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The Food and Feeding Relationships of Pelagic Fish in Loch Ness

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

By DAVID S. MARTIN

Loch Ness and Morar Project



ADRIAN J. SHINE

Loch Ness and Morar Project

 

Introduction 

This paper is concerned with the food and feeding relationships of the pelagic Charr Salvelinus alpinus and Brown Trout Salmo trutta of Loch Ness, with some observations on the Three-spined Stickleback Gasterosteus aculeatus population also present in the loch. 

The glaciated, tectonic fault origins of Loch Ness have created a remarkably regular basin, i.e. steep sides sloping to a flat bed, with a maximum depth of 230 m and the greatest mean depth (132 m) of any British lake. 

Just over half (50.9%) of the loch's surface covers a water depth greater than 150 m, and 27.2% of the surface covers over 200 m depths.  Such a distinct pelagic zone is easily defined in relation to the other two zones, the narrow littoral/sublittoral and the deep profundal. 

Yet despite the vast volume of this open water, in terms of fish population this zone has not previously been studied. Baker (1962) speculated that the fish consisted of a layer of Charr at about 30 m depth across the loch, with Brown Trout moving offshore near to the surface from the littoral zone, but this work was never supported by sampling.  Maitland (1981) concentrated his fish survey on commercial species within the littoral.

 Since the early 1980s, members of the Loch Ness and Morar Project, in collaboration with the Ness Fisheries Board, together with Dr. Annie Duncan and Mr. R.B. Greer, have conducted a pelagic zone programme, mainly from fixed stations positioned mid-loch in the deepest water.  Echo-sounding shows fish throughout the water column but mainly in the top 40 m.  Sampling has revealed that the offshire waters to 30 m depth are dominated by Charr, with Brown Trout extending over the surface (Shine and Martin, 1988), and with larger, piscivorous Ferox Trout within and beneath the Charr layer.  Three-spined Sticklebacks are also significant residents in this zone, particularly in the South Basin, but Salmon

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p 150

parr Salmo salarand Lamprey Lampetra sp., both rarely observed, are not thought to contribute significantly.

Materials

The material for this study is based on 108 Charr and 46 Brown Trout caught between 1982 and 1992, and derived from four sources.  Firstly, the majority were caught in depth-marked gill nets, of various mesh size suspended from a fixed station, mostly from evening until the following mid-morning.  The stations, mid-loch and at least 0.5 km from either shore, lay in water depths exceeding 190 m. 

Secondly, in collaboration with the Department of Agriculture and Fisheries for Scotland's (D.A.F.S.) research vessels Goldseeker and Calanus, mid-water trawling has contributed a total of 154 fish, of which 50 are Sticklebacks and so not part of this study. 

Thirdly, incidental pelagic fish have been captured on the descent phase of gill nets, which have to pass through the surface waters en route to the profundal zone.

 Finally some Charr, extracted from the stomachs of pelagic caught Brown Trout, have themselves been analysed for their gut contents.

 Offshore fish, i.e. not caught mid-loch, have not been considered in this study.

Methods 

After weighing, measuring, and sex determination of each fish, the alimentary canal was dissected, and separated into three regions: the stomach, pyloric caecae, and intestine, which were analysed separately under a light microscope, or the contents preserved in 70% ethanol for later analysis. 

The initial records exist in the form of percentage food item of the overall contents, for each of the three gut regions, but for the purposes of this study and to give a better general presentation, the figures have been converted to the following letters: D, i.e. Dominant prey item, 100% - 50% of total food contents; S, i.e. Significant prey item, 50% - 5.0% of total food contents; or O, i.e. <5.0% or only observed as an occasional food item. 

Evaluation of the gut contents has been made by a Points System, derived from Swynnerton and Worthington (1940).  Fullness of actual stomach has not been considered, however, since many fish regurgitate their stomach contents when

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p151

trapped in gill nets, and in any case the overall gut content of each fish is being considered.  For this study, the D value was allocated 2.0 points, the S value 1.0 point and the O value 0.1 points.  Such a system takes into account the contents of the complete fish from the three gut regions investigated separately. 

The otoliths were removed for ageing by Mr. R.B. Greer. 

As far as possible, fish were collected throughout the year, but winter pelagic fish were difficult to sample, since many disperse inshore to the side walls of the loch, or exist in a state of lethargy and so do not get caught in gill nets.  Those which were caught (February and March) had no gut contents for inclusion here. 

Results 

The results are summarised in Figures 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h, 1i, (appox. 38K tables)  and Figure 8 (5K graph).

Figures 1a - 1i show the breakdown of the food components for each of the three regions of the gut to emphasise the dominance (D), significance (S), or mere presence or otherwise (O) of any prey item.  This is then collectively summarised on the Points System in the final column.

The fish were numbered by the month caught, regardless of year, and in ascending weight per month.  The relationship between Length and Age is presented in Figure 8. 

It is significant that the composition of food was often different within the separate gut regions of an individual fish; e.g. Charr #72 had a 9.1 ratio of Daphnia: Bythotrephes in the stomach, but the reverse of this in the intestine.

Similarly, a prey item absent in the stomach could be dominant in the intestine; e..g. Brown Trout #9 with Daphnia and Bythotrephes.  Even equal-sized fish caught in the same gill-netting operation could have distinctly differing compositions of food within the gut; e.g. Charr #40 dominated by Chironomid larvae, and Charr #43 dominated by Bythotrephes. 

The Points System column reduces this complexity to allow patterns to emerge, as seen in Figures 2 and 3 (18K tables).

In terms of the most frequently selected prey item, the Charr diet is dominated by the larger Cladocera, Bythotrephes longimanus, Daphnia hyalina and Leptodora kindti, in that order, with Chironomid larvae an important component. 

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p161

The presence of benthic fauna is also of interest.  Unlike Charr, Brown Trout most frequently select fish, followed by aerial insects, with Daphnia and Bythotrephes the third and fourth most selected prey respectively.  Brown Trout predate very few chironomids, and benthic fauna is noticeably absent. 

Figure 3 summarises the dominance of one prey item over all the others within the gut, i.e. where the points for a single prey item are greater than the sum of all the others.  The inference is that, given a choice, the fish would prefer particular prey.  Here the preferred items were Daphnia and Bythotrephes for the Charr, and quite clearly fish for the Brown Trout. 

Figure 4 (19K table) presents the percentage of each fish species containing evidence of a prey item, regardless of the quantity of material eaten.  Thus Bythotrephes, followed by Daphnia, were taken by over 75% of the Charr sampled, with chironomid larvae and Leptodora present in over 43% and 37% of these fish respectively.  Nearly half the Brown Trout contained aerial insects to some degree, followed by Bythotrephes, fish and then Daphnia in nearly 37% of all the Trout analysed.

The diversity of prey items selected by the different salmonids is shown in Figure 5 (12K table).  Charr, in general, forage on a greater number of prey taxa than do Brown Trout, i.e. they are able to exploit a wider food source. 

It would appear that, in the months represented in this study, Bythotrephes is the most important cladoceran prey item in July and August, but is superseded by Daphnia in September, October and November.  Chironomids and Leptodora were especially present in August. Figure 6 (10K table) supports Maitland's findings (1981) that the Daphnia population in Loch Ness peaks in October and that Bythotrephes peaks in July, although Walker, Greer and Gardner (1988) suggest there is little seasonal variation apparent in the proportions of organisms eaten by Charr in Loch Rannoch. 

Discussion

The diet of the pelagic Charr in Loch Ness is dominated numerically by the larger Cladocera: Bythotrephes longimanus, Daphnia hyalina and Leptodora kindti in that order.  A similar situation is true for pelagic Charr in Loch Rannoch (Walker et. al., 1988), in Lake Windermere and other water bodies of the English Lake District (Frost, 1977), as well as in lakes elsewhere, including Austria (Steinböck, l949), Norway (Dahl, 1920), and Sweden (Nilsson, 1955), although the proportion in the diet varies.

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p161

Chironomid larvae and pupae are also an important food item for Charr at certain times of the year.  Although these larvae live in tubes of mud in the benthos, they are known to make extensive daily movements up into the water (Mundie, 1964; Dr. P. Cranston, pers. comm.).  Sergentia coracina, the predominant chironomid of the profundal benthos, migrates as a 4th-instar into the water column as a means of horizontal redistribution (Brinkhurst, 1974).  This is a remarkable feat, considering the depth of Loch Ness, but the larvae have often been taken in plankton hauls within the 30 m pelagic fish zone, in this late instar form.  By this behaviour, chironomid larvae become available to mid-water feeding Charr.

The results in Figure 1 also seem to bear witness to the fact that some of the smallest Charr (0+ age group) also have a benthic feeding mode, exploiting ostracods, copepods and caddis larvae.  The smallest Charr in this study were taken in mid-water trawls, but it could be speculated that the nets swing in towards the loch walls when the towing vessel slows and drifts whilst the nets are retrieved, and thus may catch benthic sub-littoral fish.

Mr. R.B. Greer (pers. comm.) is confident, however, that the small Charr could easily move inshore and offshore within a few hours, thus exploiting two habitats, - a feeding region and a resting region.  Indeed, Greer said "almost nothing is known of the 0+ year group pelagic fish in Scottish waters".

When feeding on offshore zooplankton, the small Charr show no preference in the size of large Cladocera (Bythotrephes, Daphnia and Leptodora) eaten, compared to the large Charr, although small profundal-caught Charr do appear to eat smaller benthic organisms than their larger counterparts, which are also piscivorous (Griffiths, Martin, Shine and Evans, 1993).

The pelagic Brown Trout of Loch Ness do not exploit the zooplankton to the same extent as do the Charr.  Although Daphnia and Bythotrephes are exploited by 37% and over 45% respectively of the Brown Trout population, and more often than the other zooplankton species, the low numbers of these Cladocera eaten makes it appear unlikely that these organisms are subject to heavy predation by Trout.  Pelagic Trout concentrate their feeding either on aerial insects, especially during the summer months (Shine and Martin, 1988), or more importantly, on fish.  Figure 7 (22K table) shows that it is the larger Ferox Trout which exploit Charr as a food source.

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Ferox Trout have been defined as the small populations of very large and old Brown Trout present in large oligotrophic waters, with exceptional individuals exceeding 90 cm in length and 20 years of age (Campbell, 1979).  In Loch Ness, the Ferox Trout lurk beneath and within the Charr layer.  Indeed a Brown Trout #15, taken at 29 m in depth-marked gill nets, had eaten a Charr 33.7% of its own body weight.

In collaboration with Dr. Annie Duncan, the Royal Holloway College's pelagic survey nets were set at between 20 m and 30 m.  Of 23 Brown Trout caught, eight were greater than 30 cm in length and ten had the remains of Charr within their stomachs.  Of the 17 Charr taken in the same nets, ten were less than 16 cm in length, and some of the larger Trout were tangled in the nets within a few centimetres of five of the smallest gill-netted Charr, obviously caught when attempting to capture these Charr.  Campbell (1979) has tabulated the feeding relationship of 17 Ferox Trout in a number of Scottish lochs, including the littoral of Loch Ness, and these show prey/predator length ratios ranging from 14.8% to 35.1%.  The Loch Ness results in Table 7 are consistent with the above work, with Charr in the stomach ranging from 8.8% to 37.1% of the Trout's own body length.

Piscivorous pelagic Charr have not been observed at Loch Ness, although they have been found in the profundal Charr population.  At Loch Rannoch, Walker et. al. (1988) recorded a benthic morph Charr containing a smaller Charr, 48% of its own body length, in its stomach.

This prompts consideration as to whether there are genetic differences between the fish populations of the pelagic zone and those of the littoral and profundal zones of Loch Ness, but special studies have revealed no genetic differences in the different zones for the Charr (Dr. Sheila Hartley, pers. comm.), nor for the Brown Trout, (Dr. A. Ferguson, pers. comm.).

The relationship between food eaten by fish, and free-swimming fauna collected in Loch Ness, shows parallels with the situation in Lake Windermere (Frost, 1977).  Of the larger forms of planktonic Cladocera in Loch Ness, Daphnia is the most frequently abundant, with Bythotrephes and Leptodora relatively scarce and infrequent (Shine, Martin and Marjoram, 1993).  Although Bythotrephes is much rarer than Daphnia in the fauna, it is consumed to a greater extent than the more abundant Daphnia.  Even the proportion of Leptodora in fish guts appears high for a relatively rare species.  Thus the relationship between the proportion of Cladocera in the plankton and those in the diet of fish is not a reflection of the abundance of individual species.

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p171

 

The selection of Bythotrephes and Leptodora, because they are large, could account for the disparity of their proportions in the fish diet and as free-swimming plankton.

 Conversely, the overwhelming zooplankton species in Loch Ness are the copepods Diaptomus gracilis and to a lesser extent Cyclops abyssorum, both present all year round.  Yet the incidence of copepods in pelagic guts is almost negligible, and Diaptomus has not been observed in the guts of the open-water salmonids, and only once as an individual in a Three-spined Stickleback, so there is a large difference between the number eaten and the supply.

Bosmina coregoni, although present in over 21% of the Charr, numbered fewer than five individuals per fish, and Brown Trout hardly exploited them at all.

If Charr, and to a lesser extent Brown Trout, actively pick out individual plankton, this suggests that a certain degree of movement occurs.  Vertical migration of the fish has been studied by Shine and Martin (1988) and Shine et. al. (1993), but incidental evidence suggests horizontal movements as well.  Brown Trout #36, a female Ferox feeding almost exclusively on Charr, contained one Pisidium sp. within its intestine.  Of course, this presumably constituted the gut contents of a previously digested benthic Charr, but of more interest, - was it the Charr moving offshore, or the Brown Trout moving inshore, which led to the fatal encounter for the smaller fish?

Migrations do occur as winter approaches.  The vertically migrating fish disappear from the open water, probably to the loch walls and littoral zone, presumably in response to the declining availability of the zooplankton, and the need to find spawning grounds.  Spawning by Charr has been observed in mid-December in the littoral zone at about 15-25 m depths.

 

Conclusion

 Judging from the various and sometimes conflicting accounts of the food of Charr in some Scandinavian waters (Dahl, 1920; Sømme, 1933a, 1933b; Schmidt-Nielsen, 1939), and then comparing these with the accounts of the food of Charr in some English lakes and Scottish lochs (Frost, 1977; Walker et. al., 1988), it would appear that the Charr is an extremely opportunistic fish, preferring certain food items but adapting and adjusting its feeding habits to the particular feeding circumstances present at the time.

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p 172

 

The large number of presumed Sticklebacks in the South Basin pelagic zone may account for a disparity between Bosmina eaten by Sticklebacks and Bosmina free-swimming in the water column, if indeed Sticklebacks do exploit this cladoceran.  It would also mean that the Sticklebacks could avoid interspecific competition with the salmonids within the pelagic zone.  The Stickleback samples from the trawl, however, are as yet unworked.

 

Acknowledgments

The authors wish to thank all the volunteers of the Loch Ness and Morar Project for their efforts in collecting the fish, and Mr. R.B. Greer and Dr. Annie Duncan for the provision of nets.  Some of the fish-netting work was supported by a Small Ecological Project Grant from the British Ecological Society.

The D.A.F.S. Marine Laboratory at Aberdeen provided the trawls used, and we are indebted to the crews of the Simrad demonstration vessel Simson Echo, the Ocean Bounty, and the D.A.F.S. vessels Goldseeker and Calanus, for the work done and material supplied.  In particular, Dr. Richard Ferro of D.A.F.S. supervised the Goldseeker operation in 1988, and we thank the Director of the Marine Laboratory, Dr. A.D. Hawkins, for his support in making the operation possible.

Ms. Jane Harper conducted many of the initial dissections, Mr. R.B. Greer worked the otoliths for Charr ageing, and Dr. Sheila Hartley and Dr. Andrew Ferguson are continuing work on the genetics of Brown Trout and Charr.

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p173

References

Baker, P.F.  (1962).  Cambridge University Loch Ness Expedition Report.  Cambridge.

Brinkhurst, R.O.  (1974).  The Benthos of Lakes. London:  MacMillan.

Campbell, R.N.  (1979).  Ferox Trout, Salmo trutta L., and Charr, Salvelinus alpinus (L.), in Scottish lochs.  Journal of Fish Biology, 14: 1-29.

Dahl, K.  (1920).  Studier over røje i ørretvand.  Norsk Jaeger- og Fiskerforenings Tidsskrift, 49: 1-16.

Frost, W.E.  (1977).  The food of Charr, Salvelinus willughbii (Günther), in Windermere.  Journal of Fish Biology, 11: 531-547.

Griffiths, H.I., Martin, D.S., Shine, A.J., and Evans, J.G.  (1993).  The ostracod fauna (Crustacea, Ostracoda) of the profundal benthos of Loch Ness.  Hydrobiologia, 254: 111-117.

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.

Mundie, J.H.  (1964).  Invertebrate animals.  In: Freshwater Biological Association: Thirty-Second Annual Report.  (Ed. H.C. Gilson).  Pages 30-32.  Ambleside: Freshwater Biological Association.

Nilsson N.A.  (1955).  Studies on the feeding habits of Trout and Char in North-Swedish lakes.  Report, Institute of Freshwater Research, Drottningholm, 36: 163-225.

Nilsson, N.A.  (1963).  Interaction between Trout and Char in Scandinavia.  Transactions of the American Fisheries Society, 92: 276-285.

Schmidt-Nielsen, K.  (1939).  Comparative studies on the food competition between the Brown Trout and the Char.  Kongelige Norske Videnskabernes Selskabs Skrifter, 4: 1-45.

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

Shine A.J., Martin, D.S. and Marjoram, R.S.  (1993).  Spatial distribution and diurnal migration of the pelagic fish and zooplankton in Loch Ness.  Scottish Naturalist, 105: xx-xx.

Sømme, S.  (1933a).  Hvad spiser røien?  Norsk Jaeger- og Fiskerforenings Tidsskrift,  62: 239-245, 311-318.

Sømme, S.  (1933b).  Undersøkelser over maveinnhold av røie (Salmo alpinus L.).  Er røiekultur lønnsom i Sør-Norge?  Nyt Magazin for Naturvidenskaberne, 73: 115-136.

Steinböck, O.  (1949).  Der schwarzsee ob Sölden im Ötzal.  Eine hydrobiologische studie.  Veröffentlichungen des Museum Ferdinandeum in Innsbruck, 26/29: 117-146.

Vol 105, The Scottish Naturalist: Food and Feeding Relationships of Pelagic Fish in Loch Ness p174

Swynnerton, G.H. and Worthington, E.B.  (1940).  Note on the food of fish in Haweswater (Westmorland).  Journal of Animal Ecology, 9: 183-187.

Walker, A.F., Greer, R.B. and Gardner, A.S.  (1988).  Two ecologically distinct forms of Arctic Charr Salvelinus alpinus (L.) in Loch Rannoch, Scotland.  Biological Conservation, 43: 43-61.

Received May 1993

Mr. David S. Martin, Loch Ness and Morar Project,

Loch Ness Centre,
DRUMNADROCHIT, Inverness-shire IV3 6TU.

 

Mr. Adrian J. Shine, Loch Ness and Morar Project,

Loch Ness Centre,
DRUMNADROCHIT, Inverness-shire IV3 6TU.

 

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