This document summarizes research on the impact of invasive riparian plants on salmonid fish populations in Scottish rivers. Preliminary results found that salmonid biomass was most affected by distance from the river source, while individual salmon density was positively correlated with invasive plant cover and trout density was negatively correlated. Further sampling is needed to fully understand how invasive plants may influence aquatic invertebrate communities and the proportion of terrestrial invertebrates in salmonid diets. Upcoming fieldwork in 2016 will include expanded dietary analysis of fish under 60mm and additional sampling of terrestrial and aquatic invertebrates.

1. An introduction to riparian invasive plants
• The riparian interface between terrestrial and aquatic communities is critically
important to aquatic ecosystems.
• Invasive riparian plants can influence the aquatic environment via the
contribution of primary and secondary terrestrial energy sources1.
• These allochthonous energy sources can vary depending on the density and
diversity of the riparian plant community2, consequently affecting stream biota.
• It can be difficult to tease apart the effects of these plants from other riverine
eco/morphological factors that naturally affect salmonids.
• A comparison of a native (left) and an invaded site (right) is shown below– a
dense monoculture of Himalayan balsam and associated shading can be seen at
the invaded site.
Preliminary conclusions and plans for 2016
• Salmonid biomass appears to be most affected by hydromorphological
processes such as wet width and distance from source.
• Invasive cover appears to have a small effect on individual salmon and trout
density – this will be explored further with the use of vegetation surveys to
allow invasive cover to be expressed on a continuous scale.
• It is possible that invasive plants have smaller scale effects on terrestrial and
aquatic invertebrate community structure, although further sampling is required
to test this.
• Fieldwork this summer will be focused on a full assessment of salmonid diet,
complete with multiple terrestrial and aquatic invertebrate samples, including:
• Pitfall and malaise trapping
• Drift netting
• Surber sampling
• In addition, the dietary study will be extended to salmonids under 60mm, which
may select smaller prey items due to gape size limitations3.
Acknowledgements
I would like to thank Scottish Natural Heritage for funding this research.
Gastric lavage carried out under Home Office Project Licence PPL 70/8673
References
1Pusey, B.J., Arthington, A.H. (2003) Importance of the riparian zone to the conservation and management of freshwater
fish: a review. Marine and Freshwater Research 54: 1-16.
2Claeson, S.M., LeRoy, C.J., Barry, J.R., Kuehn, K.A. (2014) Impacts of invasive riparian knotweed on litter decomposition,
aquatic fungi, and macroinvertebrates. Hydrological Processes 14: 2959-2976.
3Baglinière, J-L & Maisse, G. 1999. Biology and ecology of the brown and sea trout. Chichester, UK: Praxis Publishing
Aims
• Develop quantitative methods for teasing apart the effects of invasive plants
from underlying riverine processes.
• Quantify effects of invasive cover on:
• terrestrial and aquatic energy sources to salmonid diet
• terrestrial and aquatic invertebrate communities
• Identify the strongest predictors of salmonid biomass and density to guide
subsequent population and diet studies in 2016.
Methods
• Depletion sampling was used to estimate salmonid biomass and density.
• Gastric lavage (stomach flushing) carried out on fish over 60mm under Home
Office licence to assess dietary variations between native and invaded sites.
• Terrestrial and aquatic invertebrate samples taken concurrently using drift and
malaise traps to quantify invertebrate communities present at each site.
• A suite of physicochemical measurements were recorded to identify the
strongest predictors of salmonid biomass and density.
Preliminary results
• Total of 1670 Atlantic salmon and brown trout caught across 24 sites; subset of
391 stomach flushed.
• Biomass was unaffected by invasive cover, and was found to be strongly affected
by distance from source (χ2 (1)=12.51, p=0.00040).
• Fulton’s condition factor (a measure of fish health based upon length and
weight) was unaffected by invasive cover.
• The proportion of terrestrial invertebrates in salmonid diet was unaffected by
invasive cover – further exploratory analysis is required to confirm this.
• Total salmonid density was most strongly affected by wet width.
• Individual salmon density was positively affected by invasive cover (χ2 (1)=4.35,
p=0.0371); individual trout density was negatively affected (χ2 (1)=4.45,
p=0.0349).
Malaise trap (left) and drift net (right) in situ.
Demonstration of the gastric lavage procedure.
N=282 N=304
N=414 N=670
Linkages between riparian invasive plants, hydromorphology
and salmonid fish in Scottish rivers
Alex Seeney*1, Colin Bull1, Nigel Willby1, Philip Boon1,2
1Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, UK
2Scottish Natural Heritage, Silvan House, 231 Corstorphine Road, Edinburgh, EH12 7AT
*alex.seeney@stir.ac.uk@AlexSeeney