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Large And Small Organisms Detected In The Open Waters Of Loch Ness By Dual Beam Acoustics


Reproduced with the permission of the Scottish Naturalist
Copyright: May be used for private research. All other rights reserved

By JAN KUBECKA
Hydroacoustic Unit, Department of Biology,
Royal Holloway University of London

ANNIE DUNCAN
Hydroacoustic Unit, Department of Biology,
Royal Holloway University of London

and ALAN J. BUTTERWORTH
National Rivers Authority, Thames Region

Introduction

 A great deal of what is already known about the biology of Loch Ness derives from Sir John Murray and Laurence Pullar's Bathymetrical Survey of the Scottish Fresh-Water Lochs (Murray and Pullar, 1910) and from Dr. Peter Maitland's studies in 1977-80 on the ecology of Scotland's five largest lochs (Maitland, 1981).  Although he recorded seven species of fish in Loch Ness, Maitland suggested that only three were predominant in the open waters of the loch - Charr Salvelinus alpinus, Brown Trout Salmo trutta and Salmon Salmo salar - all largely in the top 30 m.  More recently, Shine and Martin (1988), of the Loch Ness and Morar Project, provided some evidence of the presence of fish in the profundal zone by netting Charr from 200 m deep with the profundal bivalve, Pisidium conventus, in their guts.

With the advent of commercially available scientific echo-sounders in the 1980s, the study of fish in large deep lochs was suddenly made much easier.  These instruments applied the physics of sound to be able to discriminate between 'fish' and other targets reflecting back sound echoes, to 'size' the echoes, and to count them with confidence in a known volume of water.  By various means, the position of the 'fish' target in the sound beam could be determined.  The volume of water which could be sampled for fish targets was very large indeed, - in the May 1992 survey it was about 5% of the volume of the pelagic region of Loch Ness down to 100 m depth.

Vol 105, The Scottish Naturalist: Organisms Detected in Loch Ness by Dual-Beam Acoustics p180

When Royal Holloway University of London acquired a BioSonics Model 105 Dual-Beam Echo-Sounder, we used it to carry out several preliminary acoustic surveys in Loch Ness for the Loch Ness and Morar Project, and were thrilled to find that it could be used to provide an instantaneous acoustic picture of the horizontal and vertical distributions of the fish populations, as well as size composition, in some two days of field work.  The present account outlines some of the difficulties encountered, but there can be no doubt about the power of this new tool in the hands of ecologists.

 

Acoustic Methods

Of the three preliminary acoustic surveys carried out in Loch Ness (May  1991, October 1991 and May 1992), the May 1991 survey was the best, and it is this survey which will be most discussed.  Figure 3a (5K) shows the position of the transects which were surveyed during two consecutive days.  Each transect was surveyed by three runs, one pelagial in 200 m of water and two runs over the basin walls (down to 70 m depths), in which different ping rates were applied, 2.0 pings per second  in the deeper water and 5.0 pings per second in the shallower depths.

The BioSonics Model 105 Echo-Sounder was operated from a Dory-17 boat (Figure 1, 17K photo) with a 420 kHz transducer suspended from a towing body (Figure 2, 23K photo) at 2.0-3.0 m depth and beaming vertically down to 70-100 m, at which depth the signal-to-noise ratio became too low.  Pulse durations used were 0.8 ms and single targets were identified by selection criteria.

The instrument was calibrated in May 1991, by using a long-life ping-pong ball (-42 dB) suspended below the transducer, and re-calibrated  later using a  tungsten-carbide  standard  target (-43.7 dB).  The noise thresholds applied were generally very low in depths down to 50 m, thus allowing us to record acceptable targets down to -81 dB.

The size and densities of single targets were processed using the BioSonics Echo-Signal Dual-Beam Processor (ESP_DB), whilst the total acoustic biomass was determined by the BioSonics Echo-Integration Processor (ESP_EI).  The size analysis of single targets showed a predominance of very small targets, and we made an attempt to discriminate between these tiny targets (or scatterers) and real fish targets by using two kinds of integration strata simultaneously.  The two strata were the primary, which integrated all targets from -81 dB to the largest target (fish plus the scatterers of the scattering layer), and the secondary strata, which integrated those targets larger than -55 dB (fish only).

Vol 105, The Scottish Naturalist: Organisms Detected in Loch Ness by Dual-Beam Acoustics p181

To convert echo-integrated acoustic biomass from volts squared to the number of fish individuals requires a scaling factor, which is influenced by the size of the average back-scattering cross-section of the targets.  This was 2.974*10 -8 m2/ individual for the scatterers at all depths (0-50 m) and 1.84*10-5 m2/individual for fish targets in different depths.  These values result in widely differing scaling factors (Figures 8, 7K graph and Figure 10, 7K graph).

The biomass of the fish targets was calculated using the echo-integrated squared voltages divided by a mean back-scattering cross section of 4.27*10-4 m2/kg.  This scaling factor comes from stunned Trout of various sizes exposed in front of a 420 kHz transducer (unpublished data).

 

Results and Discussion

Dual-Beam Echo-Counting and Fish Sizing

Despite the reputation of Loch Ness, the densities of pelagic targets of the size of fish were surprising low, if one takes a fish target to be >-55 dB or larger than 4.0 cm length (Love, 1971).  When the whole size composition of acoustic targets in the open water of the entire loch is considered, the undoubted fish targets contributed a very small proportion (Figure 4, 6K).  In the shallower, rather sheltered and more productive Urquhart Bay, on the other hand, the proportion contributed by fish was strikingly greater.  The average acoustic size of fish in open water of the loch was -47 dB, which, according to Love (1971), is equivalent to a length of 8.5 cm.

In every transect, many hundreds of scatterer targets were detected (Figure 3b, 8K) whose target strength ranged from -79 dB to -73 dB in 1991 (Figure 4) and -74 dB to -70 dB in 1992 (Figure 5, 6K), concentrated in the depths between 12 m and 30 m.  No-one yet knows what these are, although they are most likely to form the scattering layer described by Shine and Martin (1988).

For May 1992, Figure 6 (11K) provides a three-dimensional diagram of the frequency of target strengths of all targets in every transect from north to south.  As above, this figure also shows the small proportion of fish targets and the consistency of the peak target throughout the entire 23-mile loch, but with a tendency towards an increased number of targets per transect from north to south.  This finding corresponds to earlier acoustic records of increased acoustic counts in the South Basin, as determined by the simultaneous deployment of seventeen vessels during Operation Deepscan in October 1987 (Shine and Martin, 1988).

Vol 105, The Scottish Naturalist: Organisms Detected in Loch Ness by Dual-Beam Acoustics p185

Figure 5 compares the target strengths of the loch's scatterer targets with a 55 mm Three-spined Stickleback Gasterosteus aculeatus suspended in the scattering layer itself.  Even such a small fish, were it in the full pelagial, would be a much bigger target than the scatterers.  It is known that young Charr inhabit the open water, since in November 1992 Charr as small as between 3.5 mm and 7.0 mm were caught by mid-water trawl (Shine, Kubecka, Martin and Duncan, 1993).  However, sampling the scattering layer depths by ichthyo-plankton tow net (1.0 m diameter; 1.0 mm mesh size) in May 1992 caught just one Lamprey Lampetra fluviatilis larva, one small Charr and chironomid pupae at densities of about 0.1 individuals/m3.

Larger targets are also present in Loch Ness; between 67 m and 85 m depth of transect 17 a large shoal of large targets was detected (Figure 7, 8K chart).   The shoal was so dense that most of the targets were not single.  Tracked single targets at the edges of the shoal are shown and these had target strengths of about -35 dB to -24 dB, probably with a maximum length of one metre!  Such compact shoals of very large fish might quite well have caused earlier mis-identification of Monsters.

 

Echo-Integrations for Biomasses 

Figure 8 shows a marked trend of increasing fish density from north to south.  The same trend can be seen when the acoustic density is expressed as fish biomass (Figure 9, 7K).  The density distribution of total targets (>-81 dB or greater) was calculated by applying one scaling factor, that for small scatterers, since these were much more numerous than fish targets; the density distribution of these total targets also showed a similar north-south trend, but much less pronounced (Figure 10, 7K).

Simple subtraction of fish volts squared from total volts squared was too crude a process for estimating the density of the loch's scatterers.  This is probably caused by the highest densities of scatterers appearing as multiple targets of voltage large enough to be taken by the Echo-Signal Processor as fish targets, something which occurs more frequently in the more southern transects.  The bias caused to estimates of fish abundance by identifying and incorporating small scatterers as multiple targets is not too serious, because of their very small 'acoustic weight', and Figure 9 (7K) can be accepted as an upper estimate of fish abundance down to 60 m depth in transects along the loch.

Vol 105, The Scottish Naturalist: Organisms Detected in Loch Ness by Dual-Beam Acoustics p192

An average depth distribution of the so-called 'plankton densities',  arbitrarily defined as targets between -75 dB and -55 dB, is illustrated in Figure 11 (5K), and shows doubled densities between 12 m and 22 m.  These densities were calculated as the difference in volts squared between total V2 and fish V2 and converted to numbers/m3 by a 'scatterers' scaling factor.  Densities of fish targets (>-55 dB), given as number/ha for each 2.0 m depth stratum (Figure 12, 4K), are also concentrated between 12 m and 28 m, with few in the top 10 m water layers.  These were scaled by the same back-scattering cross section at all depths.  In the absence of better information on the identity and target strengths of the small scatterers and the smaller pelagic fish, we cannot fully exploit the flexibility of BioSonics software to discriminate further between these two classes of acoustic targets in Loch Ness, or to better calculate their relative biomasses.  The actual densities shown in Figures 11 and Figure 12 should be considered to be relative rather than absolute, because in each class of targets the span in acoustic size is very great (the dB scale is a logarithmic one), so that the use of an average back-scattering cross section can only be crude. 

Summary

 1.  The entire 23-mile length of Loch Ness was surveyed acoustically in May 1991, in October 1991 and in May 1992 using a BioSonics Echo-Sounder with an operating frequency of 420 kHz.  Detailed results are given of the May 1991 survey, since this was the best and involved acoustic runs over the pelagic region (200 m depth) in 22 transects.

2.  Acoustic densities of fish targets (>-55 dB) in the pelagic region of the loch were low, whereas there were hundreds of small scatterers (-79 dB to -73 dB).  The average acoustic size of fish was -47 dB.  The mean density (SD) of fish targets in the loch pelagial was 97 91 fish/ha down to 60 m depth.  The mean biomass of these fish was 4.23 4.00 kg/ha, giving a rough average individual size of 43.6 gm.

 3.  The density per transect of total targets and of fish targets increased from north to south.

 4.  The average loch depth distribution of both scatterers and fish targets peaked at between 12 m and 22 m for the former and between 12 m and 28 m for the latter.

Vol 105, The Scottish Naturalist: Organisms Detected in Loch Ness by Dual-Beam Acoustics p193

Acknowledgements

We should like to express our grateful thanks to Royal Holloway University of London and the National Rivers Authority for permission to use the sonar systems.  We should also like to thank the Loch Ness and Morar Project team for our work-time on their boat Ecos, and for their hospitality during our visits; also Mr. R. Bremner of Drumnadrochit for the loan of his splendid Dory-17 boat in May 1991.

References

Love, R.H.  (1971).  Dorsal-aspect target strength of an individual fish.  Journal of the Acoustical Society of America,  49: 816-823.

Maitland, P.S. (Ed.) (1981).  The Ecology of Scotland's Largest Lochs:  Lomond, Awe, Ness, Morar and Shiel.  Monographiae Biologicae, Vol. 44.  The Hague: Junk.

Murray, J. and Pullar, L. (Eds.) (1910).  Bathymetrical Survey of the Scottish Fresh-Water Lochs.  Vols. 1-6.  Edinburgh: Challenger Office.

Shine, A.J., Kubecka, J., Martin, D.S. and Duncan, A.  (1993).  Fish habitats in Loch Ness.  Scottish Naturalist, 105: 237-255.

Shine, A.J. and Martin, D.S.  (1988).  Loch Ness habitats observed by sonar and underwater television.  Scottish Naturalist, 100: 111-199.

 

Received May 1993

 

Dr. Jan Kubecka, Hydrobiological Institute,
Czech Academy of Sciences, 370 05 Ceske Budejovice, Na Sadkach 7, CZECH Republic.
Present address: Hydroacoustic Unit, Department of Biology,
Royal Holloway University of London, EGHAM, Surrey TW20 0EX.

Dr. Annie Duncan, Hydroacoustic Unit, Department of Biology,
Royal Holloway University of London, EGHAM, Surrey TW20 0EX.

Dr. Alan J. Butterworth, National Rivers Authority,

Thames Region, By-pass Road, GUILDFORD, Surrey GU1 1BZ.

 

 

 

 

Loch Ness Dual Beam Acoustics, Large and Small Organisms