The new fisheries multibeam echosounder ME70: description and expected contribution to fisheries research
1 Ifremer, Département EMH, rue de l'Ile d'Yeu, BP 21105, 44311 Nantes Cedex 03, France
2 Ifremer, Département NSE, Plouzané, France
Correspondence to V. M. Trenkel: tel: +33 240 374000; fax +33 240 374075; e-mail: vtrenkel{at}ifremer.fr.
Trenkel, V. M., Mazauric, V., and Berger, L. 2008. The new fisheries multibeam echosounder ME70: description and expected contribution to fisheries research. – ICES Journal of Marine Science, 65: 645–655.Recently, Simrad in collaboration with Ifremer developed a calibrated, multibeam, vertical echosounder (ME70) for fisheries research. We describe its capabilities and technical limitations. The ME70 has up to 45 beams with distinct frequencies in the range 70–120 kHz, spanning at most 150°. All beams are stabilized in vessel roll and pitch. It has reduced side-lobe levels, up to –70 dB (two-way) instead of the –25 dB (one-way) of conventional systems. We outline research areas for which the ME70 might provide new types of information and hence lead to novel insights. We illustrate the potential contributions with datasets collected in the English Channel and on the continental-shelf break of the Bay of Biscay. Finally, future research and developments using the new system are outlined.
Keywords: abundance, acoustics, behaviour, three-dimensional
Received 25 July 2007; accepted 25 February 2008.
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Introduction |
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 Top Introduction Characteristics of the ME70...Potential research applications...References |
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The use of acoustic methods to estimate fish abundance has a history of >50 years (see review in Fernandes et al., 2002, for their use within the ICES Area). Since the early 1950s, substantial progress has been made at all stages of the process, from measurement instruments to signal extraction and interpretation (Simmonds and MacLennan, 2005). Vertically orientated, single-beam echosounders operating at frequencies ranging from 38 to 200 kHz are most commonly used. Quantitative estimates were made possible by the introduction of standardized calibration protocols (Foote et al., 1987).
Several physical factors influence the utility of acoustic methods for abundance estimation. Beam width in combination with depth, and also pulse duration and bottom topography, determine the extent of the dead zone near the seabed for which no information is available (Ona and Mitson, 1996). The acoustic observations of different species are influenced to varying degrees by this dead zone. For example, a large proportion of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), and redfish (Sebastes spp.) is thought to reside in it for most of the time (Aglen et al., 1999). The detection range for single targets depends on echosounder performance, fish target strength (TS), and ambient- and vessel-noise levels, so may differ between vessels (Mitson and Knudsen, 2003). As beam width determines the diameter of the volume or surface insonified (sampled) at a given depth, for vertical echosounders the backscattering strengths from fish schools smaller than this diameter will be underestimated and their dimensions overestimated; therefore, the bias in evaluating small schools increases with depth (Diner, 2001, 2007). For oblique beams, the backscattering strength decreases with increasing angle (Melvin et al., 2003; Zedel et al., 2005).
The acoustic-backscattering properties of individual fish result from the interplay between physical and biological factors. They are a function of signal frequency and depend not only on the morphology of the species, its schooling behaviour, and an individual’s orientation and body shape and size (Misund, 1997), but also physiological state (Ona, 2003). Fish reactions to noise and light emitted by a surveying vessel bias abundance estimates (Olsen et al., 1983; Misund, 1997), probably because of changes in fish tilt angle as they dive in front of a survey vessel (Gerlotto and Fréon, 1992; Handegard and Tjøstheim, 2005; Cutter and Demer, 2007).
In addition to the standard method of abundance and biomass estimation by echo-integration, in combination with fishing hauls carried out for species and size identification (see description in Simmonds and MacLennan, 2005), acoustic information has also been used to study schooling behaviour (ICES, 2000) and the potential impact of fishing on spatial distribution patterns (Wilson et al., 2003). For this, standard protocols for extracting morphological school descriptors from vertical echosounders have been proposed (ICES, 2000; Reid et al., 2000), providing school characteristics in two dimensions (depth and alongship), because no information perpendicular to the survey track is available. Therefore, neither horizontal (athwartship) avoidance reactions nor their expected impact on school morphology can be studied using single-beam echosounders. To overcome this limitation, horizontal and vertical scanning multibeam sonars have been employed (Misund and Aglen, 1992; Gerlotto et al., 1999; Soria et al., 2003). Multibeam sonars have also been used increasingly to study fish behaviour in the wild, including the determination of swimming speeds, aggregation dynamics, and spatial school characteristics (Misund and Aglen, 1992; Mayer et al., 2002; Gerlotto and Paramo, 2003; Brehmer et al., 2006), and predator–prey interactions (Benoit-Bird et al., 2004).
To propose an instrument that might be useable in situations with mainly small schools where beam width is a strongly limiting factor, Ifremer and Simrad joined forces in 2001. They developed the first calibrated multibeam echosounder (ME70) for fisheries research and installed it on the French FRV "Thalassa", a modern, noise-reduced fisheries research vessel (Mitson and Knudsen, 2003) in summer 2005.
The ME70 has two principal operational modes for observing organisms in the water column (fisheries-research mode), and for bathymetry and habitat classification (bathymetry mode). Our aim here is to describe the fisheries-research mode of the ME70, to illustrate its expected contribution, and to present some preliminary results. We also provide a detailed technical description of the new system and a discussion of its advantages and limitations, then illustrate its expected contributions to future fisheries research. The last have been grouped under three headings in relationship to the limitations of acoustic measurements: schools of small size, mixed-species associations, and individual and school behaviour. Areas for which the ME70 will not offer any improvements are not considered. Conclusions are drawn, and directions for future research are then proposed.
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Characteristics of the ME70 in fisheries-research mode |
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TopIntroductionCharacteristics of the ME70...Potential research applications...References |
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The ME70 is a calibrated, multibeam echosounder for obtaining quantitative information on organisms in the water column (Andersen et al., 2006). It can be configured with up to 45 beams spanning at most 150°; the athwartship centre angle of the fan can be adjusted from +45° to –45°. Each beam has a unique frequency in the range 70–120 kHz and a unique steering angle, except for one special configuration in which all beams have the same steering angle. The spread of steering angles through the fan is either spaced linearly or optimized for side-lobe reduction. Beam width can be selected from 2.2° to 20° for the central beam, and the width of the other beams are then adjusted depending on frequency and steering angle. Additionally, there are two specific beams, referred to as reference beams, which can be configured separately and freely in terms of frequency, steering angle, and beam width. This allows comparison with single-beam echosounders or to increase the sampling volume by adding them as wide beams (20° opening), steered at 50°. All beams can be operated as split beams. Beam emission is in groups of 1–4 beams. As with single-beam vertical echosounders used in fisheries research, the instrument provides values of volume-backscattering strength (Sv) and TS when the beams are configured as split beams.
The innovative design of the ME70 is based on a matrix of 800 transducer elements that are individually controlled, functioning both at transmission and reception. The challenges of real-time processing of huge amounts of data were met, and the resulting tool is promising and allows a high degree of user-controlled flexibility. The ME70’s matrix transducer offers up to –70 dB side lobes (two way) compared with about –25 dB (one way) for "T" array geometries of conventional multibeam systems (Figure 1a). Hence, the ME70 is a multifrequency echosounder capable of collecting data throughout the whole fan, in particular outside the spherical volume (Figure 1b), where seabed backscattering through the side lobes (Figure 1a) strongly limits fish detection in conventional multibeam systems. All beams are stabilized with respect to a vessel’s roll and pitch to keep the steering angles of all beams fixed between transmission and reception.
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The ME70 has many configuration options, each a trade-off between several factors, and their choice will depend on the objectives of the study (Table 1). Figure 2 shows a typical beam configuration ("V" configuration): Figure 2a is a schematic view of the athwartship beam distribution, the alongship distribution is shown in Figure 2b, and the steering angle and frequency of each beam in the configuration is depicted in Figure 2c. When selecting a beam configuration, various issues need to be considered, as covered below.
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Selection of the number of beams and the pulse length
As each beam of the fan has a distinct frequency, the number of beams depends on the selected pulse length and the frequency bandwidth, and vice versa. With the maximum frequency bandwidth ranging from 70 to 120 kHz and the maximum number of beams being 45, the minimum pulse length available is 2048 µs for a continuous wave (CW) signal. If the number of beams is reduced to 21, the minimum pulse length is reduced to 1024 µs.
Selection of the number of beams to be transmitted simultaneously
The options range from 1–4 beams per transmission group. The choice affects the detection range, because the source level is 12 dB higher for separate transmissions (a group of one) than with transmissions by groups of four. The extent of the blind zone underneath the vessel is also affected, because it is inversely proportional to the number of frequencies per group (see rule in Table 1).
Selection of the frequency-distribution pattern
The frequency distribution in the fan can be set up in different ways. Three options have been evaluated. To maximize the detection range, the "V" configuration has the lowest frequency in the centre and the highest frequencies in the most-steered beams. To maximize the angular resolution, the "
" configuration has the highest frequencies in the centre, and the lowest frequencies in the outer beams. To explore species discrimination using multifrequency analysis, all frequencies (beams) in the "I" configuration point, not necessarily vertically, in the same direction. The choice between "V" and "" configurations will be guided by the objectives of the scientific survey in terms of angular resolution when there is the possibility of a narrower beam opening at the vertical for the "" configuration and detection range, given the longer range at the vertical with the "V" configuration. When narrow angular resolution and long detection range are both required at the vertical, the "" configuration combined with a reduced number of frequencies per transmission group can be a suitable option, but to the detriment of increasing the length of the blind zone underneath the vessel.
Selecting the sampling volume (fan width) perpendicular to the vessel track
Three series of parameters relating to the prevailing role of the centred beam in the definition of the fan’s characteristics require determination: (i) along- and athwartship beam width of the centre beam, (ii) athwartship beam spacing, and (iii) along- and athwartship steering angle of the centre beam.
In technical terms, an appropriate weighting of all cells in the array is automatically calculated when the beam width of the centred beam is set to a given value. As a consequence, the beam widths of all other beams are a function of the frequency distribution, the beam-steering angles, and this pre-calculated weighting. However, it is somewhat restrictive to describe the fan’s characteristics only through the sampled volume and, remembering the originality of the ME70 in terms of side-lobe reduction, valuable information can be collected with a given beam configuration when the resulting side-lobe levels are included in the consideration. The results shown in Figure 3 emphasize that it can be judicious to lower slightly the angular resolution, to increase data quality and to reduce noise outside the spherical volume. As beam width increases from 2.3° in Figure 3a to 2.8° in Figure 3b, the corresponding weighting on the array reduces the side-lobe level for each beam and leads to reduced artefacts outside the spherical volume in the outer beams (–33° steering angle in Figure 3).
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The high available angular resolution (minimum 2.2° for a beam centred on 120 kHz) and reduced side lobes of the ME70 can reduce the dead zone compared with traditional echosounders with 7° or 11° beam widths. Unfortunately, the ME70 currently suffers from a limitation in beam-forming, which affects data collected in a fixed layer of 75 cm above the seabed visible in all subplots of Figure 3. Despite this limitation, the improved bottom resolution of the ME70 for an uneven seabed is evident when comparing data obtained with an ER60 sounder (7°, 120 kHz; Figure 4a) with data from the central beam of the ME70 (2.8°, 117 kHz; Figure 4b).
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In addition to collecting information with the multibeam fan, the ME70 offers the possibility of duplicating data collected with traditional vertical echosounders. For this, two additional reference beams can be used, whose frequency, beam opening, and steering angle can be set freely by the user. Only the pulse length remains the same as in the fan beams. The two reference beams are always transmitted after the fan beams, to reduce frequency leakage with the latter. Although the reference beams increase the extent of the blind zone underneath the vessel by about one or two pulse lengths, depending on whether they are in the same or separate emission groups, they can be useful for comparisons with conventional vertical echosounders operating simultaneously.
Once a particular configuration has been selected, the system can be run with default system gains calculated automatically from stored values. However, it is recommended that each configuration be calibrated. The ME70 calibration procedure is similar to that for other echosounders (Foote et al. 1987), particularly for the ER60 echosounders. To adjust the nominal gain set by default, the two parameters "gain adjustment" and "Sa correction adjustment" must be measured for each beam. If we define the gain as the ratio of the intensity measured in a given beam to that for an omni-directional transducer, the first parameter concerns the offset between the theoretical gain and that measured for the calibration sphere. The second parameter corresponds to the offset between the theoretical contribution of the shape of the pulse on the integrated response of the sphere and the measured one. A typical view of the fan in the calibration interface is presented in Figure 5a for a configuration with 19 beams and a sequence of 150–350 sphere TS measurements per beam. The resulting calibration parameters are displayed in Figure 5b. Calibration results for a beam configuration with only 15 beams and a wider athwartship sector are also presented, including standard deviations for the gain-adjustment estimates. The calibration sphere used for this test was a 25-mm diameter sphere made from tungsten carbide with a 6% cobalt binder, provided with the ME70. Owing to the small weight of this sphere, an additional weight was suspended below it to stabilize the calibration target, but this required careful attention when calibrating the outer beams, to avoid detecting the echo from the weight. Future calibrations may use larger and heavier spheres, as described in Foote (2006). Their use alleviates the need for an additional weight and their high TS values improve detections in a noisy environment. A disadvantage is that two spheres instead of one are needed to calibrate the whole fan, requiring further developments of the calibration procedure.
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To take fullest advantage of the ME70 and the five frequency ER60s installed on "Thalassa", all are configured and controlled using HERMES software, specifically designed for the purpose. HERMES stores the data from all echosounders (ME70 and ER60s) in a single file, using the HAC standard format detailed in ICES (2005).
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Potential research applications of ME70 |
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TopIntroductionCharacteristics of the ME70...Potential research applications...References |
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Some of the challenges in the acoustic method identified in the introduction may be addressed with data from the ME70. This is illustrated here with two datasets. Data from herring (Clupea harengus) schools were collected during March 2006 in shallow waters of the English Channel (Table 2). Data were also collected during March 2007 from mixed species, mainly small schools, on the continental-shelf break of the Bay of Biscay. The ME70 configurations used for each dataset are listed in Table 2. MOVIES software (Berger et al., 2005) was used to perform echo-integration and extraction of shoal parameters.
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Small schools
The size, e.g. the horizontal diameter, of single or multispecies pelagic fish schools depends on species, season, time of day, depth, and geographic location (Scalabrin and Massé, 1993; Soria et al., 2003). Schools are often grouped into clusters (Petitgas, 2003). In the Bay of Biscay, nearest-neighbour horizontal distances between pelagic schools within clusters range from 400 to 3300 m (Petitgas, 2003), and horizontal school length is often
10 m. Little is known about school width (perpendicular to survey track), but investigations with vertical-scanning multibeam sonar have shown that pelagic fish schools seem to be elongated, with school length up to twice the width for Sardinella aurita (Gerlotto and Paramo, 2003). Based on geometric considerations and simulations, Diner (2001) proposed an algorithm for correcting school-length measurements, which are increasingly overestimated with depth because of the increased sample volume. The correction is valid if the true school length is at least 1.5x the sampled beam diameter (e.g. school length must be 
12 m at 100-m depth for a 4.5° beam width, and 18-m long if beam width is 7°). Hence, a small school is one whose length is less than 1.5x the beam diameter at the given depth. The definition of "small" depends both on the depth of the school and the beam width. Similar, non-negligible depth effects might exist for abundance estimates based on echo-integration, owing to negatively biased backscattering cross sections of small schools, as defined above, at greater depths (Diner, 2007).
The reduced beam angles (minimum 2.2°) of the ME70 are expected to provide several benefits, including improved estimates of school length and backscattering cross-section estimates for smaller schools, i.e. those too small for the conventional 7° or 11° beams. Of course, using a smaller beam width only reduces the number of schools that fit the aforementioned definition of "small". For the mixed species data, in the central beam of the ME70 (beam opening 4.3°), 30% of encountered shoals were <1.5x the beam width at the depth of the shoal. For the 7° single-beam measurements at 70 kHz, the corresponding value is 65%. In that dataset, most schools were near the seabed at depths of around 200 m.
Based on the use of several beams, an additional estimate of school width, and therefore the entire school geometry, is obtained. The mixed example shows the effect of increasing the spatial resolution on observed school structure (Figure 6). It also illustrates the effect of beam stabilization for the ME70. Although the image from the ER60 is smeared perpendicular to the survey track, the image reconstructed from the three central beams of the ME70 is much sharper.
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A drawback of using small beam widths is that the ping rate might have to be increased or the vessel slowed to maintain an overlap between subsequent samples. The effect of non-overlapping, along-track samples appears for the herring school situated in shallow water (Figure 7). For large schools, unsampled areas along the survey track are not a problem, but for small schools, this might lead to biased estimates of school-morphology parameters.
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Mixed species associations
Pelagic species form mixed or closely located schools in many shelf seas. For example, in the Bay of Biscay, anchovy (Engraulis encrasicolus), horse mackerel (Trachurus trachurus), mackerel (Scomber scombrus), sprat (Sprattus sprattus), and pilchard (Sardina pilchardus) are commonly found together in varying species combinations related to geographic location (Massé, 1996). Tropical pelagic species are also often found in mixed schools, with individuals of similar size but different species occurring together (Fréon, 1984). Similar observations have been made in the Bay of Biscay, where small horse mackerel were caught together with large anchovy of similar size (VMT, unpublished data).
Many attempts have been made to identify monospecific schools based on morphological (school length, perimeter, density, etc.) and contextual information (location, depth, time of day, etc.; Scalabrin et al., 1996; LeFeuvre et al., 2000; Lawson et al., 2001). Although this approach has allowed species classifications for particular datasets, its general applicability seems to be hampered by the fact that school morphology varies with environmental characteristics (Scalabrin and Massé, 1993; Muiño et al., 2003; Soria et al., 2003), fish size (Iglesias et al., 2003), and maturation state (Mackinson, 1999), perhaps even more so than species.
At least two alternative strategies have been proposed to advance acoustic-species classification: inclusion of the third dimension for morphological-school description (Fernandes et al., 2002), and use of multifrequency information (see review in Misund, 1997). So far, broad groups, e.g. plankton, fish with and without swimbladders, have been distinguished successfully based on differential sound-scattering over a range of discrete echosounder frequencies (e.g. 18, 38, 120, and 200 kHz; Korneliussen and Ona, 2003), or using the response difference between 38 and 120 kHz (Woodd-Walker et al., 2003).
The ME70 is not only multibeam (multi-angle), but also multifrequency in the range 70–120 kHz. This frequency range may be useful for collecting information needed for species identification. Multifrequency data from the vertical and steered beams are from a single transducer array and do not suffer from the common problem of horizontally displaced transducers not sampling exactly the same scene (Korneliussen and Ona, 2002; Korneliussen et al., 2008). Tank measurements have shown that anchovy and sardine TS values do not vary much between 70 and 120 kHz (flat frequency response), although the absolute levels differ between the two species (Conti and Demer, 2003). This might be a general result. Simmonds and Armstrong (1990) found that it was possible to distinguish between herring, mackerel, and gadoids in the range 27–54 kHz, although they did not investigate higher frequencies. From the literature, we might expect the frequency response of many species to be flat over the range covered by the ME70 (70–120 kHz), which would be favourable for abundance estimation, but ineffective for species classification. Therefore, the new information provided by the ME70 might be most effective if used in combination with data over a wider range of frequencies from single-beam echosounders. A subset (3 nautical miles) of the mixed-species dataset illustrates the potential of combining information from the ME70 and ER60s (Figure 8). The example includes both diffuse and dense shoals. The frequency response derived from ER60 data could be combined with information from the ME70 on fish-density distribution within shoals and shoal morphology. The three-dimensional shape of the flat layer on the right side of Figure 8, though unidentified, may be indicative of a single species. However, frequency responses, particularly in shallow water, are often variable because of the reduced overlap of the volume sampled by the different frequency ER60s, which is caused by the position of the transducers on the hull of the ship, maximum distance 1 m on "Thalassa", but also variable beam widths, when considering 18 kHz. In deeper water, the difference in steering angle of the transducers (maximum 1° on "Thalassa") can also lead to some variability in the frequency-response curve. The roll-and-pitch-stabilized three-dimensional image obtained with the ME70 will be valuable in determining which echoes were well sampled by all frequencies, and consequently for filtering the data to obtain more accurate frequency response curves. In addition, improved in situ single-target measurements, from the narrower beam widths and the simultaneous use of different frequencies, as proposed by Demer et al. (1999), might also help species identification.
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Individual and school behaviour
The potential impact of avoidance behaviour on abundance estimates derived from acoustic measurements has long been a concern (Olsen et al., 1983; Gerlotto and Fréon, 1992; Vabø et al., 2002). Reactions to an approaching survey vessel have been observed using multibeam echosounders (Gerlotto et al., 1999; Gerlotto and Paramo, 2003; Soria et al., 2003). In contrast, Fernandes et al. (2000) compared acoustic measurements from the FRV "Scotia", which was built to ICES reduced-noise standards, with those from an autonomous underwater vehicle, and concluded that herring did not seem to avoid noise-reduced survey vessels. However, Ona et al. (2007) found that such a vessel induced stronger vertical-diving reactions in herring than a conventional survey vessel. Using such a conventional survey vessel, Gerlotto and Paramo (2003) found that fish schools near a vessel track, most likely round sardinella, were nearly twice as long as they were wide, whereas those located
30 m from the track line had similar width and length. Vertical reactions (i.e. diving responses) have also been observed for anchovetta (Gerlotto and Fréon, 1992), herring (Vabø et al., 2002), and cod (Handegard and Tjøstheim, 2005). The ME70 may contribute to behavioural studies of individual fish and schools, be it natural or in reaction to a vessel. As previously demonstrated, three-dimensional measures of fish schools as a function of horizontal distance from the vessel track allow behaviour to be visualized and quantified. Consider again the herring-school example (Figure 7). The school might have reacted to the vessel, because individuals on the vessel track were lower in the water than those to the side (Figure 7a). Moreover, as the vessel progressed over the school, the depths of individuals increased (Figure 7b). As all ME70 beams can be configured as split beam, the movements of individual fish along and across the track, i.e. within and across beams, can be measured and tracked. Studies of fish movements might facilitate progress on characterizing fish-reaction behaviour and the remote identification of species or age groups. For example, Kang et al. (2006) found that the variability of swimming angles within schools of walleye pollock (Theragra chalcogramma) was related to the dominant age group present in the school.
The configuration of steering angles of the ME70 fan is somewhat flexible, ranging from all beams (with different frequencies) pointing in the same direction ("I" configuration, see above), to each beam having a separate angle and also a separate frequency. A configuration with multiple "I"s, i.e. the same range of frequencies at different steering angles, is not possible, but multiple, slightly tilted "I"s, with non-overlapping frequency ranges, are possible. Such a configuration would allow exploration of whether changes in tilt angle as a result of avoidance reactions lead to changes in backscattering, which differ between steering angles, as derived theoretically by Cutter and Demer (2007). In their simulation study, in which all beams had the same frequency, those authors found that sideways and diving responses of fish schools were visible as a difference in backscattering strength between central and steered beams (>30°). As the ME70 beams have different frequencies, the overlaying frequency response might blur the picture somewhat.
Further, the potential biases in abundance estimates attributable to school reactions can be evaluated more directly. The values calculated for the shoals in Figure 7 were at least four times as high in the outer beams (athwartship angle ± 20°) than in the more central beams.
Trawl efficiency is another topic that might be evaluated from ME70 observations, for example using observations similar to those shown in Figure 9. When the survey vessel passed over a fishing trawl in action, individual fish were clearly visible in the trawl opening (Figure 9b). Therefore, assessing the number of fish inside and outside the trawl might allow trawl efficiency to be estimated.
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The next steps
As shown here, the calibrated multibeam echosounder ME70 is a powerful complement to other sampling devices, such as traditional multifrequency echosounders. However, to achieve its maximum potential, a few technical challenges need to be surmounted. For example, reliable bottom detection must be achieved in all beams, a problem particularly acute for the outer beams where the non-oblique, incidence angle reduces bottom contrast. Moreover, tools for visualizing the ME70 data in two, three, and four dimensions are essential (Mayer et al., 2002).
The dependence of Sv and TS on frequency and incidence angle need further exploration and quantification. Hazen and Horne (2003) found that tilt-angle variation was the most important factor for changes in simulated TS values, even when compared with body length or depth, and its importance was confirmed by experimental work on krill (McGehee et al., 1998).
The current ME70 provides three-dimensional views for fish schools. However, research is needed to develop pertinent descriptors for the amorphous shapes of fish schools which are not well described by spherical or cylindrical bodies (Gerlotto and Paramo, 2003); school width and length metrics are not sufficient to capture the entire geometry.
Algorithms also need to be developed to connect shoals and individual fish across ER60 and the ME70 beams. Improved precision for tracking of individual fish pursuing the approach proposed by Handegard (2007), who uses several types of information simultaneously, e.g. phase angles, echo intensity, ranges, and times, will provide further insights into fish behaviour and its effects on biomass estimation. It will be necessary to track individual fish not only within a beam but also across beams, which might be complicated by the potential impact of observation angle and frequency on TS values.
The ME70 is operational for studying the pelagic zone. However, a noise problem currently limits its application in the layer extending to 75 cm above the seabed. This problem may be surmounted with enhanced beam-forming techniques and faster computers for real-time processing.
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Acknowledgements |
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We thank Simrad for fruitful collaboration, and Noel Diner for initiating the development of the ME70. The development of the ME70 was supported by the European funding scheme IFOP. We are also grateful to Niels Olav Handegard, Dave Demer, Chris Wilson, and an anonymous referee for useful comments on an earlier version of this manuscript.
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References |
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TopIntroductionCharacteristics of the ME70...Potential research applications...References |
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