The authors present the findings of a restoration project in Loch Enoch in Scotland. There are historical references that brown trout was present in Loch Enoch up to the 1920s but it is believed the acidity of loch triggered the disappearance of Salmo trutta. The recent observed reduction in the acidity of L. Enoch to a level close to that found in nearby lochs with trout populations, suggested that trout might now survive in L. Enoch. For a population to survive, all stages in the life-cycle of a species must be able to develop. Accordingly, tests were undertaken, first with eggs and fry. The availability of food was also studied. In October 1994, 3,000 yearling trout of L. Grannoch origin which had been reared in a local hatchery were distributed throughout the loch. The fish population was studied from 1995-98. The authors conclude that survival of the trout population is possible if the acidity of the loch water remains low

Collen, Peter

Harriman, Ron

Keay, Elspeth

Morrison, B.R.S

Watt, Alan

English

Aquatic Commons

RESTORATION OF BROWN TROUT TO LOCH ENOCH 3 
RESTORATION OF A BROWN TROUT (SALMO TRUTTA L.) 
POPULATION TO LOCH ENOCH, AN ACIDIFIED LOCH 
IN GALLOWAY, SOUTH-WEST SCOTLAND 
PETER COLLEN, RON HARRIMAN, BRIAN MORRISON, 
ELSPETH KEAY AND ALAN WATT 
(P. Collen, R. Harriman, B. R. S. Morrison, E. J. Keay and A. W. Watt, 
Freshwater Fisheries Laboratory, Faskally, Pitlochry PHI6 5LB, Scotland.) 
[Tel: 01796 472060; Fax: 01796 473523; 
E-mail: COLLENP@MARLAB.AC.UK]) 
Introduction 
The decline and, in some waters, the disappearance of brown trout Salmo 
trutta populations in south-west Scotland have been attributed to surface-
water acidification (Harriman et al. 1987; Maitland et al. 1987). From changes 
in the diatom flora and levels of heavy metals in sediment cores, Flower et al. 
(1987) have calculated water quality changes over the past 200 years for lochs 
in this area. For Loch Enoch (Fig. 1, and front cover illustration) they 
concluded that the pH of the loch water had decreased from 5.4 around 1840, 
to 4.8 in c. 1930, then more rapidly to 4.4 in 1982. 
Historical references confirm that L. Enoch was frequented by anglers 
(Lyell 1873; Harper 1908; Macdonald 1927; McBain 1929), but McBain 
refers to local reports of trout disappearing from the loch. Although Maitland 
et al. (1987) suggested that trout might have become extinct by 1890 at the 
latest, our discussions with local anglers indicate that they may have been 
present in the 1920s. None was taken in gill nets set across bays in the loch in 
1978, 1984 and 1991. 
Studies of surface sediments in L. Enoch (Battarbee et al. 1988) and water 
chemistry (Harriman et al. 1995) indicated a decrease in the acidity of the loch 
during the 1980s (Fig. 2) and this raised the possibility of a successful 
reintroduction of trout. 
Loch Enoch 
Loch Enoch (Grid ref. NX 445 851) has an area of 50 hectares, a maximum 
depth of 36 metres and lies at an altitude of 493 metres in the Loch Doon 
granite intrusion in south-west Scotland. It has an irregular shoreline and a 
catchment consisting mostly of blanket peat and large areas of rock outcrops. 
Purple moor grass Molinia caerulea is the dominant plant, though ling 
Calluna vulgaris and bog myrtle Myrica gale are locally abundant. 
Feeder streams are few and small, probably flowing only during heavy 
precipitation. The outflow, the Eglin Lane, has a 2.2 metre fall, impassable to 
trout, about 350 metres downstream from the loch. Between the fall and the 
4 PETER COLLEN ET AL. 
RESTORATION OF BROWN TROUT TO LOCH ENOCH 5 
FIG. 2. Long-term changes in the pH of Loch Enoch (1978-1999). 
6 PETER COLLEN ET AL. 
loch the stream is 2.5 metres wide, with a mainly cobble substratum covered 
by dense mats of aquatic liverworts. Suitable spawning sites are limited. 
The chemistry of L. Enoch is typical of acidified lochs in the area, having 
low pH (mean c. 4.8), a low calcium concentration (mean c. 23 micro-
equivalents per litre) and a mean non-marine sulphate concentration of 55 
microequivalents per litre. Marine salts dominate the ionic composition. 
Because the water is exceptionally clear (mean total organic carbon c. 2 mg 
per litre), toxic forms of aluminium are high, giving a mean value of 61 (ig per 
litre, while organically-complexed forms average only 16 μg per litre (Table 
I). 
Preliminary tests on survival of eggs and fry in the Eglin Lane 
The observed reduction in the acidity of L. Enoch to a level close to that 
found in nearby lochs with trout populations, suggested that trout might now 
survive in L. Enoch. For a population to survive, all stages in the life-cycle of 
a species must be able to develop. Accordingly, tests were undertaken, first 
with the most acid-sensitive stages of the trout, i.e. eggs and small free-
swimming fry. Fish fry and eggs used for these survival tests came from 
resident trout in two nearby acid lochs (Fig. 1): Loch Grannoch, mean pH 
4.59 (range 4.27 to 5.04) and Loch Dee, mean pH 5.62 (range 4.73 to 6.60). 
These values refer to monthly samples taken from 1991 to 1999. 
Survival of fry 
In June 1991, approximately 2,000 hatchery-reared fry of Loch Dee origin 
were released over a 200 metre stretch of the Eglin Lane above the fall, at a 
density of 5 per m2. Eleven days afterwards, electrofishing indicated that the 
density had dropped to 0.03 per m2, and 90 days after stocking only three fish 
were recovered. In the following three years, trout were caught by 
electrofishing but densities never exceeded 0.01 per m2. 
Also in June 1991, in order to observe introduced fry more closely, ten fry 
of Loch Dee origin were put into each of five polythene boxes measuring 
480x290x230 mm, with a 21x14 mm panel of 2 mm mesh at either end and a 
21x13 mm perforated polythene sheet in the lid. The boxes were attached to 
metal stakes in the Eglin Lane above the fall and contained three stones of up 
to 200 mm diameter to provide cover for the fish. A similar number of fry 
from the Lodge Burn at Loch Grannoch was introduced into a further five 
boxes in the Eglin Lane. 
Within two days, only 46% of the Dee fry were alive and this fell to 10% 
after 23 days. The corresponding survival figures for Grannoch fry were 96% 
and 28%. After 90 days all the Dee fry had died but almost 20% of the 
Grannoch fry survived. The length of these fish ranged from 32 to 47 mm and 
condition factors were 0.70 to 1.22. 
RESTORATION OF BROWN TROUT TO LOCH ENOCH 7 
Survival of eggs 
Eggs from trout in L. Dee and L. Grannoch, obtained on the same day in 
November 1993, were transported to the L. Enoch outflow above the fall 
within 24 hours of fertilisation. Three groups of ten egg-boxes, modified from 
a design by MacCrimmon et al. (1989) and each filled with 20 mm gravel and 
containing 50 eggs, were buried in the substratum. Five of each set of ten 
boxes contained eggs of Dee origin and five contained eggs from Grannoch 
fish. Three additional boxes were buried separately and used to determine the 
stage reached in development. One group of ten boxes was removed when the 
eyed, post-hatching and swim-up stages were reached. 
At each stage of development, the survival of Grannoch eggs and fry was 
better than that of Dee eggs and fry. By the time the alevins were ready to 
leave the gravel, only 10% of the Dee fish were alive compared with 30% of 
the Grannoch fish. 
The availability of food: invertebrates present in Loch Enoch 
In order to determine what food items would be available for introduced 
fish, samples of benthos were collected from twelve sites in the littoral zone 
of the loch during 1993 to 1995. These contained a wide range of taxa (Table 
2), of which many are known to be acid-tolerant and were found in the 
8 PETER COLLEN ET AL. 
stomachs of trout from Loch Dee (Morrison 1997). Several species of water 
boatman (Corixidae) were also taken in these samples. Zooplankton tows 
mainly collected the copepod Eudiaptomus gracilis, which is not normally 
eaten by trout, and the small cladoceran Bosmina coregoni. Of the larger 
species that are more commonly found in the stomachs of trout, Bythotrephes 
longimanus, Polyphemus pediculus and Eurycercus lamellatus were present, 
but in small numbers. 
Restocking Loch Enoch 
The experiments with the eggs and fry in the loch outflow suggested that some 
survival of these early stages was possible. Since these are the stages in the 
life-cycle which are most sensitive to acid conditions it was felt that the 
introduction of older fish from the same source might be even more 
successful. 
In October 1994, 3,000 yearling trout of L. Grannoch origin which had been 
reared in a local hatchery were transported by helicopter to a netted-off bay of 
L. Enoch and then distributed throughout the loch by boat (Fig. 3). All fish 
were released within four hours of arrival at the bay and the lengths and 
weights of 72 fish were recorded. 
Fish recaptured during the next four years (1995-1998) 
To assess the results of the restocking, gill nets of 32 mm, 44 mm and 50 mm 
stretched mesh were set in the littoral zone. Fish were caught in July 1995 (7 
fish), August 1996 (97 fish; see Fig. 3) and September 1997 (104 fish). 
Lengths and weights were recorded and condition factors determined (Table 
3); examination of scale samples verified that all fish were of the same age. 
The small number of trout captured one year after release, in 1995, was 
probably due to their small body size in relation to the mesh sizes of the nets. 
The fact that relatively large samples of fish in good condition were 
obtainable two and three years after introduction was evidence that the loch 
could support a trout population, and the survival of at least a small proportion 
of alevins from planted eggs demonstrated its potential to continue. The 
condition of the fish and the range of food items that had been eaten (see later) 
support the view that, at least for this size of trout population, there is 
sufficient food available in the loch environment to enable the fish to grow 
well. 
On 18 November 1998, a small sample of fish was netted from the loch. 
Eggs taken from them were fertilised at the loch and immediately placed in 
the Eglin Lane, using the previously described egg-box method. In May of the 
following year, five boxes were removed and were found to contain 0, 4, 4, 8 
and 12 swim-up fry. 
RESTORATION OF BROWN TROUT TO LOCH ENOCH 9 
FIG. 3. Loch Enoch. Above: Trout were transported to the loch by helicopter and 
released into a small area enclosed by nets, prior to distribution from a boat into various 
parts of the loch. Below: A sample of trout caught by gill nets in 1996. The Swiss army 
knife at bottom left is 9 cm in length. 
10 PETER COLLEN ET AL. 
RESTORATION OF BROWN TROUT TO LOCH ENOCH 11 
Stomach contents of trout recaptured from Loch Enoch 
The stomach contents of subsamples from the 1996 and 1997 catches were 
analysed using a modification of the points system of Hynes (1950) to obtain 
information on diet (Table 4). Twenty-three fish netted on 18 November 1998 
contained very few food items; the results are not included in the table. 
Table 4. Percentage volume of stomach contents found in 38 trout from Loch Enoch in 1996 and 
27 trout in 1997. Items occupying more than 10% of stomach volume are shown as bold 
numbers. 
Corixids are not normally an important part of the diet of trout (Frost & 
Macan 1948) and comprised less than 1% of the contents of the stomachs of 
trout from nearby L. Dee (Morrison 1997). However, in 1996, corixids were 
relatively abundant (16.8%) in the stomach samples of trout from L. Enoch. 
This may have been due to the absence of fish from the loch for a long period 
of time (Lyle & East 1989). Consequently, species such as the carnivorous 
Glaenocorisa propinqua and Arctocorisa carinata (Table 2) would be able to 
increase in numbers and become predators of major importance, feeding on 
zooplankton and small benthic organisms (Savage 1989). The abundance of 
12 PETER COLLEN ET AL. 
corixids in the stomachs of L. Grannoch trout (Maitland et al. 1987), the loch 
from which the trout for L. Enoch were obtained, is probably linked to the 
decline in the fish population there during the past 30 years. In L. Enoch, trout 
caught in 1997 contained very few corixids, presumably because the 
introduced trout had greatly reduced their numbers. 
The high proportion of terrestrial insects in the stomachs of trout recaptured 
in L. Enoch (Table 4) may indicate either that there was a need to supplement 
the aquatic food supply or that terrestrial food was more readily available. In 
the 7-year L. Dee study already referred to above, 45% of the stomach 
contents of trout taken in September comprised terrestrial insects. From May 
to August the proportion varied from 10 to 16%. The occurrence of such a 
high proportion of terrestrial insects in the L. Enoch trout caught in August 
and September is therefore not unusual. In fact, terrestrial insects normally 
provide an important food source for trout in the late summer (Maitland & 
Campbell 1992). 
General conclusions 
In terms of water chemistry, L. Enoch currently provides a borderline 
environment for trout. A similar conclusion might be reached for L. 
Grannoch, and long-term acclimatisation of the local population to acid 
conditions may explain why the eggs and fry taken from L. Grannoch 
survived better, albeit marginally, than eggs and fry taken from L. Dee. In 
fact, the mean pH of water in L. Grannoch (pH 4.59) is lower than that of L. 
Enoch (pH 4.75) and the mean level of the toxic form of aluminium is higher 
at 181 μg per litre compared with 61 μg per litre for L. Enoch. On the positive 
side, however, L. Grannoch has a higher total organic carbon content which 
results in a greater proportion of available aluminium being present as non­
toxic aluminium complexes, and in a reduction in the toxicity of the remaining 
ionic aluminium to trout (Roy & Campbell 1997). In recent years the acid 
neutralising capacity (ANC) of a water has been used as a surrogate for the 
combined effect of pH, aluminium and calcium, e.g. in critical load 
methodologies (Harriman & Christie 1995). On this basis L. Grannoch, with a 
mean ANC of -6 microequivalents per litre, would be considered slightly less 
sensitive to acid input than L. Enoch, with a mean ANC of -10 
microequivalents per litre (Table 1). Loch Grannoch also has another 
advantage. The Lodge Burn (mean pH 5.05 (range 4.46 to 6.24); ANC 
23 μequiv. 1-1) provides a viable spawning stream which enables eggs and fry 
to avoid damaging acid conditions. Thus, in the long-term, the resident trout 
population of L. Grannoch is probably more secure than the recently 
introduced population in L. Enoch. 
Survival of trout introduced into L. Enoch has been proved by sampling 
over a period of four years. Many reintroduced fish caught in late summer and 
RESTORATION OF BROWN TROUT TO LOCH ENOCH 13 
autumn contained well-developed eggs, and fertilised eggs planted in egg-
boxes produced a small number of live fry. Successful natural spawning by 
the trout has yet to be demonstrated. The continuing reduction in acidic 
deposition in the UK is likely to result in further reduction of the acidity of L. 
Enoch. This may be sufficient to allow the survival of eggs and fry to the 
point where a sustainable trout population is reached. 
Acknowledgements 
We would like to thank staff from Forest Enterprise for providing eggs and 
fish and for help in coordinating the use of helicopters for transporting fish. 
We would also like to acknowledge the assistance of our colleagues Alistair 
McCartney, David Moore, Jill Rowbottom and Elaine Taylor, and former 
colleagues Alan Cowan and Graeme Lowery. 
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Restoration of a brown trout (Salmo trutta L.) population to Loch Enoch, an acified Loch in Galloway, South-West Scotland

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Restoration of a brown trout (Salmo trutta L.) population to Loch Enoch, an acified Loch in Galloway, South-West Scotland

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