Not to be cited without prior reference to the authors ICES CM2002/K:10

Improving acoustic surveys by combining fisheries and ground discrimination acoustics

Steven Mackinson, Steven Freeman, Roger Flatt and Bill Meadows

The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk, NR33 0HT, UK. Phone: +44 (0)1502 524295. Fax: ++ 44 (0)1502 524511. e-mail: [email protected]

Abstract

Field surveys are notoriously costly. Whilst continuous acoustic surveys provide good value in terms of their data richness and spatial coverage when compared to point sample surveys (e.g. trawls, dredges, grabs), there is scope to improve survey methods and provide value added data. We present technical details and an example application of an approach that maximises survey efficiency and data richness through the integrated use of two acoustic tools; a fisheries echosounder and ground discrimination system. Simultaneous, integrated operation of both systems provides a more cost- effective use of survey time by eliminating the need to conduct independent coverage. The scientific benefit is that the approach removes the temporal confounding of spatial data that results when trying to compare data from two independent surveys. i.e. it enables fish to be placed with their habitat by linking information about seafloor composition directly with fisheries data.

Key words: acoustic surveys, cost-effective, integrated technology, spatial distribution

Introduction

The ability of fisheries and ground discrimination acoustic surveys to offer continuous, high resolution observations through the water column, provides a spatial and temporal data richness that cannot be achieved by point sampling methods such as trawls, grabs and dredges. The Achilles heel of acoustic surveys lies in the identification/ classification of acoustics targets; and, although progress is being made on automated methods, (Haralabous and Georgakarakos, 1996; Reid, 2000; Hammond and Swartzman, 2001; Hammond et al. 2001) there is a long way to go before acoustics can be relied upon without the need for ground-truthing through fishing and sediment sampling. It is therefore advisable the acoustic surveys be viewed as a necessary compliment to, rather than a substitute for, conventional surveys.

Acoustic hardware and associated post-processing systems are now commonly used in fish abundance estimation (Misund, 1997; Rivoird et al. 2000), species distribution mapping (Swartzman et al. 1992, Masse et al. 1996; Mackinson et al. 1999; Bahri and Fréon, 2000; Maravelias, 2001), behaviour studies (review by Fréon and Misund, 1999), and observations of physical attributes of the seafloor (Freeman?, Bax et al. 1999). Many field surveys typically utilise several acoustic tools. For example, a single survey might use a split beam echosounder to determine fish distribution and abundance, sidescan and or multibeam sounder to develop a composite picture of seafloor roughness/ hardness, and a second echosounder for ground type discrimination. Issues such as interference of different operation frequencies, optimal vessel speed for operation and availability of transducer sites might preclude the simultaneous use of the tools. Thus, to provide information from each tool it will be necessary to survey the same transect line more than once. In this instance, the reliability of interpretations made from comparison of the data collected from each tool will suffer as a consequence of the temporal and (to a lesser extent) spatial displacement that arises. Moreover, the extended time (and cost) requirements will prohibit fulfilling other potential survey objectives.

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We present here technical methods and example application of combining a fisheries echosounder and ground discrimination system for simultaneous operation during acoustic surveys. Comparison is made between the integrated technology survey and surveys where the tools are used independently.

Technical Methods

Overview

A calibrated (Foote et al. 1987) Simrad EK500 provided the platform for linking fisheries and ground discrimination acoustics. Two split-beam (4 quadrant) transducers, with operating frequencies of 38kHz and 120kHz were used in our integrated surveys. Transceivers were configured (see below) so that returning echo signals were channelled simultaneously via two processing routes: (i) to the Simrad EK500 scientific echosounder for recording of fish targets in the water column, and (ii) to a Quester Tangent Corporation (QTC) Seaview-4 acoustic ground discrimination system for classification of bottom substrate type. Date/Time stamps and GPS data form the common link allowing us to match the EK500 and QTC data.

Configuration for integrated operation of EK500 and QTC (based on personal communications with John Preston, QTC inc.)

There are two possible configurations for linking the EK500 and QTC ground discrimination system. The preferred method uses the signal from one quadrant of the split beam transducer. The alternative, which suffers from limitations (discussed below), sums the signal from all 4 quadrants.

One quadrant method (preferred)

To connect a QTC Seaview-4 to the EK500, the QTC View transducer cables black and clear wires are tied in directly to the EK500’s transducer cable, to only one quadrant of the junction box (Figure 1). The transducer cable shield wire must be grounded to the transducer junction box because the EK500 transducer (according to manufacturer) does not like any direct shield contact. To examine for any grounding issues, the raw wave should be displayed and scrutinized using QTC-CAPS (calibration and processing software). If the echo baseline appears to ramp up or has lots of noise, check and reposition the shield wire.

QTC recommend that best recording performance is achieved with a ping rate of <5 per second (the Seaview-4 system integrates over 5 pulses by default, and many GPS systems update at 1 Hz). Consistent with operation of standard fisheries acoustic surveys using the EK500, we use a ping rate of 1 per second, and have found the results to be satisfactory.

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(a)

EK500 QTC echo sounder 38 or 120 khz

Signal pair

Shield wire to Transducer ships earth 38 or 120Khz

(b)

A N M B L C

K QTC signal pair connected to D phase channel one – refer to manual for other channels J E F H

Figure 1. (a) Configuration for combined use of EK500 and QTC. QTC transducer cable connected to one quadrant only of EK500 split-beam transducer cable. (b) Plug connections inside EK500 for signal pair. Transducer connector pin numbers for single quadrant connection. Note: connections same for any frequency.

Since the dynamic range of the input amplifiers is fairly limited, an attenuator is required when the echosounder has a high power output and is operated in shallow water (20-30m). Attenuation avoids saturation that would otherwise distort the shape of the pulse and lead to incorrect measurement. QTC recommend an attenuation of 20dB at 38kHz and 10dB at 120kHz. To verify that these are suitable, raw waveform ‘qwv’ files should be recorded and loaded into QTC-IMPACT software to be inspected for clipping (Figure 2). Clipped wave forms mean that the signal is no longer linear and features characteristic of a normal signal are lost. The fact that QTC Seaview-4 can produce clipped echoes has been known for several years (Preston pers. comm.). However, with proper use of attenuators, clipping of echoes occurs infrequently enough to be of little concern. QTC are able to advise on attenuation levels required for any particular sounder.

(a) (b) ) m ( h t Dep

Figure 2. Wave forms derived from raw signal files at 38kHz, logged in QTC-CAPS and displayed in QTC-IMPACT. (a) good signal showing high dynamic range with a rich component of envelope variability. These pulse shapes would lead to a high success rate in classification. The pulse to

3 Not to be cited without prior reference to the authors ICES CM2002/K:10 pulse variability is also high in these instances. (b) clipped signal resulting in loss of envelope information over the peak of the pulse which has been flattened out due to saturation.

A data-acquisition challenge for the integrated system is that the QTC-Seaview’s relatively slow sampling rate is not optimal for the short echoes that occur in shallow water with the narrow beam width (7º for 120kHz transducer). Since the echoes are so brief, using a reference depth slightly deeper (5-10m) than the average depth lengthens the echoes and improves functional capability of QTC- IMPACT.

Sum of four quadrants method (limited)

An alternative configuration is for the sum of the signals from all 4 quadrants of the split beam to be used by QTC Seaview-4, through direct connection to the boards inside the EK500. On the basis of previous experience we know a limitation to this method concerning reduced transducer receive beamwidth, and therefore, do not recommend it. Briefly, examination of the wave files when summing the signals from 4 quadrants revealed an extremely limited variability in pulse shape (Figure 3) and extensive clipping (loss of information) of echo signals. When this type of echo (Figure 3) is encountered the main variability is amplitude, resulting in little scope for classification. The combination of short pulse and narrow beamwidth found at 120kHz, when operated in shallow water, all reduce the capability to acoustically discriminate ground type.

h t Dep

Figure 3. Effect of narrow transducer beamwidth and short pulse. Ping to ping variability between the 3 consecutive echoes is small and lacks discriminative information.

Example application: Acoustic surveys on the Dogger bank

To evaluate the utility of the integrated technology, trial acoustic surveys were undertaken on the Dogger Bank in the North Sea during June 2001. Using information from a larger survey assessing the behaviour, distribution and abundance sandeels (Ammodytes marinus) (Freeman et al. this symposium; Mackinson et al. 2002) we chose a site where sandeels were known to occur in abundance. The survey area (400m x 400m) was acoustically surveyed along 5 north-south parallel transect lines spaced 50m apart. Acoustic data were recorded at both 38kHz and 120kHz. Vessel speed during surveying was 5 knots.

Acoustic recordings of fish targets were made with the Simrad EK500, and post-processed using EchoView software (Sonardata inc.). We classified acoustic targets believed to be sandeels on the basis of knowledge of their behaviour, characteristic school formations and comparison of the schools images produced from the two operating frequencies (N.B. In part due to their lack of swim bladder, sandeel schools ‘mark’ much stronger at 120kHz than 38kHz when observed at the same volume

4 Not to be cited without prior reference to the authors ICES CM2002/K:10 backscattering threshold). Trawl fishing performed verification of acoustic marks. Appendix 1 details other post processing settings.

Acoustic properties of the seabed were recorded by the QTC ground discrimination system and later split in to discrete classes during post-processing using C-IMPACT software (see Freeman et al. for detailed methods). Classification of the acoustic signatures reflected from the seabed embrace properties including bottom substrate, surface features (e.g. sand ripples), and the presence of benthic organisms within or on the seafloor surface. Data recorded at 38kHz and 120kHz were processed separately and used to identify spatial changes of different seabed types. Those with different acoustic characteristics were ground-truthed using Day grabs (0.1m2). Two grab samples were taken at each station and a description, photograph and sub-sample (~ 20cm3) for particle size analysis (PSA) of the sediment were recorded.

To emphasise the improved data quality of the integrated method, we compare the spatial and temporal characteristics of data recorded in June 2001 with that recorded from an identical piece of cruise track recorded during independent operation of the EK500 (at 120kHz) and QTC (operating through a Furuno single beam echosounder at 200kHz) in June 2000.

Results

The spatial displacement of data that occurred in June 2000, when fisheries and ground discrimination acoustics were performed independently on an identical section of cruise track is plainly apparent from Figure 4a. Maximum and mean separation distances of the cruise tracks were 0.82 km and 0.54 km respectively. Time separation between coverage of the cruise track section was 10 hours; fisheries acoustic surveys started at dawn and lasted 7 hours, thereafter ground discrimination surveys immediately commenced. There was, of course, no spatial or temporal displacement during integrated surveys conducted in June 2001 (Figure 4b). Furthermore, substrate classes detected were very much the same as those in June 2000, demonstrating that the integrated method did not compromise functional capability of the ground discrimination system.

Application of the integrated technology to a fine scale trial survey on the Dogger bank provided highly resolved information on the spatial relationship between distribution of sandeel schools in the water column and seabed sediment (Figure 5). Since sandeels bury in the sediments at night, their distribution is closely linked to the spatial extent and patchiness of sediment types (e.g. Wright et al. 2000). Extension of our trials indicated acoustic changes in seabed properties that were coincident with the time that sandeels ‘disappeared’ from the water column (Freeman et. al. this conference).

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Figure 4. Comparison of echo survey cruise tracks during (a) independent operation of EK500 and QTC and (b) integrated operation

Figure 5. Fine scale integrated acoustic survey (a) 3-D spatial relationship of sandeel schools with and sediment classifications, and (b) cruise track with seabed classes.

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Discussion

Benefits

Configuration of an EK500 with a QTC ground discrimination system provides parallel processing routes that permit the same echo data to be used for acoustic recordings of fish targets and classification of bottom substrate type. By eliminating the need for covering the same ground more than once, the integrated technology provides a more cost effective use of survey time. More importantly (from a scientific perspective), it provides considerable value-added information since spatial and temporal displacement of the data no longer exists. With integrated surveys, fish are placed directly in context of their habitat. The extra available survey time can be used to double the coverage of acoustic surveys or be put to good use for alternative objectives.

A drawback of conventional fisheries acoustic surveys is the difficulty in discriminating echoes from fish close to the seabed with echoes from the bottom substrate or other objects close to the bottom (such as wrecks). The integrated technology may help to shed some light on this problem; since QTC is designed specifically to determine bottom features, comparison of QTC classifications with fisheries echo traces offers the ability to more closely scrutinise dubious objects near the seafloor. Conversely, the fisheries sounder data is useful for identifying potential errors that arise with the QTC, such as when strong fish targets produce ‘false bottoms’.

Limitations

Differences in optimal vessel survey speed may mean that optimistic projections of surveying times being halved are not realised in practice. Nonetheless, time savings will undoubtedly be made. Depending on sea state, typical fisheries acoustic surveys are conducted from 4.5 to 10 knots (average about 7). To maximise resolution of the seabed features, ground discrimination surveys are recommended to be conducted at approximately 5 knots. A compromise that we have found appropriate for our surveys is to run at 4.5 to 6.5 knots depending on sea state.

Footprint and pattern of the beam are also important concerns for ground discrimination systems. Circular beam patterns of at least 10º degrees width at –3dB return strength are preferred (QTC pers comm). The beams of our 38kHz and 120kHz transducers are both circular with widths of 7º degrees at –3dB. Whilst some small amount of coverage of the seabed is lost in comparison to our previously used 200kHz, 10º beamwidth transducer, it is adequately compensated for by the benefits of running simultaneously with two frequencies. Comparison of the substrate classifications from data recorded with 200Khz and 38Khz transducers (Figure 4) revealed substantial correspondence.

Collection of multiple frequency data provides extended opportunities for data analysis. For the fisheries acoustic data, comparative analysis of fish school images at 38kHz and 120kHz, is a key feature used to discriminate sandeels from other fish schools. The comparative multifrequency approach is the focus of a new EU project aimed at improving discrimination of acoustic targets (SIMFAMI; http://simfami.marlab.ac.uk). For ground discrimination purposes, alternative frequencies can be analysed separately to provide independent classifications of seabed types. The higher power output and bottom surface penetration of the lower frequency, makes the 38kHz suited to deeper waters, whilst the 120kHz is considered more effective for detecting seabed surface properties in shallower waters. Concatenation of the 38kHz and 120kHz acoustic data sets provides capability of providing a classification incorporating the combined acoustic properties. The only other study where a similar technique is presently being investigated is in the Bering Sea by NOAA (Bill Collins, pers comm.)

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Applications

Potential applications of the integrated technology are many. Details of our successful trials on the Dogger Bank, attest to the obvious benefits outlined. Extended observations revealed detailed information on the spatio-temporal behaviour patterns of sandeels; as presence of sandeels in the water column diminished toward darkness, simultaneous acoustic changes were detected from the seabed, suggesting that sandeels were buried in the sediment (Freeman et al. this conference). The methods of acoustic observations of the presence/absence of sandeels within the sediment could be equally applied to other mobile and non-mobile benthic faunas that form densely packed aggregations such as, the sublittoral clam (Ensis) and the common mussel (Mytilus edulis).

Employing the combined technology as standard operating procedure will provide improved spatial information on fish habitat utilisation patterns at various time scales; information that is critical to the effectiveness spatial harvest control tactics (Hilborn and Walters, 1992; Mundy et al. 1985). In particular, utilising the technique during international bottom-trawl survey (IBTS) will provide value- added information at little extra cost (personnel time required for data processing).

References

Bahri, T. and Fréon, P. 2000. Spatial structure of coastal pelagic schools descriptors in the Mediterranean Sea. Fish. Res. (48)2: 157-166. Bax, N., Kloser, R., Williams, A., Gowlett-Holmes, K., and T. Ryan. 1999. Seafloor habitat definition for spatial management in fisheries: a case study on the continental shelf of southeast Austalia. Ocean. Acta. 22 (6), 705-719. Foote, K. G. Knudsen, H.P. Vestnes, G. MacLennan, D.N. and Simmonds, E.D. 1987. Calibration of acoustic instruments for fish density estimation: a practical guide. ICES coop. Res. Rep. 144. Fréon, P. and Misund, O.A. 1999.Dynamics of pelagic fish distribution and behaviour: effects on fisheries and stock assessment. Fishing News Books, Oxford, England. 348 p. Hammond, T.R. and Swartman, G.L. 2001. A general procedure for estimating the composition of fish school clusters using standard acoustic survey data. ICES J. Mar. Sci. 58 (6): 1115-1132. Hammond, T.R., Swatrzman, G.L. and Richardson, T.S. 2001. Bayesian estimation of fish school cluster composition applied to a Bering Sea acoustic survey. ICES J. Mar. Sci. 58 (6):1133-1149. Haralabous, J. and Georgakarakos, S. 1996. Artificial neural networks as a tool for species identification of fish schools. ICES J. Mar. Sci. 53: 173-80. Hilborn, R, and Walters, C.J. 1992. Quantitative fish stock assessment. Chapman and Hall. 569pp. Mackinson, S., L. Nøttestad, S. Guénette1, T.J. Pitcher, O.A. Misund and A. Fernö. 1999. Cross-scale observations on distribution and behavioural dynamics of ocean feeding Norwegian spring spawning herring (Clupea harengus L.). ICES J. Mar. Sci.56: 613-626. Mackinson, S., Turner, K., Righton, D. and Metcalfe, J.D. 2002. Estimating the abundance of sandeels on the Dogger bank, North Sea: using acoustics to estimate apparent dredge efficiency. Submitted to ICES J. Mar. Sci. Maravelias, C.D. 2001. Habitat associations of Atlantic herring in the Shetland area: influence of spatial scale and geographic segmentation. Fish. Ocean. 10(3): 259-267. Massé, J., Koutsikopoulos, C., and Patty, W. 1996. The structure and spatial distribution of pelagic fish schools in multispecies clusters: an acoustic study. ICES J. Mar. Sci. 53: 155-160. Misund, O.A. 1997. in marine fisheries and fisheries research. Rev. Fish Biol. Fish. 7:1-34.

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Mundy, P.R., Quin II, T.J. and Deriso, R.B. 1985. Fisheries dynamics: harvest management and sampling. Washington Sea Grant Program. WSG85-1. 60p. Reid, D.G. [ed.] 2002. Report on echo trace classification. ICES coop. Res.rep. 238: 115pp. Rivoirard, J., Simmonds, E.J., Foote, K.G., Fernandes, P. and Bez, N. 2000. Geostatistics for estimating fish abundance. Blackwell science, Oxford, England. 206 p. Swartzman, G. Huang, C. and Kaluzny, S. et al. 1992. Spatial analysis of Bering Sea groundfish survey data using generalized additive models Can. J. fish .Aquat.Sci. 49 (7): 1366-1378. Wright, P., Jensen, H and Tuck, I. 2000. The influence of sediment type on the distribution of the lesser sandeel (Ammodytes marinus). J. Sea Res. 44. 243-256.

Appendix.1. Calibration and post processing settings for . EK500 Calibration settings EchoView Post-processing Frequency 38kHz 120 kHz Type: ES38 Abs coeff. 9dBkm 38 dBkm Pulse Length: medium medium Bandwidth wide wide Power 2000 W 1000 W 2-Way Beam Angle -20.8 dB 20.5 dB Sv Transd. Gain 27.12 dB 24.65 dB Angle Sens.Athw. 22.0 21.0 Angle Sens.Along 22.0 21.0 3 dB Beamw.Along 6.8 dg 7.1 dg 3 dB Beamw.Athw. 6.3 dg 7.4 dg

Alongship Offset -0.48 dg 0.02 dg Athw.ship Offset -0.18 dg -0.16 dg TVG 20LogR 20LogR Ping Interval 1 second 1 second Heave compensation Yes Yes Bottom exclusion 0.5m Surface exclusion 7-10m depending on the penetration of surface aeration caused by rough seas Volume backscattering –60db to –70db depending on the density of threshold in the water column

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