Introduction
Recently, the planting of broadleaved trees along rivers
and streams has been widely promoted. One of the major selling
points is a perceived benefit to salmonid fish and fisheries.
A good example is the recent booklet Restoring and Managing
Riparian Woodlands by Scottish Native Woods. However,
when this issue is investigated in depth the expected benefits
to fisheries are not that clear.
Riparian woodland is claimed to benefit fish for the following
reasons:
· Leaves and twigs falling into streams provide food
for invertebrates which the fish then eat.
· Trees shade streams creating a better (cooler) temperature
regime for fish
· Trees prevent bank erosion, resulting in narrower
deeper streams and less siltation.
These points are now considered in more detail.
The value of leaf litter to salmon
In streams, leaf litter is attacked and broken down by bacteria
and fungi which are in turn consumed by invertebrates which
ingest the decaying leaf material. The really fine detritus
which ultimately results is eaten by other types invertebrates
which filter it from the water. It is assumed, therefore,
that if there is more litter there will be more invertebrates
and hence more young salmon.
However, stream invertebrates do not only depend on dead
terrestrial material. Some species eat living algae which
grows on stones and others eat decomposing algae and it’s
eventual breakdown products. Weight for weight, algae and
algae detritus is more nutritious than leaves.
A paradox arises in that trees planted to add food for particular
invertebrates can, by shading, reduce the amount of algae
in a stream and thereby reduce those invertebrates which depend
on algae. This could be a problem because those invertebrates
which depend on algae are readily eaten by salmon. Studies
of salmon diet by Maitland (1965) and Egglishaw (1967) found
that algal grazers are very important, notably Baetis,
a type of mayfly, and Hydroptila, a grazing caddis
larvae. Both studies also found orthoclad chironomid larvae
to be important. While perhaps primarily detritus feeders,
Hawkins et al. (1982) reported that in North America, in streams
from which trees had been clear felled, numbers of orthoclads
tended to rise, as did Baetis and grazing caddis.
The negative impact of shade on Baetis is in fact
well known (Hynes 1970). Shade has even been found to impact
on the filter feeding Simulium (blackfly) larvae
(Hynes 1970), another important food of salmon (Egglishaw
1967)
Of course, studies of the diet of salmon in unshaded streams
where species like Baetis are abundant does not prove
that these are inherently the best food. However, studies
on shaded streams do. For example, in British Columbia, Bilby
and Bisson (1987) described changes in a stream after trees
were clear-felled. Algae production increased, but the reduced
litter fall meant that the total amount of carbon in the stream
fell dramatically. Despite this coho salmon fry grew much
faster, which they attributed to the increased abundance of
Baetis. The larger caddis larvae which had eaten leaves were
of lesser value to the small fish. Nearer to home, O’
Grady (1993) found in over 20 Irish streams the numbers of
juvenile salmon in shaded reaches averaged only 19% of that
in adjacent unshaded reaches.
It seems accepted that thick shade is undesirable. Intuitively,
it may be imagined that there should be some intermediate
point where an optimum invertebrate production is achieved.
However, this is as yet only a guess. The Forestry Commission’s
Forest and Water Guidelines do in fact recommend 50%
shade, but as Mackenzie (1996) pointed out, even this figure
does not appear to have been established on any scientific
basis.
Until thorough research is done on this subject, the value
of tree planting in adding to salmon food must be considered
unproven. One thing which research has proven is that the
level of nutrients in a stream with it’s consequent
impact on algae growth and indeed on the rate of decomposition
of leaves, is a much more important control on invertebrate
production than the amount of terrestrial organic material
(Egglishaw 1964, Hicks et al. 1991). Serious attempts
at increasing stream productivity should consider fertilisation
as opposed to planting trees.
Invertebrates falling off overhanging trees
It is often assumed that tree living invertebrates falling
off branches into the water will add to salmon food. While
insects do fall off trees, studies have failed to demonstrate
a consistently higher level of terrestrial invertebrate input
from a tree canopy than for an open stream. For example, trees
shelter streams and so might reduce the amount of terrestrial
insects blown off the surrounding land. Heather moorland for
example is rich in insects.
In addition studies have consistently shown that terrestrial
flies are of little importance to salmon, a bottom dwelling
species, as opposed to trout, a mid water species. Reports
extolling the virtues of this form of production often refer
to “salmonids”, when they really mean trout.
Debris Dams
Pools formed by dams made from fallen trees are often cited
as a benefit of riparian woodland. The value of such pool
habitats is increasingly being recognised on the Pacific coast
of North America. Naturally, information in American literature
ultimately influences opinion here.
Indeed a number of Pacific salmon species do seem adapted
to utilise such environments, especially juvenile coho. There
rivers tend to be very steep and trees much taller than Europe,
meaning that many of the pools in their rivers were naturally
created by log jams. In behavioural terms, our salmon are
very different. They are not pool dwellers, but prefer fast
riffling water, a niche occupied in North America by juvenile
steelhead. Again, such pools have not been shown to benefit
salmon here.
Reduced summer temperatures
It is often said that trees are beneficial because their
shade prevents summer temperatures reaching lethal levels.
This indeed is true in the lower latitudes at which salmonids
are found (Portugal, California). However, in cool Scotland
there is a lack of evidence to show that overheating is a
problem. In fact it is the case that juvenile salmon actually
do best in quite warm water, 16 degrees being optimum. If
there is a general problem in Scotland, it is that optimum
growth conditions are scarcely achieved. In one of the few
studies of this type, Egglishaw and Shackley (1982) found
that in the Shelligan Burn, an unshaded burn near Glenalmond
College, one of the years of highest production in the 1970s
was 1976, a notably hot and dry year. Also the record smolt
run in 30 years from the Girnock Burn, a treeless monitored
stream in Deeside, occurred in 1977, after the 1976 drought.
If more proof were required, consider the upper reaches of
the River Dart in Devon. From personal experience, the physical
shape of the tributaries (granite substrate) is similar to
many Scottish streams and the banks generally bare. However,
the summer sun is very much stronger than in Scotland, yet
many streams these hold almost as high salmon densities.
Trees prevent erosion
Tree roots are an effective means of binding river banks.
In rivers with gravel beds and sandy banks, trees can play
a valuable role preventing them widening and becoming very
shallow. That is, as long as old trees are not allowed to
topple over and set off a chain reaction of erosion.
However, in the Tay catchment, stream “overwidening”
is generally not a danger. Many streams, though not all, have
boulder beds inherited from the ice age and are inherently
stable. In fact if there is a problem in some of our tributaries,
it is that they are too stable, the Tilt being a good example.
Short stretches of active bank erosion are necessary to provide
gravelly fords, essential for salmon spawning. Also while
older salmon parr do well in deeper bouldery reaches, the
existence of reaches only a few centimetres deep, especially
in the headwaters, may provide essential habitat for fry.
There is not, therefore, a general erosion problem in this
district which needs tackling by tree planting.
Trees provide “cover”
It is sometimes thought that the roots of trees like alder
provide valuable shelter for small fish. In wooded streams
with a fine substrate and few crevices in the bed this may
indeed be true. However, in such streams the shade from trees
tends to suppress the growth of grasses and emergent vegetation
which also provides cover and helps to maintain undercut banks
etc. The net result is that in gravel or sand bedded streams
trees actually tend to reduce fish cover. In higher gradient
bouldery streams, the dominant type in the Tay catchment,
the impact is likely to be neutral.
Trees absorb water
One point which is often overlooked is that trees soak up
and lose large volumes of water through evapotranspiration
in hot weather. This is a serious consideration, especially
when the whole width of the floodplain may sometimes be considered
for planting. In our thin soiled upland areas gravelly floodplains
are important groundwater stores to keep burns flowing. The
long term impact of planting such areas needs serious consideration.
Conclusions
Some readers of this article may find it rather disappointing.
The planting of trees appeals to the popular imagination for
it’s own sake and is generally assumed to be a good
thing.
However, it is the case that benefits of riparian woodland
to salmon have not been demonstrated. In fact the weight of
actual evidence would suggest otherwise, at least if trees
are allowed to be prolific. While there may be a point at
which some trees might increase salmon production, at this
moment we do not really know whether this is the case or what
this point might be.
Even if having some trees is found to be beneficial, the
management required to prevent deterioration to some worse
state is likely to be prohibitive. In a climate like Perthshire,
a very costly effort might be required prevent trees from
taking over.
Therefore, the conclusion for the moment must be to maintain
the status quo, at least for streams under 10 metres
wide, until this subject is properly researched. It should
be remembered that there are many treeless rivers and streams
in Scotland which are very productive of juvenile salmon.
Recent declines in adult salmon abundance have nothing to
do with the absence or presence of trees. If the wooding of
our streams turns out to be a mistake, it will be costly to
rectify.
References
Allan J.D. (1995) Stream Ecology. Chapman &
Hall. London.
Bilby R.E. and Bisson P.A. (1987) Emigration and productivity
of hatchery coho salmon (Oncorhynchus kisutch) stocked
in streams draining an old-growth and a clear-cut watershed.
Canadian Journal of Fisheries and Aquatic Sciences,
44, 1397-1407.
Egglishaw H.J. (1964) The distributional relationship between
the bottom fauna and plant detritus in streams. Journal
of Animal Ecology, 38, 19-33.
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.
HMSO, Edinburgh.
Egglishaw H.J. and Shackley P.E. (1982) Influence of water
depth on dispersion of juvenile salmonids, Salmo salar
L. and Salmo trutta L., in a Scottish stream. Journal
of Fish Biology, 21, 141-155.
Hawkins C.P., M.L. Murphy and N.H. Anderson (1982) Effects
of canopy, substrate composition and gradient on the structure
of macroinvertebrate communities in Cascade Range streams
of Oregon. Ecology, 63, 1840-1856.
Hicks B. J., J. D. Hall, P. A. Bisson and J. R. Sedell (1991)
Responses of Salmonids to Habitat Change. In Influences
of Forest and Rangeland Management on Salmonid Fishes and
their Habitats (ed. W.R. Meehan), pp. 483-518. American
Fisheries Society Special Publication 19, Bethesda, Maryland.
Hynes H.B.N. (1970) The Ecology of Running Waters.
University of Toronto Press, Toronto.
Mackenzie N.A. (1996) The riparian woodland ecotone.
Scottish Native Woods, Aberfeldy.
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,
34, 109-33.
O’ Grady M.F. (1993) Initial observations on the effects
of varying levels of deciduous vegetation on salmonid stocks
in Irish waters. Aquaculture and Fisheries Management,
24, 563-573.
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