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Spatial Distribution and Diurnal Migration of the
Pelagic Fish and Zooplankton in Loch Ness

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

By
ADRIAN J. SHINE

Loch Ness and Morar Project 

DAVID S. MARTIN
Loch Ness and Morar Project 

ROSALIND S. MARJORAM
Loch Ness and Morar Project 

Introduction

The first suggestions of an off-shore population of Charr Salvelinus alpinus in Loch Ness were made as a result of echo-sounding by Dr. P.F. Baker (Baker and Westwood, 1960).  Echo-sounder transects were also made as a part of the Institute of Terrestrial Ecology's multi-disciplinary survey during 1977-80 (Maitland, 1981). 

Throughout the 1980s, the Loch Ness and Morar Project conducted a pelagic programme of qualitative echo-sounding, gill-netting and trawling, designed to reveal the basic distributions of fish within the water column and along the axis of the loch (Shine and Martin, 1988).  Species taken by pelagic gill-netting and trawling consist mainly of Charr, with some Brown Trout Salmo trutta and, particularly in the southern basin, Three-spined Sticklebacks Gasterosteus aculeatus.  From 1988 onwards, more quantitative acoustic methods have been applied, which have improved our understanding of the distribution, numbers, biomass, and diurnal behaviour of the fish population. Observations have also been made of the vertical and horizontal distribution of zooplankton. 

Most of these exercises were carried out as part of 'Operation Echo', a series of collaborations with the Simrad Company and the Marine Laboratory of the Department of Agriculture and Fisheries (D.A.F.S.) at Aberdeen.  Simultaneous surveys were also carried out by the Hydroacoustic Unit from Royal Holloway University of London, and are described separately (Kubecka, Duncan and Butterworth, 1993).

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p197

Methods 

The data presented here has been obtained from several sources: from a fixed station established on deep-water moorings, and from various longitudinal and transverse runs with a variety of vessels (Figure 1, 9K). 

Gill nets, ranging in mesh size (stretched diagonal) from 30 to 90 mm, have been deployed from the fixed station to depths of over 30 m. 

Most trawling was carried out in 1988 by the D.A.F.S. trawler Goldseeker, using a 'sprat' trawl with a 12 mm codend.  The trawl was towed at 20-30 m depth at 2.0 - 2.5 knots with a vertical mouth opening of approximately 6.0 m.  Depths were established by a Simrad Trawlink acoustic system.  There were four hauls, totalling four hours of trawling. 

In November 1992, material was made available from a large trawl undergoing configuration trials by D.A.F.S. aboard the research vessel Calanus. 

Plankton observations are based for the most part upon vertical hauls at 6.0 m increments using a closing plankton net with a 28 cm diameter, one metre in length with a 0.25 mm mesh size.  Hauls were taken around mid-day and midnight.  Numbers are expressed per cubic metre, with no allowance for any net factor.  The hauls presented for 1983 were taken with a 31-litre Patalas water sampler. 

Longitudinal results are from the Royal Holloway Clarke-Bumpus sampler (15 cm aperture) towed at 20 m depth for distances of one nautical mile, separated by intervals of one nautical mile. 

A Focal Industries Optical Plankton Counter, model OPC-IT, was used in 1990 to investigate particles within the scattering layer and water column.  This had a sampling aperture of 3.0 x 22 cm and a 640 nanometer light source with beam dimensions of 0.4 x 2.0 cm. 

Particles were detectable from 0.25 mm to 2.0 cm, and counts were made by towing over one minute intervals at depth increments through layers detected acoustically.  A D.A.F.S. high-speed plankton net was attached to the towed body, and depths were measured using a Simrad H.P.R. 310T transponder. 

Temperature structure was measured by a variety of instruments, ranging from a Windermere Profiler to a Lowrance single thermistor on a depth-marked wire.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p198

 

Acoustic Methods

Most fixed station observations were made using conventional chart-recording single beam echo-sounders.  A Lowrance X-16 (50 kHz) was used from the fixed station in the North Basin in 1991 and 1992.  This has the facility to reduce paper speed, to the extent that one hour could be represented by 2.0 cm of chart, thus permitting monitoring over several days to be presented in a manageable form. 

Much of the basic distribution of targets within the water column may be overviewed in this way.  There is, however, a difficulty in quantifying the numbers of fish and their sizes.  Even echo-sounders capable of target strength measurement with a single beam cannot quantify the true target strength, since a small fish in the centre of the beam may appear stronger than a large fish off axis.  Acoustic surveys using echo-integration therefore required scaling factors derived from fish catches. 

In the 1980s three new acoustic methods capable of in situ target strength assessment were refined, and between 1988 and 1992 the Project was fortunate to participate in the first practical use of these systems in Britain's freshwaters.

All three systems exploit sophisticated software processing, in particular to apply 'single fish echo criteria'.  This criteria rejects echoes below a given noise threshold and those of short length likely to be noise.  In addition, echoes larger than the criteria at various points along the peak are classified as multiple targets.  All systems utilise echo-integrators to total the acoustic biomass, while relating the measured target strength distribution to this.  The differences lie in the way this target strength distribution is obtained; detailed descriptions and comparisons of the three systems are given below:

 

1.  H.A.D.A.S. Hydroacoustic Data Acquisition System.

See Craig and Forbes (1969).  Figure 2a.(4K)

This is an indirect statistical method for use with a single beam echo-sounder.  Developed by Dr. Torfin Lindem of Oslo University, the software package utilises the algorithm proposed by Craig and Forbes (1969) to remove the effect of the beam pattern from the received echoes.

In general, the echoes are divided into classes, with the strongest assumed to come from the largest fish in the centre of the beam, with the second strongest coming from the second largest fish in the centre, together with the largest fish in the first off-centre classification.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p200

This process is applied progressively throughout the various size classes, and therefore requires a large number of echoes (1,000+) and a good percentage of targets passing the single fish criteria.  The proportions between resolved single fish and multiples are established through echo-integration, thus allowing the total density to be calculated.  The equipment was originally developed for the Simrad EY-M sounder, and has been used in a number of surveys (Jurvelius, Lindem and Louhimo, 1984;  Hartmann et al., 1987).

On 24th July 1988 Dr. Lindem brought an advanced version of H.A.D.A.S. to Loch Ness, where it was linked to a Simrad EY200 sounder (49 kHz) calibrated with a copper sphere.  The equipment was mounted with an in-hull transducer aboard the Simrad vessel Simson Echo, and two runs of approximately 5.0 km were made in the South Basin between Fort Augustus and Invermoriston.

The first run was by day at 15.00 hrs and the second at dusk (20.00 hrs).  Single fish resolution was mostly 90-100% and never below 70%   By day 4,790 single fish echoes were resolved, and by night 1,600.  Trawling was carried out simultaneously in the same area by the Goldseeker.

 2.The Dual Beam Method (Biosonics Inc.).

See Ehrenberg (1978); Traynor and Ehrenbrg (1979).  Figure 2b (4K)

This is a direct acoustic method of target strength determination, transmitting pulses on a narrow beam element and receiving them on both this transducer and through a wider surrounding beam.  The ratio between the signals received by the two beams provides the off-axis angle of the target.  The Royal Holloway Hydroacoustic Unit brought a Biosonics 105 Dual Beam sounder to the loch in May 1991, October 1991 and May 1992.  During the May visits the equipment was used simultaneously with the EK500 Split Beam equipment described below.  The Dual Beam findings are described in an accompanying paper by Kubecka et al. (1993).

 

3. The Split Beam Method (Simrad EK500).

See Bodholt, Nes and Solli (1988); Ehrenberg (1979).  Figure 2c (4K)

This is another direct method of in situ target strength assessment, and relies upon the phase difference of echoes received by separate elements of the transducer, thus locating the target within the beam and compensating accordingly.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p201

The Simrad EK500 utilises four transducer elements, and was first used at Loch Ness with a hull-mounted transducer aboard the Simson Echo in July 1990.  Subsequently it was installed with both 38 kHz and 120 kHz transducers rigged at 1.0 m depth aboard the Project vessel Ecos in May 1991 and 1992.

In May 1992 a series of 0.5 nautical mile runs, totalling 10.5 n-miles, were made in the North Basin both by day and at approximately midnight.  The distributions quoted are the mean of these runs.

On 24th May a length run was made along the axis of the loch.  The EK500 results are produced through a colour printer which tabulates target strength distributions within chosen depth layers.  The number of accepted single fish echoes is also recorded, together with the SA (area back-scattering coefficient) or integrated value expressed in m2 per n-miles2.

In the estimation of fish densities the sigmas (linear values of back-scattering cross-section for individual fish) of targets in the logarithmic dB scale are calculated thus:

ó= 4pi (10. TS/10)

Where TS is target strength.

The percentage of sigmas per target strength group is then divided into the SA to give fish per n-mile2 and converted into fish/ha.

Where fish lengths are suggested they are based upon the formulae of Lindem (1984):

TS = 20 Log L-68   for small fish

TS = 20 Log L-67   for larger fish

Where L is fish length in cm.

On occasion, Love's (1977) empirical formula is also used:

TS = 18.4 Log L -1.6  Log F -61.6

Where L is fish length in cm and F is acoustic frequency in kHz.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p202

 

 

Results

Fish

Figure 3 (13K) provides a backdrop to the summer vertical and diurnal distributions.  The record was taken from a fixed mooring in the North Basin over a 24-hour period, and shows individual fish targets ranging to 30 m throughout the day.  A well defined 'scattering layer' makes regular migrations nearly to the surface at midnight.  The layer appears particularly dense at depth within the 33oecho-sounder beam, but as it approaches the surface in the narrower part of the beam it can be seen to be composed of targets which are detected as individuals.  The larger fish do not migrate to the surface at night.

System Comparisons

The H.A.D.A.S. and EK500 results quantify this process.  A comparison between the peak target strengths within the scattering layer shows the H.A.D.A.S. (49 kHz) at -54 dB, the EK500 Split Beam (38 kHz) at -63 dB, and the EK500 Split Beam (120 kHz) at -70 dB (Figure 4, 16K).  The Biosonics Dual Beam (420 kHz) peak was at -75 dB (Kubecka et al., 1993).

Of these results, the -54 dB and -63 dB are reasonably compatible with fish between 4.0 cm and 2.0 cm respectively.  The frequency of other targets within the distribution falls fairly steadily with increasing strength, and shows less variation between the systems.

Overall target strength distributions are shown in Figures 5a and 5b (12K).  The strongest target detected was -30.75 dB at 38 kHz on the EK500, thus suggesting a fish length of approximately 75 cm.   There is a secondary peak at approximately -40 dB to -45 dB (10-20 cm fish) on the H.A.D.A.S. and the EK500 (38 kHz).

 

Catch Data

A total of four hours of trawling in the scattering layer yielded 42 Charr of standard lengths between 4.0 cm and 27 cm, plus some Sticklebacks.  The hauls are combined in Figure 6a (20K graphs) and show a peak of around 10 cm, doubtless influenced by the selectivity of the sprat net. Figures 6b, 6c, and 6d (7-18K photos) picture the fish of the pelagic.  Gill-netting shows the maximum length of Charr to be approximately 30 cm, and the larger fish to be Trout, Figure 6d with the largest caught measuring 58 cm (Martin and Shine, 1993).

These larger fish could be expected to evade the trawl, while smaller ones could escape through the meshes.

Smaller fish were not taken until November 1992, when a very large trawl was undergoing trials from Calanus.  Due to the closing of the codend meshes by stretching and further obstruction by leaves, 62 Charr down to 3.5 cm were retained, together with similar quantities of Sticklebacks.  The 10 cm peak in the trawl samples would give target strengths of -48 dB (Lindem, 1984) or -45.7 dB (Love, 1977), which coincides quite well with the secondary peak in the target strength distribution noted above.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p210

Fish Numbers and Depth Distributions

The H.A.D.A.S. runs were made in the South Basin in August 1988, and showed total numbers of nearly 1,500 fish/ha (Figure 7a, 17K graphs).  The mean of the May 1992 EK500 runs in the North Basin showed a total of 340 fish/ha
(Figure 8a, 14K).  Both these estimates include scattering layer targets.  The remainder amount to no more than 20 fish/ha in the North Basin (EK500 38 kHz,  Figure 8b) and approximately 400 fish/ha in the South Basin (H.A.D.A.S. 49 kHz (Figure 7b).

The depth distribution shows all target strength classes to be most numerous around the area of the scattering layer, but with substantial numbers of the larger fish above it.

 

Vertical Migration

The dusk 5.0 km run with the H.A.D.A.S., and the EK500 midnight runs (Figures 9a and 9b, 12K graphs), clearly show the smaller targets migrating to the extent that they are almost absent (too shallow to be surveyed) in the EK500 records.The larger fish migrate to a much lesser extent; indeed it would appear that the migration becomes progressively less with increasing size.  It is interesting that, despite the drop in mean numbers from 341/ha to 31/ha at night, the EK500 SA figure (total integrated value) has barely changed, showing the very low 'acoustic weight' of the scattering layer targets (Figure 10).

 

North-South Density Gradient

The EK 500 length run, made in May 1992 from north to south along the axis of the loch, showed patchy numbers but a reasonably consistent target strength distribution.  There was a clear increase in fish numbers from north to south (Figure 11, 15K and Figure 11a , 25K charts).  This confirms previous qualitative observations throughout the 1980s, particularly in October 1987 (Shine and Martin, 1988).

While this run was being made, a Clarke-Bumpus zooplankton sampler was towed at approximately 20 m, close to the scattering layer.  Settled volumes of zooplankton also showed a definite increase towards the south.  A mean of only 8.0 fish/ha was found above the scattering layer, as opposed to 419 fish/ha within it.  However, a substantial portion of the biomass (mean SA 2.64) lies above the 20-40 m scattering layer depths (mean SA 6.39).  This again emphasises the low biomass of the majority of individuals in the scattering layer.

 

Overall Estimates

The patchy distribution makes overall estimates speculative.  The mean integration figure apportioned between the various size classes of the target strength distribution gives the numbers per hectare.  A mean weight can be estimated through the length/weight charts (Figure 6) by converting the target strength classes to lengths.  The mean weight is then multiplied by the numbers given by the integration.

Based upon the target strength distribution, given by the 10.5 nautical mile runs and the mean of the length run integration figures, the mean numbers are 427 fish/ha and the biomass 3.1 kg/ha.  Thus the resident pelagic fish stock would be 2,433,900 with a biomass of 17,670 kg.  This estimate is exceeded by its statistical confidence limits, because of the patchiness of the numerical distribution.

 

Zooplankton

In Figures 12a, 12b, 12c, 12d and 12e (14K graphs) plankton diurnal migration diagrams are presented, which show that from at least March onwards a pronounced vertical migration takes place involving the copepods Diaptomus gracilis and Cyclops strenuus abyssorum.  These are the dominant species.  The Cladocera are present in much fewer numbers, and Bosmina coregoni in particular appears at the greater depths (Figure 12b).  Daphnia hyalina appears to migrate to some extent while Bosmina does not.  The larger predators, such as Leptodora kindti, Bythotrephes longimanus and Polyphemus pediculus, have not been caught in sufficient numbers to be sure of their movements, although underwater camera work in 1992 showed diurnal migration in Leptodora.  In Lake Huron (Canada), however, Bythotrephes has been reported by Vanderploeg, Liebig and Omair (1993) to lie in a narrow diurnally-migrating horizontal band and to favour Cladocera as prey.

In Loch Ness, Cyclops has occasionally been observed to be concentrated at the surface by day, the reverse of the usual pattern (Figure 12b).  Total numbers are seen to increase as the summer advances.  Large variations also occur through horizontal transport, particularly due to internal seiche movements, for example in October 1985.  Larger numbers have been associated with denser scattering layers (Marjoram, 1993).  Levy, Johnson and Hume (1991) also report changes in fish distribution due to internal seiche.

By day, total zooplankton numbers are usually greatest just above the scattering layer, but sometimes coincide with it.  Migration of the copepod element of the zooplankton is very similar to that of the scattering layer.

During the length run (Figure 11, 15K), zooplankton numbers increased from north to south. All species were more numerous in the south, although there were differences in percentage composition.   Cyclops increased from 26% to 64% while Diaptomus decreased from 45% to 28%.  Bosmina decreased from 8% to 2%.

South Basin tows with the optical plankton counter within deep acoustic scattering layers (Figure 13a, 46K chart and graph) do show increases of particle numbers, mostly approximately 0.25 mm, within the layers.  However, the greatest peaks in particle densities can be from depths where there are no acoustic layers at all.  The plankton net attached to the optical counter yielded the results shown in Figure 13b (7K).  The most notable feature is the great preponderance of Bosmina at these greater depths.

 

Scattering Layer Movements

The nightly rise appears to be light triggered, commencing as the light falls to approx 1.0 m/s at 10 m depth (Marjoram, 1993).  The dawn sinking occurs as the light reaches the same level.  It has already been observed that the layer does not necessarily establish in the thermocline by day, but very often does (Shine and Martin, 1988).  In the summer of 1992 the mixed layer was 12oC or more, and the scattering layer did establish in the thermocline at a lower temperature.  Experiments with an inverted transducer showed that the layer does not necessarily rise to the surface at night but remains at approximately 5.0 m until dawn (Marjoram, 1993).

 

In the southern fixed station experiments of 1984 (Shine and Martin, 1988), the scattering layer was seldom entirely absent, even during north-east flowing internal seiches.  In the North Basin, however, it is notable that the scattering layer is strongest during north-east winds and may disappear entirely during south westerlies.

Frequently there is an across-loch tilt in the scattering layer.  Figure 14 (29K charts) shows tilting during a period of south-west winds, with the layer deeper on the southern shore.  Another feature is the presence of 'ramps' in the scatterers, sloping down towards the wind direction.  They are approximately 440 m long and slope at an angle of approximately 4o.

 

Discussion

The difference in target strength of the smaller scattering layer targets recorded by the various systems is of some interest.  To begin with, it would seem that the various peaks are not the result of optimum detection thresholds, since total numbers are very similar.  In an example on the EK500, 1,600 targets/ha were recorded at 120 kHz and 2,120 targets/ha at 38 kHz simultaneously, whereas the target strength peaks were at -70 dB and -62d B respectively (Mr. Erik Stenerson, pers. comm.).

Differences could result from calibration or time-varied gain (T.V.G.), but results have been comparable on three separate occasions.  It may be that resonance effects are responsible.  There has been little acoustic work on very small fish.

Burczynski, Michaleitz and Marrone (1987), in a survey of Rainbow Smelt Osmerus mordax, found that Love's (1977) formula appeared to underestimate the length of 5.6 cm fish by approximately 2.0 cm.  Another point is that very few of the fish surveyed will be on the beam axis and so be in true dorsal aspect.

This will reduce signals even though the beam pattern may have been compensated.  Mr. R. Johnson of Biosonics (pers. comm.) suggests that the reduction will be greatest at the higher frequencies, which is consistent with the pattern of our observations.  The low target strengths, especially at the higher frequencies, have introduced the suggestion that the individuals of the scattering layer are too small to be fish.  Other candidates are chironomid larvae and pupae, larger zooplankton such as Leptodora or perhaps some unrecorded crustacae.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p227

Shine and Martin (1988) reported 4th instar chironomid larvae, particularly Sergentia sp., taken in the pelagic during plankton hauls.  Larvae have also been observed in quantity in the stomachs of pelagic Charr (Martin and Shine, 1993).  Chironomid pupae have been collected by tow-nets within the scattering layer, particularly in May.  It has been observed that, when placed in containers, the pupae do not necessarily emerge for two or three days.  Therefore it seems possible that they may adopt a planktonic migration strategy while preparing for final emergence.  Hauls taken later in the year, however, do not always yield significant numbers of chironomids even though the scattering layer is strengthening. 

Experiments have been conducted by introducing quantities of 4th instar chironomid larvae and pupae into an echo-sounder beam (Lowrance Mach II, 50 kHz) at ranges up to 18 m.  Only occasional traces were obtained, thus suggesting that detection was dependent on targets close to the beam axis, perhaps in multiples.  Nevertheless, detection can be achieved and chironomids must be regarded as an important component of the pelagic community.  Work should be undertaken to establish the target strength of chironomids.

Leptodora may be dismissed as a candidate, since it is not present in winter.  In March 1991, for example, during a plankton diurnal, a scattering layer was observed in the absence of Leptodora (Figure 12a). The optical plankton counter showed the greatest concentration of smaller particles in zones not producing acoustic reflections.  Therefore zooplankton are unlikely candidates.

 With regard to other species, not hitherto recorded from Loch Ness, it should be born in mind that no plankton net, nor any other sampler, in ten years of work has ever shown traces of any such organism; nor has any been found in the sediments.  No pelagic Trout or Charr examined (Martin and Shine, 1993) had consumed anything unusual. 

 The trawling of 3.5 cm Charr in the pelagic during November 1992 suggests a very low growth rate.  In Martin and Shine (1993) it is shown that the 1+ Charr are between 4.0 cm and 7.0 cm long.  The new material includes 0+ fish smaller than this (Mr. R.B. Greer, pers. comm.) 

We have observed spawning Charr in December, which emphasises how small 0+ fish could be in the earlier summer months.  No significant genetic differences have been observed between Loch Ness pelagic and benthic Charr (Dr. Sheila Hartley, pers. comm.) and a fecundity study by Meacham (1993) shows consistent

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p228

egg size increase from summer to December.  It seems unlikely, therefore, that there is a separate spawning population giving rise to such small fish in the autumn. 

A further point is that in the earlier months, as in March 1991, the scattering layer is only detectable on more powerful echo-sounders.  This suggests an increase in individual target strength (i.e. growth) as the year proceeds. 

The scattering layer diurnal migration is similar to that of the copepods, but these do not occur significantly in the fish diet, which is similar for Charr between 4.0 cm and 30 cm.  Bythotrephes and Daphnia are the main prey items, followed by Leptodora.  Only the smallest Charr and some of the Sticklebacks contained some copepods.  It was found by Fryer (1957) that Cyclops strenuus abyssorum is predatory upon Diaptomus gracilis.  Loch Ness Cyclops have also been observed to feed on animal material, including Diaptomus (Mr. K.W. Heath, pers. comm.).  A limited amount of plant material is also consumed, and perhaps this is why Cyclops maxima may sometimes be observed near the surface by day.  Diaptomus shows the greatest diurnal migration, and this could be a response to predation by Cyclops and other zooplankton rather than by fish.  In any case, the scattering layer generally lies below the copepod maxima.  It seems reasonable, by contrast, to suppose that the much smaller numbers of Daphnia are a reflection of fish predation observed by Martin and Shine (1993). 

At the same time, the smaller range of Daphnia migration may account for the decrease in the migration tendencies of the larger Charr.  The migration of the scattering layer, if composed of fish, is perhaps a little surprising if one of the main food sources is Daphnia, which does not migrate to the same extent. 

It could, however, be an energetic strategy similar to that exhibited by the Sculpin Cottus extensus, a larval fish of North America, which is a benthic feeder during the day and rises into the warmer surface waters at night.  This speeds digestion and so growth rate (Wayne and Neverman, 1988).  The scattering layer migration could be a predation response, but none of the pelagic Charr had consumed fish, and only a few of the smallest Trout take Charr of <6.0 cm. 

The autumn Daphnia peak noted by Maitland (1981) and ourselves (Figures 12d and 12e) could perhaps be explained as the larger Charr move inshore to spawn, thus reducing the predation pressure.  Other factors could be that the very large water mass of Loch Ness retains its heat longer than smaller lakes.  Also there is generally an injection of allochthonous material (potential food) during equinoctial storms which bring the rivers into spate. 

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p229

The Bosmina do not seem to migrate at all, and are consumed by many of the Charr, although seldom in significant numbers.  The considerable numbers of Bosmina observed from the thermocline downwards suggests that they are filter feeding a detrital or microbial food source.  In this connection, the persistent north/south gradient in fish, and often in zooplankton, is interesting.  This presents something of a paradox,  since George and Jones (1987) had found that both conductivity and chlorophyll-a maxima lie consistently in the North Basin.  They suggested that this was caused by the slightly richer northern catchments.  An explanation could be that allochthonous organic material brought in by rivers of the much larger, and wetter, southern catchments is much more important than the primary productivity. 

If this material is processed through the microbial element, it could find its way to the zooplankton, particularly the filter feeding Cladocera, and so to the fish. 

In this respect, the results of the length run along the axis of the loch are somewhat surprising, in that the cladoceran Bosmina decreases in percentage towards the south.  This species is the one most likely to be utilising allochthonous inputs, and so might be expected to form a greater percentage of the plankton composition in the south.  However, it should be borne in mind that the samples were from a depth of 20 m, and that some thermocline tilt might have been present due to a light north-east wind.  This could have affected the observed distributions.  Numerically all species increased in the South Basin.

The increase in fish numbers towards the south is particularly pronounced in the smaller targets of the scattering layer.  During the 'steady state' produced by the normal south-west winds, these scatterers would spend the majority of their time close to the thermocline and therefore in the south-west return current produced.  By contrast, a number of length runs have shown that zooplankton numbers at the near surface increase down wind (unpublished data).

 The deeper scatterers would only rise into the north-east surface drift for the six hours of the short northern nights.  Shine and Martin (1988) have shown that the scattering layer targets move passively in water currents.  Therefore their position in the return current will tend to increase their density towards the south.  By contrast, during periods of north-east winds, it is noticeable that the scattering layer beneath the northern fixed station strengthens appreciably.  The other structures noted in the scattering layer, such as the cross-loch tilts, are consistent with the circulation of the surface drift due to Coriolis forces diverting the wind drift to the right (George, 1981), i.e. to the southern shore in a south-west wind. 

 

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p232

 

The 'ramps' are of interest, and may represent some form of internal mixing 'fronts' or a reaction to vertical movements induced by internal waves.

 

Summary

Hydroacoustic in situ target strength assessment techniques have been applied to the pelagic zone of Loch Ness.  The bulk of the targets are distributed down to the thermocline at densities from 300/ha to 1,000/ha.  Over 80% of these targets are of very small size, forming a distinct scattering layer.  In contrast to the larger fish, this layer makes nightly migration to the near surface.

The various hydroacoustic systems give different target strengths for the small scatterers, between -75 dB and -57 dB, possibly due to a resonance phenomena.  The disparity of target strength is much less for the larger size classes.  The identity of individuals comprising the layer is discussed, and juvenile Charr together with chironomids are suggested.

Over 80% of the zooplankton standing crop are copepods which make pronounced diurnal vertical migrations. 

The cladoceran Daphnia, which is a main food source for the pelagic Charr, does not migrate to the same extent, and perhaps this accounts for the lesser migration shown by the larger fish. 

Bosmina, a small cladoceran filter feeder, exhibits no vertical migration, and is often found deep within the thermocline and below it.  It is speculated that the cladocerans, particularly Bosmina, may utilise allochthonous organic material through the microbial element of the plankton.

There are consistently larger numbers of fish in the South Basin, and greater zooplankton densities are sometimes associated with this.  This is a paradoxical observation, since George and Jones (1987) had previously established that conductivity and chlorophyll-a concentrations were greater in the north.  Again, it is suggested that this is due to allochthonous inputs.

It has been noted that the scattering layer can be more dense in the North Basin with north-east winds, and it is suggested that this results from the individuals spending most of the time in the deeper return current caused by the surface drift.  Structures, such as tilts and ramps, in these scatterers show the close association of physical events in the vicinity of the thermocline with the distribution of the scattering layer. 

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Acknowledgements

Most of our material has resulted from the efforts of volunteers aboard the Loch Ness and Morar Project vessel Ecos, owned and skippered by Mr. John Minshull.  We also acknowledge the support of the Loch Ness District Salmon Fisheries Board, and particularly Mr. W. Hastings, the bailiff. 

The Simrad company has loaned most of the acoustic equipment, and we are also grateful to the crew of this company's demonstration vessel Simson Echo.  Particular help was given by Mr. Barry Pardey, Mr. Erik Stennerson and Mr. David Wilson.  Dr. Torfin Lindem operated the H.A.D.A.S. system. 

D.A.F.S. Aberdeen assisted for some days in August 1988 with their trawler Goldseeker, and Dr. Richard Ferro has also made material available from trawling trials aboard Calanus in 1992. 

Invaluable advice has been given by Dr. J. Simmonds of D.A.F.S. and Dr. Annie Duncan of Royal Holloway College.  Special thanks are due to Mr. R.A. Bremner of the Official Loch Ness Exhibition Centre for his continued support and provision of a headquarters for the Loch Ness and Morar Project. 

References

Baker, P.F. and Westwood, M.  (1960).  Underwater detective work.   Scotsman, 14th September 1960.

Bodholt, H., Nes, H. and Solli, H.  (1988).  A new echo-sounder-system for fish abundance estimation and fishery research.  In:  Proceedings of the International Council for the Exploration of the Sea.   C.M. 1988/B:11. Session P. [Aberdeen: Simrad Company].

Burczynski, J.J., Michaleitz, P.H. and Marrone, G.M.  (1987).  Hydroacoustic assessment of the abundance and distribution of Rainbow Smelt in Lake Oahe.  North American Journal of Fisheries Management,  7: 106-116.

Craig, R.E. and Forbes, S.T.  (1969).  Design of a sonar for fish counting.  Fiskeridirektoratets Skrifter, 15: 210-219.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p234

Ehrenberg, J.E. (1978).   The dual-beam system, - a technique for making in situ measurements of the target strengths of fish.  In:  Proceedings of the Acoustics in Fisheries Conference.  Faculty of Maritime and Engineering Studies,  Hull College of Higher Education, 26-27 September 1978.  University of Bath, England.

Ehrenberg, J.E. (1979).  A comparative analysis of in situ methods for directly measuring the acoustic target strength of individual fish.  Journal of Oceanographic Engineering, 4: 141-152.

Fryer, G.  (1957).  The food of some freshwater cyclopoid copepods and its ecological significance.  Journal of Animal Ecology, 26: 263-286.

George, D.G.  (1981).  Wind-induced water movements in the south basin of Windermere.  Freshwater Biology, 11: 37-60.

George, D.G. and Jones, D.H. (1987).  Catchment effects on the horizontal distribution of phytoplankton in five of Scotland's largest freshwater lochs.  Journal of Ecology, 75: 43-59.

Hartman, J., Dahm, E., Dawson, J., Doering, P., Jurgensen, S., Lindem, T., Loffler, H. and Raemhild, G.  (1984).  European Inland Fisheries Advisory Commission (EIFAC).  Experiments on Pelagic Fish Stock Assessment by Acoustic Methods in Lake Tegel.  EIFAC Occasional Paper No. 17: 1-26.  Rome:  Food and Agricultural Organisation of the United Nations.

Jurvelius, J., Lindem, T. and Louhimo, J.  (1984).  The number of pelagic fish in lake Paasivesi, Finland, monitored by hydroacoustic methods.  Fisheries Research, 2: 273-283.

Kubecka, J., Duncan, A. and Butterworth, A.J.  (1993).  Large and small organisms detected in the open waters of Loch Ness by dual-beam acoustics.  Scottish Naturalist, 105: 175-193.

Levy, D.A., Johnson, R.L. and Hume, J.M.  (1991).  Shifts in fish vertical distribution in response to an internal seiche in a stratified lake.  Limnology and Oceanography, 36: 187-192.

Lindem, T.  (1984).  European Inland Fisheries Advisory Commission (EIFAC).  Inter-calibration Exercise, Lake Constance, 23-29 October 1983. Hydroacoustic Data Report.  Geneva:  Food and Agriculture Organisation of the United Nations.

Love, R.H.  (1977).  Target strength of an individual fish at any aspect.  Journal of the Acoustical Society of America, 62: 1397-1403.

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.

Marjoram, R.S.  (1993).  An Investigation of the Identification and Behaviour of an Acoustic Scattering Layer in Loch Ness, Scotland.  B.Sc. Dissertation, Worcester College of Higher Education.

Martin, D.S. and Shine, A.J.  (1993).  The food and feeding relationships of pelagic fish in Loch Ness.  Scottish Naturalist, 105:  149-174.

Vol 105, The Scottish Naturalist: Pelagic Fish and Zooplankton in Loch Ness p235

Meacham, N. J. (1993).  The Fecundity and Associated Ecological Factors of the Arctic Charr, Salvelinus alpinus, and Brown Trout, Salmo trutta, in Loch Ness, Scotland.    B.Sc. Dissertation.  University of Hull.

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

Traynor, J.J. and Ehrenberg, J.E.  (1979).  Evaluation of the dual-beam acoustic fish target strength measurement method.  Journal of the Fisheries Research Board of Canada, 36: 1065-1071.

Vanderploeg, H.A., Liebig, J.R. and Omair, M.  (1993).  Bythotrephes predation on Great Lakes' zooplankton measured by an in situ method: Implications for zooplankton community structure.  Verhandlungen der Internationalen Vereinigung für Theoretische und Angewandte Limnologie, 25: 349.

Wayne, A.W. and Neverman, D.  (1988).  Post-feeding thermotaxis and daily vertical migration in a larval fish.  Nature, 333: 846-848.

 

Received June 1993

Mr. Adrian J. Shine, Loch Ness and Morar Project,
Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.
 

 Mr. David S. Martin, Loch Ness and Morar Project,
Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.

 

Miss Rosalind S. Marjoram, Loch Ness and Morar Project,
Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.

 

 

 

Loch Ness Spatial Distribution and Diurnal Migration