The document summarizes a PhD study investigating the impacts of invasive riparian plants on juvenile salmonids in low order streams. The study monitored 24 sites across 6 rivers, with paired control and treatment sites where invasive plant coverage exceeded 50%. Over two years, the study collected biotic and abiotic samples to assess differences between native and invaded sites, including fish surveys, invertebrate samples, and vegetation surveys. Preliminary analysis found invasive cover had no effect on overall fish biomass or diet, but positively influenced salmon density and negatively influenced trout density, possibly due to differences in habitat preferences between the two species under conditions of bank instability from plant dieback. Further analysis of samples is ongoing to better understand impacts on a fine scale

1. FBA News, No. 70 Winter 2016 05
The riparian interface between terrestrial and
aquatic communities is critically important
to aquatic ecosystems. Riparian ecosystems
are some of the most diverse and complex
habitat types (Naiman et al., 1993), and in
providing a physical buffer between land
and water, they are of particular importance
to the health, water quality and associated
community structure of the waterways
they border. Invasion of the riparian zone
by non-native flora is commonly perceived
as a negative occurrence and can also
have a sizeable economic impact, with
an estimated £1.7 million spent annually
to control riparian Japanese knotweed
(Fallopia japonica) in Scotland (Williams et
al. 2010). Given the number of ways in which
invasive riparian plants can affect structure
and function in low order streams, it is
important to consider how they may also
affect salmonid fish, especially considering
their high economic value to angling, and the
status of the Atlantic salmon (Salmo salar)
as a protected species in the UK.
Invasive riparian plants can influence
the aquatic environment via shading
and lowering water temperature, whilst
altering the quality, quantity and timing of
terrestrial resources to streams (Claeson
et al., 2014). Rapid growth enables these
invasive plants to outcompete native species
for resources such as nutrients and light,
leading to the formation of dense stands
called monocultures (Figure 1, right). These
stands die back at the first sign of winter
frost, leaving riverbanks exposed to the
elements and leading to physical changes
to the structure of the river as the bank
is weakened and collapses, consequently
altering instream habitat availability for
salmonids. However, increased shading in
riparian zones is also shown to curb (but
not inhibit) instream growth of aquatic
macrophytes, and may also encourage
growth of more mat-like plants, the roots
and branches of which can provide more
sources of refuge for invertebrates and small
fish (Zefferman, 2014). These plants may
also provide bankside cover for salmonids
during the summer months, offering
shade and refugia from predation whilst
also supplying an additional energy source
via terrestrial invertebrates. Conflicting
arguments such as these suggest that
these invasive riparian plants cannot simply
be categorised as good or bad.
Whilst searching for field sites for my
PhD project in early 2015, the height and
depth of the previous year’s knotweed
stands were easy to see. These stands were
often wider than the small tributary streams
they bordered and as such, tracking the
spread of these plants along river corridors is
quite a simple process (Figure 2). However,
relating the presence of the invasive stands
to hydrological and fluvial processes and
furthermore, to aquatic biota, is more
difficult. Understanding the complex links
between the dynamics of a river system and
the flora and fauna associated with it requires
a synoptic knowledge of the type and
magnitude of factors involved, particularly
when evaluating habitat availability for
salmonid fish.
As someone who is interested in the
ecology and life history of salmonids, the
chance to investigate habitat and food
availability for juvenile Atlantic salmon
and brown trout (Salmo trutta) in invaded
low order streams presents a fascinating
opportunity to link invasive riparian plants
to a range of biotic and abiotic stressors
that may affect fish populations. Monitoring
both aquatic and terrestrial invertebrate
communities is also critically important,
as both groups provide essential sources
of energy for salmonids. Changes in the
physical and chemical composition of riparian
plants may affect both invertebrate groups,
altering the type and volume of food source.
The aim of my PhD research is to tease apart
the effects of invasive riparian plants from
underlying riverine processes, quantifying not
only the direction, but also the magnitude of
these effects, which may take a step towards
justifying the large amount of funding that is
While studying Veterinary Science at the Royal Veterinary College, London, Alex Seeney was fortunate enough to undertake a placement
at the London aquarium which sparked a fascination with aquatic ecology and fish biology. This prompted him toward the Freshwater
and Coastal Sciences MSc at Queen Mary University of London, where he was supervised by Drs Eoin O’Gorman and Guy Woodward. Alex
examined the effects of warming on individual organism mass-abundance scaling, with fieldwork carried out in a geothermally-heated
catchment in Hengill, southwest Iceland. So taken with this work was he, that he followed his supervisors to the Silwood Park campus of
Imperial College, and continued to work on diatom and macroinvertebrate samples from Hengill. Since 2014, he has been based at Stirling
University, examining the effects of invasive riparian plants on juvenile salmonids in low order streams, on a project funded by Scottish
Natural Heritage, and is supervised by Dr Colin Bull and Professor Nigel Willby (both at Stirling University) and Professor Phillip Boon
(Scottish Natural Heritage).
The riparian invasion: salmonid friend or
foe?
Figure 1. A comparison of typical native plant dominated (left) and invaded (right) sites. Photo: Alex Seeney.

2. FBA News, No. 70 Winter 201606
currently being provided to treat such plants
along Scottish rivers (and elsewhere).
Field sites were selected based on
suitable habitat for adult spawning and
juvenile survival. Sites were located on
small, low order streams, and were chosen
in communication with fisheries trusts to
ensure plentiful populations of salmon and
trout in tandem with established stands of
Himalayan balsam (Impatiens glandulifera)
and Japanese knotweed. A pair of control
sites were located upstream from a pair of
treatment sites on each stream, giving a total
of 24 sites across 6 rivers. Treatment sites
were chosen under the criteria that invasive
plant coverage must exceed a minimum of
50 % (based on a visual assessment).
To assess the impacts of invasive riparian
cover on juvenile salmonids, I have collected
a range of biotic and abiotic samples over a
2 year fieldwork period across 2015-16. In
order to monitor morphological changes at
sites, thalweg profiles (showing the main
flow of the river) and cross-sectional riverbed
profiles were recorded each year, in tandem
with pebble counts to monitor changes in
grain size diversity and distribution. Full
depletion electrofishing surveys were carried
out during August 2015 and 2016, covering
a minimum of 100 m2
where possible at
all sites. Weights and fork lengths were
recorded for all salmon and trout, and a
small sub-sample (approximately 20 fish
per site) were anaesthetised and a gastric
lavage (stomach flushing, Figure 3) was
performed to assess diet composition. This
procedure was regulated under Home Office
licence (Home Office Project Licence PPL
70/8673).
A full range of terrestrial (malaise and
pitfall traps, Figure 4) and aquatic (Surber
and drift) invertebrate samples were taken
during the summer of 2016 to assess the
terrestrial and aquatic communities present
at each site. These samples will be used to
assess how the abundance and diversity
of both aquatic and terrestrial invertebrate
communities (and therefore the availability
of prey items for salmonids) differs between
native and invaded sites. Furthermore,
vegetation surveys were carried out at all
sites during the summer of 2016, which will
allow invasive coverage to be assesed.
Statistical models have been used to
evaluate the magnitude and direction of
a variety of effects on salmonid biomass,
density and dietary composition. Invasive
cover appeared to have no effect on biomass
or density, which were driven most strongly
by physical parameters: distance from
source and wet width, respectively. Similarly,
invasive cover also appeared to have no
effect on the ratio of terrestrial to aquatic
invertebrates in stomach contents. However,
when fish density was analysed at individual
species level, salmon density appeared to
be positively influenced by invasive cover,
whilst brown trout density appeared to be
negatively influenced (Figure 5).
A likely explanation for this can be
found by considering the differing habitat
Figure 3. A demonstration of the gastric lavage procedure. Photo: Alex Seeney.
Figure 2. Japanese knotweed towering over the water. Photo: Alex Seeney.

3. FBA News, No. 70 Winter 2016 07
requirements for juvenile salmon and trout.
Whereas salmon show a preference for
gravel substrates during the day and aquatic
plantsatnight,troutshowarelianceoncover
provided by stream margins throughout
(Riley & Pawson, 2010). Although further
sample analysis is required to confirm these
trends, it is possible that the winter dieback
and reduction in bank stability associated
with invasive riparian cover may lead to
a reduction of marginal stream habitat
during the winter for juvenile trout, whilst
having less of an effect on the in-stream
plant cover and gravel substrates used by
salmon. Further analysis of substrate grain
size distribution and vegetation surveys is
necessary to investigate this hypothesis, as
Japanese knotweed and Himalayan balsam
may alter in-stream habitat differently
depending on their coverage.
The range of samples collected over
the past two years provides an exciting
Biological Invasions 16: 1531-1544.
Naiman R.J. Décamps H. & Pollock M.
(1993). The role of riparian corridors in
maintaining regional biodiversity. Ecological
Applications 3: 209-212.
Riley W. & Pawson M. (2010). Habitat
Requirements for Juvenile Salmonids in
Chalk Streams: How will Management Best
Address Conflicting Interests?, in Salmonid
Fisheries: Freshwater Habitat Management
(ed P. Kemp), Wiley-Blackwell, Oxford, UK.
Williams F., Eschen R., Harris A.,
Djeddour D., Pratt C., Shaw R.S., Varia
S., Lamontagne-Godwin J., Thomas
S.E., Murphy S.T. (2010). The economic
cost of invasive non-native species on Great
Britain. CABI, Wallingford, 198 pp.
Zefferman E. (2014). Increasing
canopy shading reduces growth but not
establishment of Elodea nuttallii and
Myriophyllum spicatum in stream channels.
Hydrobiologia 734: 159–170.
opportunity to investigate how invasive
riparian plants may be affecting juvenile
salmonids on a fine scale. Stomach contents
and invertebrate samples from both years
will be used to assess how different salmonid
age classes are utilising the available habitat
and food sources between invaded and
native sites. Furthermore, the use of electivity
indices will highlight any preferential feeding
regimes between salmon and trout, giving
an insight into the effects of invasive riparian
plants on species and age-specific groups of
juvenile salmonids.
Alex Seeney
alex.seeney@stir.ac.uk
References
Claeson S.M., LeRoy C.J., Barry J.R.,
Kuehn K.A. (2014). Impacts of invasive
riparian knotweed on litter decomposition,
aquatic fungi and macroinvertebrates.
Figure 4. Malaise (left) and pitfall (right) traps in situ. Photo: Alex Seeney.
Figure 5. The contrasting effects of invasive riparian cover on Atlantic salmon and brown trout densities. Red * denotes mean values.