The findings for the spring and summer of 2000 and spring
of 2001 are shown in Figures 5 & 6. These show that during
the spring, the temperature of the water leaving the dam is
generally considerably colder than would naturally be the
case. In both years there was warm weather in early May which
succeeded in raising temperatures at Invervar into the teens,
while the water leaving the dam did not exceed 8 degrees.
Other spot recordings taken in warm afternoons in May 2000
suggest that temperature suppression was indeed still evident
at the Kenknock site, but that by Meggernie (10 miles from
Lubreoch) the effect had attenuated. Reduced temperatures
were also found in the neighbouring River Lochay downstream
of the power station which receives water from Stronuich Dam.
It is also interesting to note that during the spring and
early summer, the water temperature at Lubreoch remains stable
for periods and increases in a stepwise manner. The stable
periods tend to coincide with warm periods, as evidenced by
the temperatures at Invervar. The jumps occur when ambient
temperatures would appear to reduce. This indicates that thermal
stratification occurs in Loch Lyon when it is warm and calm
(i.e. a layer of warm water, a “thermocline”,
floats over colder denser water in the bottom of the loch).
However, cooler weather or wind causes the water in the loch
to mix, causing bottom temperatures to rise.
Presumably temperature must have an impact on the growth
of juvenile salmon, and the insects on which they feed (see
below), on the upper Lyon. However, it is the case that there
is no really noticeable increase in growth of juvenile salmon
by Meggernie, so temperature cannot be the only factor at
In order to account for the slow growth of juvenile salmon
it is necessary to investigate the insects in the river on
which the fish feed.
Full analysis of aquatic insect populations is an involved
process. Owing to their life cycles, many insect larvae are
present or are only of a catchable size for a short part of
the year and this differs according to species. Thus sampling
really needs to be repeated throughout the year. Also, because
different species prefer different types of physical habitat,
these also must be sampled.
So far it has not been possible to rigorously sample the
Lyon invertebrates. However, in June-August 2001, some samples
were obtained, and notwithstanding the incompleteness of the
data, they do give an indication of something unusual.
Larvae were sampled by the “kick sampling” method.
A stout fine meshed net is held in the current and the stones
and gravel immediately upstream are disturbed by kicking and
shuffling the feet. By trying to restrict each sample to a
standard area and time (2 minutes in this case) some crude
measure of abundance may be obtained. It should be stressed,
however, the strength of current, looseness of stones, swimming
ability of larvae etc all conspire to vary the sampling efficiency
Owing to time constraints larvae were generally not identified
to species level, but rather to broader groups. For an initial
scoping study this is adequate.
The following groups or species of larvae were identified:
Ephemeroptera (the “mayflies”)
These are known to anglers as the “olives”.
In subsamples identified to species level, Baetis rhodani
(the “large dark olive”) is most common,
Baetis muticus (the “iron blue”) also being
found. These agile larvae are good swimmers and eat algae
off the surface of stones plus rotting detritus.
Relative to most of our invertebrates Baetis
have short life cycles and are “multivoline”
– i.e. they produce more than one generation per year,
unlike most insects in our streams. This means that instead
of only being abundant during a short season, Baetis
larvae can be abundant for most of the year. Because of
this Baetis rhodani is regarded as being one of the most
successful freshwater invertebrate in Britain.
In a number of studies baetids have been found to be a
most important food item for juvenile salmon, especially
fry. This is due to two factors. They are available throughout
the year, and also as they swim on the top of stones they
are easy prey.
These will almost certainly be Ephemerella ignita
the “blue winged olive” or “sherry spinner”
to trout anglers. This larvae lives on detritus (decomposing
organic matter – algae, leaves etc) and crawls on
the bed as opposed to swimming. It also has a distinct seasonal
abundance (summer only). It has less inherent value as food
for juvenile salmon than Baetis.
These are the smallest of the mayflies and live amongst
the sediment and debris on the bed. The also eat detritus.
All examples caught are probably Caenis rivulorum.
This group of mayflies eats algae off stones or detritus.
They have a characteristic flattened streamlined shape to
allow them to cling to stones in fast currents. They are
not particularly good swimmers like Baetis and
mainly crawl. They include flies like the March Brown (Rithrogena
semicolorata) and a large bright yellow fly (Heptagenia
sulphurea) that is sometimes called the “Mayfly”
in Scotland or the Yellow May Dun on chalkstreams. Heptagenia
are associated with rough streams especially.
There are many different species of stoneflies in our
rivers. However, apart from one species Perla bipunctata
(the large stonefly), a large carnivorous larvae of very
distinct appearance, the stoneflies were not differentiated
further. Stoneflies are associated with water of very high
quality. Some, like Perla are carnivorous and others
like Leuctra, which were probably the main group
encountered, eat detritus. Stoneflies can be an important
food for salmon.
This family of caddis flies does not build a case to live
in and is of a grub-like appearance. Hydropsyche
feed on fine detrital material or plankton which is filtered
from the current in fine silk nets which they spin on stones
for that purpose. They live very much in or under the substrate.
Ryacophila and Polycentropus
These also are caseless caddis larvae, but are predators
of other insect larvae. Again they crawl amongst and under
the stones and so may be of lesser value to salmon.
These include many species of which have not been identified
further. Cased caddis exibit a range of feeding behaviours.
Some only eat detritus but some do scrape algae too.
Diptera (Flat winged flies)
There are many different species of chironomids and these
have not been differentiated. Likewise their habitats also
vary. Chironomids are small larvae and often prolific
and so have widely been found to be a most important salmon
food. They are often associated with silt deposits into
which they burrow.
Simulium larvae attach themselves by one end
to stones or plants by sticky threads. They sway in the
current and filter detritus from the water. They can be
prolific being multivoltine, and can be an important food
for salmon. Being attached in exposed places, they are picked
off by juvenile salmon.
Gammarus (freshwater shrimps)
These again are detritus eating, living on dead leaves
and other matter accumulated on the river bed.
Results of invertebrate survey
The numbers of different types of invertebrates caught at
different locations are shown on page 34.
The general pattern is that in the main stem of the Lyon
detritivores such as Ephemerella and Hydropsyche
caddis flies are most abundant. It should be noted, however,
that at Pubil and Stronuich, Ephemerella were not
observed on 13 June. However, they were present at Pubil on
3 July and also in an unrecorded sample at Stronuich on 16
July, many of which were small. They were very abundant at
Cashlie, 500m below Pubil, on 5 August. Ephemerella
is known to have a long egg incubation followed by rapid larval
growth in summer (Brittain and Saltveit, 1989). The delayed
hatching of Ephemerella in the upper Lyon may be
related to the lower water temperature experienced in spring.
In the tributaries of the Lyon, Ephemerella and
Hydropsyche were scarce, the fauna generally being
dominated by Baetis. This difference was not only
relative, but absolute. In the main stem Baetis were
generally scarce. The only exception was the site at Milton
Eonan which was also unusual in respect of Heptagenia
and especially Simulium which were scarcely found
elsewhere in the upper Lyon. However, the current at this
site was much stronger than at any of the others and so there
might have been local habitat factors at work.
In fact looking more widely at samples from other upland
rivers in the district, it can be seen that Baetis
tend to be plentiful. The more “upland” the stream
the greater the dominance by Baetis appears to be.
Ephemerella become more numerous as streams get larger
and presumably richer in detritus. It is notable that the
Ephemerella production in the Tay at Kenmore is vast.
The stonefly Perla bipunctata was not found at all
in the main stem of the Lyon but was found in all the Lyon
tributaries. Looking across the sites, this species seems
to occur in the type of unenriched upland streams where the
Baetis are most dominant.
A notable stream is the Allt Chiorlaich, where in 2000 fry
grew by far the fastest (Table 1, p. 34). Owing to drought
this stream was actually dry in May 2001. On sampling in early
July after it had started flowing again few invertebrates
were found. However, on another sample on 5 August, it was
found to have a very high number of Baetis present.
A number of studies have shown that Baetis, if present,
are a very important component of the diet of young salmon,
especially fry (Maitland 1965, Egglishaw 1967), as can Simulium
(Egglishaw 1967). It is tempting to speculate that the apparent
lack of these groups in the mainstem of the Lyon might be
a major element in the salmon’s problems.
The lack of these invertebrates may in some way be related
to the profuse growths of filamenteous algae. Algal growth
can smother every stone in the river for over 20 miles on
occasions (personal observations). However, in the Lyon tributaries
the stones “sparkle” and have only ever a fine
coating of algae. Presumably in the tributaries, the invertebrates
find such algae as there is palatable and probably succeed
in grazing it down. Perhaps in the main stem they find the
algae unpalatable. This algae has been tested and found not
to contain toxins. However, it is apparently the case that
filamenteous algae is rarely eaten when alive (Allan 1995).
Simulium have also been found to do badly where algae
is profuse as they cannot obtain anchoring sites (Boon 1988).
A possible cause of the profuse growth of algae is the fact
that the Lyon is regulated. Indeed, as referred to earlier,
proprietors on the Lyon complained about the amount of “slime”
in the river even in 1965 which they then believed was a consequence
of the water originating in reservoirs.
It is the case that reservoirs can have profound effects
on the ecology of rivers downstream, especially those from
which water drains from the bottom of the dam. In a review
based on American experience as early as 1963, Neel included
among possible impacts, profuse algal growth, temperature
and chemistry changes and changes to the flow regime and sediment
movement. Regarding chemistry, the discharge of iron, manganese
and sulphides were specifically mentioned.
Boon (1988) reviewed studies of the downstream impacts of
reservoirs in the U.K. He remarked on the paucity of studies
relative to North America, citing a number of studies in northern
England and Wales, but surprisingly for the number of dams,
none at all in Scotland.
The biological effects of dams varies greatly according to
local factors. In some instances Baetis have increased,
in others decreased (Brittain and Saltveit 1989). For example,
in the River Elan, Wales, which was dammed a century ago to
supply water to the Midlands, mayfly species have been severely
reduced including Baetis. In this river the stones
are coated in a curious deposit containing iron and manganese,
which was considered responsible. As referred to earlier,
the stones in the upper Lyon are in fact coated in a black
precipitate which testing has shown contains aluminium and
manganese. The significance of this is as yet unknown.
Downstream of dams there can often be an increase in invertebrates
which live on detritus. Increases in Ephemerella
are commonly reported (Brittain and Saltveit 1989) as have
increases in Hydropsyche (Boon 1988). Brittain and
Saltveit quote a German example where Baetis and
Heptagenia decreased and Ephemerella increased,
similar to the Lyon. In general the stone clinging Heptagenia
appear to be adversely affected by profuse algal growths below
dams (Brittain and Saltveit) as are Simulium, even
though they as filter feeders might be expected to do well
The life cycles of invertebrates have also been found to
be modified by changes in temperature regime. Decreased spring
and summer temperatures can delay hatching and reduce growth,
and in some cases can result in the elimination of particular
species (Boon 1988). A temperature impact on Ephemerella,
is noticeable in the upper Lyon.
The absence of Perla bipunctata, as here, has been
reported elsewhere (Boon 1988).
On the basis of numerous studies performed elsewhere, it
appears possible that the differences in invertebrate fauna
between the mainstem River Lyon and it’s tributaries
are some consequence of the damming of the headwaters and
subsequent regulation of the river. While cause and effect
have not been truly established it is tempting to speculate
that the differences in invertebrate fauna have resulted in
a combination of reduced growth rates and abundance of juvenile
salmon, effecting a loss in salmon production from this system.
Exactly, what the cause is will require further study to
determine how invertebrate populations and algal abundance
changes seasonally along the river and it’s tributaries.
This then needs to be related to factors like temperature,
hydrology and water chemistry. When the mechanisms are clearly
understood, then perhaps some ameliorative action might be
Allan J. D. (1995) Stream Ecology. Chapman &
Boon P.J. (1988) The impact of river regulation on the invertebrate
communities in the U.K. Regulated Rivers: Research and
Management, 2, 389 – 409.
Brittain J.E. and Saltveit S.J. (1989) A review of the effect
of river regulation on Mayflies (Ephemeroptera). Regulated
Rivers: Research and Management, 3, 191-204.
Egglishaw H. J. (1967) The Food, Growth and Population Structure
of Salmon and Trout in two Streams in the Scottish Highlands.
Freshwater and Salmon Fisheries Research 38.
Maitland P.S. (1965) The feeding relationships of salmon,
trout, minnows, stone loach and three-spined sticklebacks
in the River Endrick, Scotland. Journal of Animal Ecology,
Neel J.K. (1963) Impact of Reservoirs. In Limnology in
North America (ed. D.G. Frey), pp. 575-593. University
of Wisconsin Press, Madison, Wisconsin.
Thanks are due to the Tay Foundation for purchasing the temperature
loggers used in this study and the FRS Freshwater Fisheries
Laboratory for arranging the chemical testing of algae and
sediment samples. I also wish to thank Jane Keay of FFL for
invaluable assistance in the identification of invertebrates.