Usability problems of acoustical fishing displays

Usability problems of acoustical fishing displays Stella Mills

This paper discusses the usability problems of acoustical fishing aids in small fishing vessels with particular reference to interpretation and comprehension of data. After a brief account of how the problems arise, some design principles are derived which are then applied, by means of an heuristic analysis, to a typical display. The paper concludes with a discussion of how, in the last year or so, manufacturers themselves have tried to address the problems arising from the heuristic evaluation. Although good progress has been made, problems of user support still need to be addressed.

Keywords: usability, acoustical fishing displays, screen design principles

In the UK, a fishing vessel of registered length between 16.5 m and 24 m is often known as a 'single ticket' vessel since only one certificated officer is required to be on board ~. Such vessels usually carry a number of fishing aids of which the (vertical) echosounder, the (horizontal) sonar and the netszonde, a sonar display produced by transducers on the net, are acoustical systems which are used in conjunction with each other (and often with other systems such as the navigation system) for navigating the seabed and other fishing purposes. Figure 1 shows a typical operation of such a system for a single-ticket vessel. The skipper in the wheelhouse would use the echosounder to check depth levels as well as searching in the vertical plane for fish. The sonar would be used to scan horizontally and spherically for fish and it would also indicate approaching obstacles on the seabed. The transducers on the net, the netszonde, would give feedback on the amount of fish entering the net and its position in relation to a shoal.

Department of Information Technology, Cheltenham and Gloucester College of Higher Education, The Park, PO Box 220, Cheltenham, GL50 2QF, UK Paper received: 20 February 1995; revised: 30 May 1995

However, since sonar was first used for fish detection in the 1960s, manuals 2'3 have been used to help alleviate the problem of interpreting the displayed information. This paper attempts to explain the reasons for interpretative problems and indicates how some manufacturers have recently developed systems which can help in the understanding of the output from sonar systems. It is helpful to have a short account of how an acoustical display works and how the interpretative problems arise and while the immediate discussion below will relate to the echosounder, the points made are relevant to the sonar and the netszonde, since all three aids work on the same principle and have similar output displays.

BASIC SYSTEMS DESIGN In its top level form an acoustical fishing aid consists of a transmitter, a transducer, a receiver and a recorder, which for the purposes of this paper, will be assumed to display its data on a colour VDU which may be backed up with a monochrome or colour paper plotter. The transmitter sends an electro-magnetic pulse to the transducer which converts this to a sound pulse. The transducer receives the echo of this pulse and converts it back into an electro-magnetic pulse which is sent to the receiver where it is amplified and passed on to the recorder. The recorder normally fires the transmitter and also records the time between transmitting a pulse and receiving its echo 4. Figure 2 shows a typical output from an echosounder where a fish shoal can be seen in the upper half of the output near the left-hand edge. Thus, the types of problems encountered are twofold: first, those associated with the echo returning and being transmitted to the VDU; and, secondly, those to do with the representation of the data on the VDU. Since to some extent at least, the second is dependent on the first, the two problems will be considered in tandem. In theory, the transducer sends a single acoustical beam in its transmitting direction which will be returned when it

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Usability problems of acoustical fishing displays: S Mills

Depth from s

•

Wireless data transler

tDe~a(:ho~

8o~r clete~ of f~h ,~

i

Detection of fish and bottom ,.~Jx)olof f~h

Distance to bottom

Figure 1 Diagrammaticview of a trawl system (Reproduced by courtesy of Simrad)

Figure 2 Typical output from an echosounder (note the fish shoal on the left below the depth output) (Reproduced from The Echosounder and Fishfinder Handbook, Ferhurst Books, tel. 01903 882277, price £10.95) is reflected back by hitting some target which could be the seabed, a shoal of fish or plankton, etc. However, in practice there are a number of secondary beams (or side lobes) which also are transmitted and the echoes from these can cause confusion on the VDU screen. Secondary beams reduce the power of the main beam and should be reduced as much as possible. Thus a good quality transducer is the first essential for achieving good system integrity since it will reduce to a minimum the confusion caused by the echoes of the secondary beam. When an echo is passed from the transducer to the receiver, it is amplified many times 5 (often more than several thousands4). The amount of amplification will vary as those echoes from a distance will be much weaker than those from close to the transducer. The adjustment can be specified by the user or can be processed automatically by embedded software--this latter method is called Time Varied Gain (TVG) and is the usual choice of single-ticket skippers.

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Noise generated by the propeller (in particular, its zinc anodes) can be a major problem if the transducer is not fitted in the correct position on the hull. Reverberation can cause 'false' echoes to appear on the VDU, although TVG will reduce this problem. Other sources of noise are from dolphins, other vessels' echosounders if the vessel is working with the fleet at close quarters, and, for a netszonde, the trawl itself will cause reverberation. One of the most common causes of noise is air bubbles on the transducer which can occur in a rough sea or echoes from air bubbles in the water or from plankton. In addition, 'false' echoes can be obtained by secondary reflections from the target. Thus it can be seen, that the data representation on the VDU may not be truly representative of the target area through interference from noise and secondary reflections. Modern technology has been successful in solving some of the more gross problems, principally through filtering techniques, but some misinterpretation can still happen. However, these problems can be helped by using an echosounder with good resolution, which essentially means using the best frequency, beam width and pulse rate for the chosen target area. For example, a narrow beam with short pulses will give better fish detection near the seabed than a wider beam due to the 'dead fish area' (i.e. the area from which no signal in phase is returned) being reduced (Figure 3). Transducer

Seabed Dead Fish Area

Figure 3 'Dead fish' area of a beam from an echosounder

Usability problems of acoustical fishing displays: S Mills A wider beam with a longer pulse (for example, around 200 kHz) will be more useful for pelagic (mid-water) fishing especially if this is near the sea surface. From this, there follows the maxim that 'frequency determines depth range '6. For example, to penetrate to around 1000m in depth requires a frequency of 50 kHz. Thus it is usual for echosounders to have dual frequencies of around 50 kHz and 200 kHz, in order to obtain the most accurate searching over an optimum depth range• The data on the VDU are colour-coded, usually such that a strong pulse returning from a hard surface, and thus sometimes called a 'hard' signal, is coloured red while a pulse returning from a soft surface is coloured blue, and is sometimes known as a 'soft' signal• Pulses of moderate strength are coloured on a scale of the colour spectrum and thus appear yellow to green ('harder' to 'softer'). Thus rock on the seabed will return a 'red' ('hard') signal whilst a shoal of mackerel will return a 'blue' ('soft') signal since mackerel do not have a swimbladder. Fish with swimbladders will return a 'red' signal since the swimbladder, being filled with air, reflects up to 90% of the source pulse strength, thus returning a strong signal• However, these basic indications are hardly sufficient to utilize the echosounder to its full capacity and, without specific training, there is inevitably a learning period for the user which, from conversations with single-ticket skippers, appears to be anything up to two years• Some manufacturers now run training courses but these are often abroad (e.g. in Norway) and require the skipper's time as well. Since the skippers have an intuitive rudimentary knowledge of interpreting the output from acoustical displays, the need for training is not given priority. That interpretation of the echosounder is still causing problems is indicated by a caption to a plate in a text first published in 19927: Fig. 3.6 Echogram produced by a netszonde on a pelagic trawl. Scale at left denotes distance (in m) below the headline, not true depth: see text for explanation. [Italics added for emphasis] Thus the main problems which cause interpretative difficulties for the skipper are the results of noise and other equipment inefficiencies, lack of training in the use of the equipment to gain maximum information and lack of interpretative knowledge. General interface design guidelines can also help with information presentation which aids interpretation and to these we now turn.

INTERFACE

DESIGN

Wilkinson 8 makes the important point that 'there is much more to designing a good display than just making it visible. The operator must understand the presented information and convert it into the correct decisions and/or control actions'. Consequently, the output of the VDU must be designed to accommodate not only the environmental conditions but also the physical and mental limitations of the operator, who, Wilkinson continues, 'must [receive] information which is accurate and specific, so that he can

take the correct actions in the correct manner at the correct time'. In 1990, the Department of Trade and Industry in the UK, following a Directive 9 from the EC, introduced an Initiative to increase awareness in the British computing industry of the importance of producing computer systems which are usable• For the last ten years or so, developers have been trying to produce software which does indeed do what Wilkinson insists it should and during this time a number of writers have attempted to define usability in a system-orientated way. However, a measure of the difficulty of this task is that the implementation of this aspect of the EC Directive has been postponed. In February 1995, it is still only a Draft International Standard 1°. While Shackel ~1 is generally credited with the first definition of usability and it can be claimed that a number of later authors have built on this (e.g. Booth 12 and Dix et al.13), Wilkinson certainly gave 8 the kernel of the definition in 1974. Indeed, he continues this paper by pointing out that 'the type of action expected of [the operator] while or after receiving information should be analysed'• Thus while quantitative, qualitative and check reading only require 'the act of reading the display', displays for setting and tracking require consideration of the operator's response. In addition, displays will be used in a combination of these ways 8. Thus a skipper of a small fishing vessel may use the echosounder to check water depth and the sonar to identify a shoal of fish suitable for capture. He will then utilize the depth data to assist in correct positioning (identified by the netszonde) of the net to catch the fish indicated by the sonar. Abramowski ~4 devoted much of his work to displays with 'descriptive' (i.e. text) information, which clearly does not concern us here, but he makes the important point that no display should detract from the skipper's work of 'commanding and manoeuvring the ship'. Since the information displayed must be secondary to, although supportive of, these tasks, no information should be displayed which 'is beyond the operator's human capability to comprehend • . . instantly and simultaneously'. In order to achieve this, it follows that 'information supplied should be easy to absorb and clear in all r e s p e c t s . . . ' . Osga ~5lists some problems that were found from a study of software used aboard the Navy of the United States of America. Not all are relevant to acoustical displays but the following points are applicable: Users must memorize numerous ... procedure outcomes. Visual feedback ... is non-existent. No help information on-line. It is also helpful to reiterate Osga's comment that 'related information is divided among numerous small displays' and not integrated to support decisions, since this is pertinent to the problem of interpreting the echosounder, sonar and netszonde simultaneously in pelagic trawling• A corollary to this is the point made earlier by Harre 6 that in order to integrate information between displays, it is necessary 'for certain items of equipment to talk and listen to each other'. If this were possible then integration of information could reduce the number of displays which at present are necessary.

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Usability problems of acoustical fishing displays: S Mills Pfendler and Widdel t6 conducted an experiment on a ship which found that a colour display 'diminishes the vigilance decrement in a monotonous monitoring situation'. In other words, colour has a stimulating effect and helps to prevent 'missed signals' but, as the authors point out, colour-coding, which is central to echosounders, can lead to interference from different colour hues. In addition, Christ ~7 showed that the most effective technique for aiding visual searching is colour coding. A prolonged discussion of the use o f colour, particularly for coding, is beyond the scope of this paper, but we should note that there are conventions ~8 which associate certain colours with certain actions. For example, red is used for dangerous actions while yellow indicates caution with a less dire consequence from the action. As we have seen, the colours used for coding the returning impulses from echosounders are generally standardized with red representing the strongest signal and blue the weakest. This follows the convention of red being a 'strong' colour while blue is a 'weaker' one. It can be seen that the interface of echosounders is heavily dependent on colour for easing identification of targets and on training and experience for interpreting the output beyond the basic analysis through colour. Another important aspect is that of the user's knowledge of the waters in which the echosounder is being used, since this can help with identification. However, from the foregoing discussion, some general principles, which are not intended to be totally exclusive, can be formed which should be followed in order to achieve an ergonomically acceptable interface. It should be noted that there are no legal requirements for the usability of the design of an interface for an acoustical system, although there are standards for the electronic component design.

DESIGN PRINCIPLES FOR THE INTERFACE OF ACOUSTIC

DISPLAYS

It is assumed throughout this section that a transducer of sufficient quality is used in order to achieve returning signals of sound integrity.

Principle 5 An on-line help should be included where appropriate. ~5

Principle 6 Operational errors should be rectifiable without con15 sequence.

Principle 7 Users should be able to master the system and use it effectively. ~9

Principle 8 Users should be given adequate training to use the system effectively and efficiently. ~9

Principle 9 Colour should be coded to convention and used consistently regardless of manufacturer.~8

Principle 10 Principles of informational ergonomics should be applied concerning readability and comprehensibility, t4 It is possible to indicate whether or not acoustical aids of the type carried by single-ticket fishing vessels satisfy these Principles and, as such, are ergonomically acceptable by using an heuristic analysis 2° on a typical interface such as that shown in Figure 2. This was carried out by one expert j3 applying the above principles to ten echosounders, two sonars and one netszonde on different small fishing vessels. The 'expert' had received 14 hours of training in interpreting an acoustical display from two skilled users before conducting the analysis, which was carried out using typical displays from the 13 pieces of equipment. Since the output from all of these is typically the same, it is not surprising that the resulting compliance of the principles was independent of whether the display was from an echosounder, sonar or netszonde. Thus for the 13 displays, the results can be summarized in Table 1, from where it is apparent that there is a need to improve the usability of acoustical displays with particular

Table 1 Findings from an heuristic analysis

Principle 1 The presentation is of great importance when decision making is necessary from the interpretation of the information displayed, s

Principle 2

Principle Compliance of aid Comments 1

2 3

Information must be within the user's capability to comprehend instantly and simultaneously. ~4

Principle 3 Information supplied should be easy to receive and absorb and clear in all respects) 4

Principle 4

9

Presentation of information should be consistent between displays regardless of manufacturer. 6

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10

Difficulties remain with identification of actual output Output is not understoodinstantaneouslyand No other tasks can detract from understanding the output Output is not clear from the interpretative No viewpoint All relevant aids use the samecolour-coding Yes No on-line help exists for operation No Yes with Proviso Proviso: errors are rectifiable only if known Users do not feel that they are 'masters' o! No the system and effective interpretation of the data is difficult to achieve In most cases, users receive no training, No generally although training is available from certain manufacturers but outside the UK Colour-coding convention is consistently Yes used Systems' interfaces lack comprehensibility No Yes

Usability problems of acoustical fishing displays: S Mills respect to comprehensibility (Principle 2 and Principle 10), interpretation of the data displayed (Principle 3) and on-line help for operational tasks (Principle 5). In addition, while the systems are used, users could feel that they were not 'master' of the system as they could be missing information through weak interpretation and comprehension skills (Principle 7). Clearly training would alleviate most of the problems, but such training must be readily available in order to be cost-effective. Even so, the problem of interpretation of colour-coded data will only be helped, not completely solved, by training and experience. One of the problems associated with training is that single-ticket fishing skippers are generally self-employed and to undertake training which is not required by law is seen as an unnecessary cost and loss of time which could be spent fishing. The best solution for the future is to use some form of distance learning, such as videos and on-line tutorials, which the skipper could use in his spare time. The latter suggestion has yet to be developed, but the former is used for advertising in an educational way by at least one sonar manufacturer and this could be extended further into a full tutorial. So far in this paper, the output signals from the echosounder, sonar and netszonde have been considered synonymously but in very recent years, two manufacturers have attempted to solve the interpretative problems of the netszonde by introducing a more complicated system of transducers on the trawl which removes the traditional acoustical output on the display and replaces it with a graphical representation (object coding) of the trawl which can, amongst other features, interface on to the electronic chart to give its position as well as incorporate data from the sonar which is used for identifying fish outside the net. These systems, which are too expensive at present (1995) for use on single-ticket vessels, claim increased usability over the traditional netszonde, supported by the sonar and echosounder, so it is pertinent to consider these developments further.

MANUFACTURERS' DEVELOPMENTS IMPROVING USABILITY

FOR

The integrated trawl instrumentation system, as one of the modern developments of the netszonde is often known, utilizes two sensors for monitoring the height of the net, two sensors for monitoring the spread of the trawl doors and the trawl wings respectively, a depth sensor, a temperature sensor and three sensors for monitoring the catch in the mouth of the trawl, midway to the codend and in the codend itself (see Figure 1). These sensors allow the skipper to know the distance and direction of the trawl from the vessel and, by interfacing to the Global Positioning System (GPS), the trawl system will reproduce its position in latitude and longitude on the video plotter. This is very useful for avoiding wrecks and other hazards and, as importantly, the skipper always knows where the trawl is. Even under difficult operating conditions, the system will automatically monitor the trawl so that the trawl's opening and the distance between its doors are optimal for the best catch. This in turn reduces towing time and fuel costs as

well as increasing the catch. Thus the skipper always knows the exact position of the trawl, how much fish is in the trawl and the mode of its behaviour. It is worth noting that the system maximizes the catch for both demersal and pelagic trawling since the height and depth sensors give the information for the skipper to position the net exactly where he wishes -- either on the seabed or in the right midwater position. The temperature sensor can be used to follow a particular thermoclime if required. Figure 4 shows a typical output on the VDU which bears little resemblance to, and is obviously more natural than, a traditional acoustical display. Clearly, these systems have progressed well beyond the colour-coded displays from a traditional netszonde which most skippers still use. We saw above that acoustical displays did not satisfy those Principles concerned with comprehensibility, interpretation and the user feeling 'in control' of the system. Although it is still too early to obtain definitive evidence since the systems are not yet widely used by single-ticket skippers, first tentative feedback is suggesting that users are finding the trawl system much more comprehensible and easier to interpret as the output is a realistic diagram 2~. This also increases confidence so that the skipper feels more 'in control'. Thus we can see that Principles 2, 3, 7 and 10 are being addressed and seem likely in the near future to be satisfied. While training is still not readily available by distance learning, the new trawl system reduces the need for interpretative training so that indirectly Principle 8 will become less significant. On-line help has still not been addressed. It is not only in trawl systems that technological developments have taken place. Sonars are now available which are omnidirectional and tilt from + 10 degrees to - 9 0 degrees, taking the horizontal as zero degrees. In addition, full coverage from port to starboard is possible and beams can be trained on any intermediate position. The sonars most suitable for single-ticket vessels use a 14in VDU and have a maximum range of 1700 m. A beam of 12 degrees gives better resolution and a frequency of 57 kHz allows good short-range coverage. The spherical transducers utilized are ceramic and give the same beam pattern in every direction without secondary (side) lobe distortions. Digital-controlled transmission is used to reduce side lobes

COURSE:

32"

SPEED:

3.2KI1

POS:

59 24.72 N 010 29.12 E

90/10/29

16:40:10

NORMAL RATE MAX ACTIVE SENSORS MANUAL INPUT AUDIO ALARM SYSTEM SETUP

368" 1 2 " 3 m / m ~

11.6m SOFT

HARD

7.8 Min

Figure 4 Typicaloutputfroman integratedtrawl instrumentationsystem (Reproducedby courtesyof Simrad)

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Usability problems of acoustical fishing displays: S Mills while new frequency correlation filters and interference filters considerably reduce other noise, especially from other local sonars operating around the same frequency. Frequency modulated transmission and receiving, reverberation controlled gain and a new ping-to-ping filtering technique all aid the reduction of reverberation echoes. The sonar utilizes a long hoist length which protects the system from noise and turbulence generated by the vessel itself and poor weather. The system interfaces with the integrated trawl system discussed above, the echosounder (from the same manufacturer), the wind sensor and the GPS, while the target tracking output from the sonar can be transmitted to the video plotter. These improvements are laudable and, while they considerably decrease the risk of misinterpretation of the sonar output, they do not remove the basic lack of knowledge required to interpret the output. Thus while Principle 7 may be better satisfied, the other Principles concerned with training and comprehensibility of output may still remain problematical. The provision of on-line help still has to be addressed. It must be stressed, however, that these systems are still very new and data are not yet available in sufficient quantity to enable us to be precise in evaluation. As with sonars, technology has improved the performance of echosounders so that single-fish detection is possible as well as good bottom detection and discrimination. Echosounders exist which claim to calculate the fish size, abundance and movement together with the distribution of fish by weight and abundance. These echosounders require the use of split-beam technology at, say, 18 kHz, 38 kHz, 70 kHz and 120 kHz, which shows the fish size in a selected layer (made possible by the different frequencies) and the fish location in the acoustical beam, thus optimizing the chance of navigating the trawl into the correct position for capture. As with more traditional echosounders, the latest models use split-screen technology to allow the presentation of different frequencies in up to 12 different colours for easier identification of fish. In addition, the output now includes digital presentations of fish abundance as well as the traditional presentation of sea depth. The headrope and footrope of the trawl can be shown together with layer markers for the size distribution range of the fish. Interfacing is possible to the sonar (for adding depth), together with two-way information with the trawl system, and information from the video plotter can be received by the echosounder for display or printout on a colour or monochrome printer. This allows later comparison of catch sizes with location. These improvements in facilities and clarity of presentation are to be commended, but as they have not yet been in use long enough for reliable feedback from independent testing, it is difficult to be precise or conclusive as to their improvement in comprehensibility and interpretation. Certainly, the better quality of display and the different layering allowed through split-beam technology must improve interpretation but the fundamental problems associated with lack of training still remain. The possible easing of the situation by on-line help has not yet been addressed. Thus, as with the new sonars, Principle 7 may be better satisfied but it is too early to be conclusive about other improvements.

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It is worth noting that in other applications, such as seabed mapping, such techniques as using artificial neural networks have been profitable in easing interpretation problems of acoustical displays 22 but these are costly, if only in terms of the neural software, and are likely to remain so, in the immediate future at least. Consequently, it is unlikely that this approach will help with interpretation in the near future as the cost places it beyond the purchase of a skipper of a small fishing vessel. Indeed, with this last point in mind, one manufacturer has just (1995) produced a low-cost netszonde monitoring system which filters out the returning primary and secondary signals and stores these digitally for display and information retrieval purposes. That these problems are being addressed in general and technology is facilitating more information at an increasingly cheaper rate, is very pleasing and augurs well for the future of alleviating the problems of comprehensibility and interpretation.

CONCLUSION It has been shown by means of an heuristical analysis that traditional acoustical displays used in small fishing vessels still do not satisfy the basic principles of interface design given above and the users of the resulting systems experience problems of interpreting the data shown on the display. Technology now exists to eradicate the problems of transmission such as noise, and some manufacturers are using this advanced technology to object code the data rather than use the traditional colour coding. It is too early in the development of such systems to state that object coding will provide a total solution to the interpretative problem but early indications show an improvement in satisfying the basic design principles.

REFERENCES 10lsen "s Fisherman's Nautical Almanack. E T W Dennis, Scarborough,

1994 2 Azhazha, V G and Shishkova, E V Fish Location by Hydroacoustic Devices. Israel Program for Scientific Translations, Jerusalem, 1960 3 Yudanov, K I Interpretation of Echograms of Hydroacoustic FishFinding Instruments. Israel Program for Scientific Translations, Jerusalem, 1967 4 FAO Fishing Manuals Echo Sounding and Sonar for Fishing. Fishing News Books, Farnham, UK, 1980 5 Mitson, R B Fisheries Sonar. Fishing News Books, Farnham, UK, 1983 6 Harre, I 'Multi-beamechosoundersfor inshore to high-seasapplications and the related data processing--anequipment survey'. HYDRO "92, Eighth Biennial International Symposium of The Hydrographic Society,

Copenhagen, 30 November-3 December 1992 7 MacLennan, D N and Simmonds, E J Fisheries Acoustics. Chapman and Hall, London, 1992 8 Wilkinson, G R 'Ergonomics in Ship Design' J. Nay. 1974, 27(4), 471-478 9 EEC Directive dated 29th May 1990, 90/270/EEC 10 Draft International Standard. ISO DIS 9241--11. Ergonomic Requirements for Office Work with Visual Display Terminals (VDTs):--Part 11: Guidance on Usability, 1994 11 Shackel, B 'The Concept of Usability'. Proc. IBM Software and Information Usability Symposium, 1981, 1-30 12 Booth, P An Introduction to Human Computer Interaction. Lawrence

Erlbaum, Hove, 1989 13 Dix, A, Finlay,J, Abowd,G and Beale, R Human Computerlnteraction. Prentice Hall, London, 1993

Usability problems of acoustical fishing displays: S Mills 14 Abramowski, C 'An Automated Integrated Ship's Bridge Layout'. J. Nay. 1976, 29(1), 82-89 15 Osga, G 'User-Computer Interface Issues for Future Ship Combat Consoles'. Proc. Human Factors Society 33rd Annual Meeting, 1989, pp. 1079-1083 16 Pfendler, C and Widdel, H 'Vigilance performance when using colour on electronic displays'. Percept. Motor Skills 1986, 63, 939-944 17 Christ, R E 'Review and analysis of colour coding research for visual displays'. Human Factors 1975, 17, 542-570 18 Widdel, H and Post, D L Color in Electronic Displays. Plenum Press,

New York, 1992 19 Health and Safety Executive Display Screen Equipment Work, Health and Safety (Display Screen Equipment) Regulations 1992, Guidance on Regulations L26. HMSO, London, 1992 20 Lingaard, G Usability Testing and System Evaluation. Chapman and Hall, London, 1994 21 Mair, J 'Skippers visit Scanmar'. Fishing News 1994, 4202, 10-11 22 McCleave, B W, Owens, J K and Ingles, F M 'Analysing depth sounder signals with artificial neural networks'. Sea Technol. 1992, 33(3), 39-42

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