosa sapidissima) and two size classes (one sea-winter and multi-sea-winter) of Atlantic salmon (Salmo salar). 76.2% of targets were assigned to the American shad distribution; Atlantic salmon accounted for 15.64%, and river herring 8.16% of observed targets. Shad-sized (99.0%) and salmon-sized (99.3%) targets approached the fishway almost exclusively during the day, whereas river herring-sized targets were observed both during the day (51.1%) and at night (48.9%). This approach demonstrates how multibeam sonar imaging can be used to evaluate community composition and species-specific movement patterns in systems where there is little overlap in the length distributions of target species.

Observer bias and subsampling efficiencies for estimating the number of migrating fish in rivers using Dual-frequency IDentification SONar (DIDSON)

Fixed-location, side-looking, multibeam, sonar techniques offer a practical approach to estimate the numbers of migrating fish in rivers that are too large or occluded for traditional sampling methods, such as weir trapping, visual observation techniques, and netting. While this technology has been used to enumerate salmonid escapement in coastal river systems of western North America, little use and evaluation has occurred in inland waters such as the Great Lakes, where rivers and runs of fish are considerably smaller than those along the Pacific coast. We use a “Dual-frequency IDentification SONar” (“DIDSON”) imaging sonar system to investigate the error and variability among nine people performing fish counts. There was no significant difference found among observers’ estimates of fish abundance per DIDSON file; however, the total count of all fish differed from the benchmark value by as much as 26%. Post-processing simple fish counts from DIDSON raw data is labour-intensive and costly. Three subsampling methods of fish passage estimations were developed and evaluated for their accuracy and precision for daily and seasonal time frames. The random and systematic subsampling methods had similar seasonal and daily accuracy and precision with few exceptions. Automation-assisted counting was much more accurate and efficient for seasonal estimates. A ratio of approximately 2:1 was found for the automated to manual fish counts and this varied little among years. The DIDSON multibeam sonar unit is useful in estimating potamodromous fish migrations for large tributaries of the Great Lakes. DIDSON image processing costs can be minimized through suitable subsampling approaches. The automation-assisted method is the most cost-effective means of estimating moderate levels of fish passage over longer study periods. Multiple individuals can be used interchangeably for the manual post-processing of DIDSON data.

The use of acoustic cameras in shallow waters: new hydroacoustic tools for monitoring migratory fish population. A review of DIDSON technology

AbstractEuropean Union legislation, through the Water Framework Directive (2000/60/EC), as well as national legislation, such as the ‘Grenelle Environnement’ (2007) in France, requires restoration of ecological connectivity in streams to improve free circulation of migratory fish. Different methods (e.g. capture by trap or net, telemetry, hydroacoustics) are used to evaluate the efficiency of fish passes to estimate the migratory species abundance and analyse changes in their within‐river distributions. Among these methods, hydroacoustics is non‐intrusive, allowing long‐term observation and description of fish populations based on physical properties of sound in water. However, the main limit to hydroacoustic tools is their difficulty in identifying species. Initially designed for military purposes, dual‐frequency identification sonar (DIDSON) has been used in environmental management for a decade. This acoustic camera uses higher frequencies and more sub‐beams than common hydroacoustic tools, which improves image resolution and then enables observation of fish morphology and swimming behaviour. The ability to subtract static echoes from echograms and directly measure fish length improve the species‐identification process. However, some limits have been identified, such as automatic dataset recording and the low range of the detection beam, which decreases accuracy, but efficient tools are now being developed to improve the accuracy of data recording (morphology, species identification, direction and speed). The new technological properties of acoustic cameras, such as the video‐like visualization of the data, have greatly improved monitoring of diadromous fish populations (abundance, distribution and behaviour), helping river and fisheries managers and researchers in making decisions.

Use of Dual frequency IDentification SONar (DIDSON) in determination of the spatial distribution and biomass of the moon jellyfish Aurelia aurita s.l.

Use of Dual frequency IDentification SONar (DIDSON) in determination of the spatial distribution and biomass of the moon jellyfish <i>Aurelia aurita</i> s.l.

Deep pools of the Danube River: ecological function or turbulent sink?

Danube main channel deep pools are in- stream habitats of high ecological relevance. We used dual-frequency identification sonar to investigate seasonal and diel fish abundance in two hydrodynam- ically different deep pools in the main channel of the Danube River in 2008. In general, fish of different species were present in both deep pools throughout the year, irrespective of water level. High fish abundance was recorded during autumn and winter, low abun- dance during spring and summer. During low dis- charge and low temperature and particularly during low flow velocity, low kinetic energy and low shear stress, high fish abundance in densely packed aggre- gations were observed, a clear indication for the refuge capacity of deep pools. In contrast, during turbulent hydraulic conditions with high flow velocity, high turbulent kinetic energy and high shear stress, low fish abundance occurred, mainly consisting of single fish, small fish shoals and benthic fish specimens. Further- more, we could reveal that calm as well as turbulent pools fulfil the requirements as refuge and feeding habitat of eurytopic, piscivorous catfish throughout the year. The application of a multiple linear model based on the hydrological parameters discharge and water temperature as well as the hydraulic parameters flow velocity, turbulent kinetic energy and shear stress provided highly significant evidence between the observed and the expected seasonal fish abundance in the deep pools (R 2 = 0.77, P \ 0.001). For the first time these results make clear that the main channel deep pools are important refuge and resource habitats of different species of the Danube fish community. Deep pools are key habitats for the in-stream fish community of the Danube River.

Avoidance reactions of fish in the trawl mouth opening in a shallow and turbid lake at night

Fish avoidance behaviour in the trawl mouth at night was investigated in the extremely shallow and turbid Lake Neusiedl in Austria. To evaluate fish reactions, a fixed frame benthic trawl with three electricity modes (without electricity, with continuous electricity and with interrupted electricity) was used and the captured fish abundances, biomasses and size structures were compared between modes. Simultaneously, the dual-frequency identification sonar (DIDSON) monitored fish abundance and size in front of and beside the trawl mouth during all tows. White bream (Blicca bjoerkna), bleak (Alburnus alburnus) and razor fish (Pelecus cultratus) were dominant fish species in the trawl catches. We did not find differences in fish abundance or biomass when the tows with different electricity modes were compared. The length frequency distribution of fish was not significantly different between modes, but trawls with the two electrified modes contained a higher proportion of fish larger than 130mm than trawls with the non-electrified mode. Additionally, the DIDSON recordings did not display any significant differences in abundance, length frequency distribution and from length–weight relationships calculated biomass between the electrical modes. Not even the two avoidance behaviour categories used as indicators of trawling error displayed significant differences between avoidance and biomass. Our results indicate that based on trawling abundance and biomass comparisons, and supported by observations by DIDSON, all electrical modes were similarly effective for all size groups of fish. The study found minimal avoidance reactions by the dominant fish species in the trawl mouth opening when a relatively small fixed frame trawl was used to sample fish in a shallow and turbid lake at night.

Quantification of whooper swan damage to lotus habitats using high-resolution acoustic imaging sonar in Lake Izunuma, Japan

Conventional survey methods (e.g., surface surveys using optical video cameras and divers) have been widely used to detect aquatic plants and estimate their densities. However, these methods are affected by the transparency of water and observational skills of the divers and usually require considerable amounts of time for high-resolution sampling. We developed a new measurement system that combines a high-resolution acoustic imaging sonar, i.e., a Dual-frequency IDentification SONar (DIDSON) with a 1° concentrator lens, motion sensors, and GPS (global positioning system) receivers to quantify the underwater status and topography of a lake bottom effectively without the transparency of the water affecting the results. High-resolution spatial acoustic imaging was used for 3D visualization of the underwater status and bottom topography of a lake, and the numbers and depths of lotuses and bottom holes were estimated using the image processing program developed for this study. Our study revealed one specific aspect of the interaction between whooper swans foraging and the density of lotus (Nelumbo nucifera Gaertn.). The threshold water depth (approximately 1.3m) between high- and low-density areas was related to the body length of whooper swans.

Walleye pollock (Theragra chalcogramma) behavior in midwater trawls

Trawls are standard tools for surveying fisheries resources, yet they are selective in what they retain, and thus provide potentially misleading information about fish populations. In order to evaluate the potential for selective retention in a midwater survey trawl used in conjunction with acoustic surveys of walleye pollock, fish behavior was examined using an integrated approach of optical, acoustic and recapture net methods. A stereo-camera system was used to provide length, position and orientation information, and a dual-frequency identification sonar was used to track fish targets in the trawl. Fish escaping the trawl were sampled using recapture, or pocket, nets mounted to the outside of the trawl. Most fish were found to be oriented along the main trawl axis, facing the forward trawl opening. Nearest distance to the trawl panel did not appear to be length-dependent, however, at night when ambient light levels were lower, fish maintained less distance to the trawl panel compared to daytime observations. Consequently, significantly more fish escapes occurred at lower light levels. Trajectories of fish escaping the trawl were highly variable compared with fish that herded into the net, or those whose retention state was unknown. Greatest escapement into pocket nets was observed from the bottom panel of the trawl at night. These findings suggest that survey trawl samples will be less biased due to selectivity when trawls are conducted during the day.

Fine-scale distribution of pelagic fishes relative to a large urban pier

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 476:185-198 (2013) - DOI: https://doi.org/10.3354/meps10151 Fine-scale distribution of pelagic fishes relative to a large urban pier Kenneth W. Able1,*, Thomas M. Grothues1, Iris M. Kemp2 1Rutgers University Marine Field Station, Tuckerton, New Jersey 08087, USA 2University of Washington, School of Aquatic & Fishery Sciences, Seattle, Washington 98195-5020, USA *Email: able@marine.rutgers.edu ABSTRACT: Intense shading under large piers is known to negatively affect benthic fishes. However, effects on pelagic fishes are poorly known. We employed the equivalent of acoustic video, dual frequency identification sonar (DIDSON), under a kayak to evaluate the response of pelagic fishes at a large (351 × 255 m) urban pier (Pier 40) in Hudson River. A repeated measures design (322 occupations) sampled 3 transects each across both the northern and southern open water—pier edge—under pier continuum during the day and night from June 2009 to September 2010. Over 22000 individual fish, ranging from small schooling forage species (e.g. Anchoa mitchilli, Menidia menidia) to large predators (Morone saxatilis, Pomatomus saltatrix) were detected with DIDSON and verified with conventional sampling nets. Small (<250 mm) schooling pelagic fishes avoided areas under the pier where light is dramatically reduced during both day and night (when municipal lights illuminate the water away from the pier). Less abundant large (>250 mm) predatory fish responded somewhat differently from small schooling fish in that large predatory fish were slightly more abundant under the pier than in open water, but only from the edge extending to 5 m under the pier where there was still some light. Beyond that, abundance declined sharply. Together, these observations indicate that areas under large piers are suboptimal habitats for many of the abundant pelagic fishes, a pattern similar to that for benthic fishes. KEY WORDS: Pier · Shade · Edge · Pelagic fish · Distribution · DIDSON Full text in pdf format PreviousNextCite this article as: Able KW, Grothues TM, Kemp IM (2013) Fine-scale distribution of pelagic fishes relative to a large urban pier. Mar Ecol Prog Ser 476:185-198. https://doi.org/10.3354/meps10151 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 476. Online publication date: February 27, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Silver eel (Anguilla anguilla) population dynamics and production in the River Shannon, Ireland

AbstractThe European eel (Anguilla anguilla) is in decline throughout its distribution and accurate regional information, particularly on the silver eel life‐stage, is an essential component of stock recovery plans. Therefore, silver eel population dynamics and production were investigated on the Irish River Shannon. Size frequency analysis during the annual seaward migration showed that between 2008 and 2011, seasonal trends in both sex ratio and female size occurred. Catch analysis and mark‐recapture experiments were undertaken during the same period to determine silver eel production. Due to extreme discharge in 2009, no catch data was available for 17 nights, and a novel protocol was developed using a Dual Frequency Identification Sonar (DIDSON) to quantify silver eel migration. In 2008, silver eel production was estimated at 57.3 t (1.35 kg·ha−1), but an additional 18.3 t of potential migrants were estimated to have been removed by the summer yellow eel fishery. In 2009, following closure of the yellow eel fishery, silver eel production increased to 68.6 t (1.62 kg·ha−1), before declining to 62.7 t (1.47 kg·ha−1) in 2010 and 61.6 t (1.45 kg·ha−1) in 2011. Modelling, based on long‐term stocking data (1959–2011) and retrospectively estimated silver eel production (1992–2007) suggests that despite commercial fishery closure on the river, the decline in River Shannon silver eel production will continue for at least the coming decade.

3D-view Generation and Species Classification of Aquatic Plants Using Acoustic Images

In this study, we used a new integrated measurement system that combines the acoustic imaging sonar of DIDSON (Dual-frequency IDentification SONar) with concentrator lenses, motion sensors, and GPS receivers to find and identify different species of aquatic plants. Two types of experiments were performed at two lakes in Japan: Lake Yamanaka and Lake Yunoko. In the first experiment at Lake Yamanaka, the image was captured with a one-degree concentrator lens. Multi-beam image processing was used to generate 3-D images of aquatic plants. The lens used in this experiment concentrated the vertical beam width within 1 degree. The second experiment at Lake Yunoko DIDSON with a 3-degree concentrator lens was applied, and histogram and spatial spectrum analyses were performed for plant species classification. Three species of aquatic plants, Myriophyllum spicatum, Chara globularis, and Elodea nuttallii, were classified by the parameters obtained from the current methods. The ability to visualize the features of each aquatic plant for species classification, e.g., leaf and branch dimensions, was attributed to spatial spectrum and scattering analyses of the acoustic images. The high spatial resolution of the integrated DIDSON measurement system will contribute protection of endangered species in rapidly changing underwater environment.

Use of High-Resolution DIDSON Sonar to Quantify Attributes of Predation at Ecologically Relevant Space and Time Scales

AbstractPredator-prey interactions of large vagile fishes are difficult to study in the ocean due to limitations in the space and time requirements for observations. Small-scale direct underwater observations by divers (ca. &lt;10 m radius) and large-scale hydroacoustic surveys (10 s m2 to 100 s km2) are traditional approaches for surveying fish. However, large piscivorous predators identify and attack prey at the scale of meters to tens of meters. Dual-Frequency Identification Sonar (or DIDSON) is a high-resolution acoustic camera operating in the MHz range that provides detailed continuous video-like imaging of objects up to a range of 30 m. This technology can be used to observe predator-prey interactions at ecologically relevant space and time scales often missed by traditional methods. Here we establish an approach for quantifying predation-related behaviors from DIDSON records. Metrics related to predator and prey group size, prey responses to predation, predation rate, predator strategies, and the nonrandom use of landscape features by both predator and prey are described. In addition, relationships between patterns in these attributes are tested and issues regarding sampling strategies for future studies are discussed. We suggest that approaches combining direct visual observation and acoustic sampling at multiple scales are required to quantify variation in these relationships across underwater landscapes.

Reliability of Fish Size Estimates Obtained From Multibeam Imaging Sonar

Abstract Multibeam imaging sonars have considerable potential for use in fisheries surveys because the video-like images are easy to interpret, and they contain information about fish size, shape, and swimming behavior, as well as characteristics of occupied habitats. We examined images obtained using a dual-frequency identification sonar (DIDSON) multibeam sonar for Atlantic sturgeon Acipenser oxyrinchus oxyrinchus, striped bass Morone saxatilis, white perch M. americana, and channel catfish Ictalurus punctatus of known size (20–141 cm) to determine the reliability of length estimates. For ranges up to 11 m, percent measurement error (sonar estimate – total length)/total length × 100 varied by species but was not related to the fish's range or aspect angle (orientation relative to the sonar beam). Least-square mean percent error was significantly different from 0.0 for Atlantic sturgeon (x̄ = −8.34, SE = 2.39) and white perch (x̄ = 14.48, SE = 3.99) but not striped bass (x̄ = 3.71, SE = 2.58) or channel catfish (x̄ = 3.97, SE = 5.16). Underestimating lengths of Atlantic sturgeon may be due to difficulty in detecting the snout or the longer dorsal lobe of the heterocercal tail. White perch was the smallest species tested, and it had the largest percent measurement errors (both positive and negative) and the lowest percentage of images classified as good or acceptable. Automated length estimates for the four species using Echoview software varied with position in the view-field. Estimates tended to be low at more extreme azimuthal angles (fish's angle off-axis within the view-field), but mean and maximum estimates were highly correlated with total length. Software estimates also were biased by fish images partially outside the view-field and when acoustic crosstalk occurred (when a fish perpendicular to the sonar and at relatively close range is detected in the side lobes of adjacent beams). These sources of bias are apparent when files are processed manually and can be filtered out when producing automated software estimates. Multibeam sonar estimates of fish size should be useful for research and management if these potential sources of bias and imprecision are addressed.

Estimating Escapement for a Low‐Abundance Steelhead Population Using Dual‐Frequency Identification Sonar (DIDSON)

AbstractEstimating the escapement of small populations of steelhead Oncorhynchus mykiss (ocean‐migrating rainbow trout) is challenging and requires innovative methods. Difficulties arise from rare and episodic occurrence of individuals, high turbidity levels during migration, and the need to minimize jeopardy to the fish, which have led to a lack of population estimates, especially in California. Here we show that dual‐frequency identification sonar (DIDSON) can be used to produce escapement estimates for a small steelhead population in Scott Creek, Santa Cruz County, California, with virtually no impact on the fish. The DIDSON uses sound to form near‐video quality images and passively monitors fish without the need to handle them or constrict passage. We deployed a DIDSON and recorded steelhead passage over three spawning seasons (2008–2010). We used a decision support tool to analyze DIDSON images and compared the resulting estimates (153, 57, and 84) with raw weir counts (50, 23, and 40), mark–recapture estimates (293 ± 9, 126 ± 12, and 109 ± 34) generated over the entire migration period, and adjusted mark–recapture estimates (201, 74, and 85), which coincided with the period of DIDSON deployment. The DIDSON and weir estimates were restricted to a smaller sampling window due to the installation of downstream migrant traps causing increased incidences of milling fish that interfered with the DIDSON results late in the migration season. The DIDSON estimates were two to three times higher than weir estimates and 23% to 55% less than the full‐season mark–recapture estimates. The adjusted mark–recapture estimates followed the same trends as the full‐season mark–recapture estimates and were correlated with the DIDSON estimates. We conclude that DIDSON is an effective tool to generate steelhead escapement estimates, but it is important to collect data over the entire migration season and to consider fish behavior and potential species identification issues during analysis.Received October 6, 2011; accepted May 21, 2012

Spatio-temporal distribution and seasonal population dynamics of the jellyfish Aurelia aurita s.l. studied with Dual-frequency IDentification SONar (DIDSON)

Deployment of a Dual-frequency IDentification SONar (DIDSON) revealed the seasonal variations in spatial distribution and abundance of medusae of the common jellyfish, Aurelia aurita s.l. (detectable bell diameters: ca. 5 cm) in a shallow (max. depth: 6 m) Japanese brackish-water lake in between October 2008 and October 2009. The medusae were usually in patchy aggregations, more prominently so during warm seasons. Integration of the DIDSON-detected medusae with smaller ones estimated from the bell diameter composition in net hauls enabled us to study their seasonal population dynamics. Abundance was highest in June, due to recruitment, and declined to a relatively stable level in August and thereafter. The population carbon biomass was highest in October due to somatic growth of individual medusae. The annual mean biomass (63.8 mg C m) was much higher than that of mesozooplankton (26.0 mg C m), suggesting a substantial top-down impact. The DIDSON is an effective tool for studies of the spatial distribution and population dynamics of large jellyfish species like A. aurita. Concomitant net sampling to determine the bell diameter composition is required to offset the geometric limitations of this instrument.

Can acoustic shadows identify fish species? A novel application of imaging sonar data

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 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.