The riparian invasion: salmonid friend or foe?

‘Tis the season to bash balsam — if you don’t know how to, check out the definitive guide from WTT chum, Theo Pike, for guidance! Timely then for a new blog focussing on invasive plants. Alex Seeney from the Centre for River Ecosystem Science (CRESS) at the University of Stirling, is battling with balsam and knotweed from a more academic angle, and below gives an overview of some his research to date. This valuable work is supported by Scottish Natural Heritage.

Some of the most diverse and complex habitat types in aquatic systems are found at the interface between terrestrial and aquatic communities – the riparian zone. These diverse, dynamic systems provide an ecologically important buffer between land and water, and as such they are of particular importance to the health and quality of the waterways they border.

The dynamic nature of riparian zones increases their susceptibility to invasion, particularly by non-native flora, which use river networks like blood vessels around the human body, spreading through river corridors and using these systems for both short and long distance dispersal. Invasion of the riparian zone by non-native flora is commonly perceived as a negative occurrence and has a sizeable economic impact, with an estimated £5.6 million spent in 2010 to control riparian Japanese knotweed (Fallopia japonica) in Great Britain.

It is also important to consider how these plants may affect salmonid fish, especially considering the high economic value of salmonids to angling and the status of the Atlantic salmon (Salmo salar) as a protected species in the UK. My aim is to tease apart the effects of invasive riparian plants from a range of environmental factors that affect salmonids, quantifying not only the direction, but also the size of these effects. In doing so, we might take a step towards justifying the large amount of funding that is currently being provided to treat these plants. Monitoring both aquatic and terrestrial invertebrate communities is also important, as both provide essential food sources for salmonids residing in streams and rivers. Changes in the physical and chemical composition of riparian plants may affect both invertebrate groups, altering the type and volume of these food sources entering the aquatic food web.

Whilst searching for field sites for my PhD project in early 2015, the height and depth of the previous year’s invasive plant stands were easy to see. The 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. However, relating the presence of invasive plant stands to hydrological processes and furthermore, to critically important terrestrial and aquatic invertebrate food sources, is more difficult. In order to help me achieve this, a range of samples have been collected over a 2 year fieldwork period during 2015 – 16.

Field sites were selected based on suitable habitat for adult salmonid spawning and juvenile survival. Sites were located on small tributary streams, and were chosen in communication with fisheries trusts to ensure populations of Atlantic salmon and brown trout (Salmo trutta) in tandem with established stands of Himalayan balsam (Impatiens glandulifera) and Japanese knotweed. A pair of uninvaded control sites were located upstream from a pair of invaded treatment sites on each waterbody, 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).

In order to monitor physical 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 summer 2015 and 2016, covering a minimum of 100m² where possible at all sites. Weights and fork lengths were recorded for all Atlantic salmon and brown trout, and a small number at each site were anaesthetised and a gastric lavage (stomach flushing) 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) and aquatic (Surber and drift nets) 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 show how the abundance and diversity of both aquatic and terrestrial invertebrate communities (and therefore the availability of prey items for salmonids) differs between invaded and uninvaded sites. Furthermore, vegetation surveys were carried out at all sites during the summer of 2016, which will allow invasive coverage to be expressed on a continuous (as opposed to binary) scale.

The range of samples collected over the past 2 years provides an exciting 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 uninvaded sites. I also plan to use electivity indices to look for evidence of any preferential feeding habits, giving an insight into the effects of invasive riparian plants on species and age-specific groups of juvenile salmonids.

I would welcome any questions or comments, and hope to provide an update in the coming months once the bulk of these samples have been processed – over 50,000 invertebrate samples identified so far!

Alex (alex.​seeney@​stir.​ac.​uk or @AlexSeeney)