Improving Acoustic Surveys by Combining Fisheries and Ground Discrimination Acoustics. ICES CM 2002/K10

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 sonar 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. 1 Not to be cited without prior reference to the authors ICES CM2002/K:10 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 scientific echosounder 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. 2 Not to be cited without prior reference to the authors ICES CM2002/K:10 (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 seabed 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.

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