Plant invasions and trout rations: the sequel

It’s a great pleasure to welcome back Alex Seeney to the WTT Blog. Just over 18 months ago, he was one of the first of the early career researchers to contribute a post (The riparian invasion: salmonid friend or foe?) about their ongoing science. Well, Alex is now Dr Alex (congratulations) and has returned with an update which I, for one, have been eager to see. I well remember some work by Sally Hladyz on how invasive rhododendron can severely impair stream functioning; her work demonstrated that the plant supplied poor leaf litter quality and blocked out the sun, subsequently depressing decomposition rates and algal production rates meaning less food for inverts. Do balsam and knotweed exert similar influences? Over to (Dr) Alex.…. 

Invasions by non-native species are reported as one of the greatest threats to global biodiversity, and the invasion of riparian ecosystems by invasive non-native plants (INNP hereafter) presents a common and difficult challenge for river and fishery managers. Whilst the various impacts of INNP are well-documented in a range of global studies, the degree to which salmonid fish are affected by them remains poorly understood. Brown trout and Atlantic salmon are two of the most important and well-recognised species, providing a range of economical, societal and ecological benefits to the rivers they inhabit. They can act as ecosystem engineers through the disturbance and mobilisation of substrate, and recent studies even suggest that nutrients from adult salmon carcasses may directly affect their offspring, enhancing egg and juvenile survival.

A typical ​‘impacted’ stream in the summer, dominated on both banks by Himalayan balsam.

Salmonids rely on smaller headwaters as both spawning and nursery grounds, partly due to the presence of a dense riparian canopy, which offers refuge from predation and an excellent terrestrial prey source to complement their aquatic diet. However, the colonisation of riparian areas by INNP such as Himalayan balsam and Japanese knotweed can cause substantial changes to the size and structure of the bankside plant community, with implications for the timing, quantity and quality of terrestrial prey inputs entering the stream. Whilst this may have fairly immediate consequences for both the aquatic and terrestrial invertebrate communities that salmonids feed on, the significance of these changes on the ability of juvenile salmonids to acquire and consume sufficient numbers of prey items is relatively unknown.

A ​‘control’ study site dominated by native vegetation. Depletion sampling was used to quantify juvenile salmonid populations.

We surveyed populations of juvenile salmonids at 24 study sites in catchments across central Scotland. Sites were selected based on suitability and accessibility for both juvenile and adult salmonids, and were chosen in consultation with local fisheries trusts to ensure that a healthy and representative population of juvenile salmonid fish would be present. On each stream, a pair of control sites were located upstream from a pair of invaded sites containing established stands of either Himalayan balsam or Japanese knotweed, (sites were separated on average by 0.35km). To quantify the prey ingested by juvenile salmonids, we used stomach flushing to remove the stomach contents from a subset of fish caught in electrofishing surveys, and subsequently preserved and identified the aquatic and terrestrial invertebrates obtained in these samples. To investigate the contribution of invertebrate taxa groups to salmonid diets, we used the Manly-Chesson index, which allows the relative selection of specific groups of taxa to be quantified in relation to the availability of all prey items in an environment.

A fine brown trout specimen obtained during electrofishing surveys.

Where banks had greater infestation with Himalayan balsam, salmonids tended to significantly increase their selection for mayflies while consuming fewer chironomids. No such relationship was observed for Japanese knotweed. At sites with greater riparian INNP cover, the replacement of a native riparian tree canopy by a smaller INNP riparian canopy may expose the stream channel to sunlight for prolonged periods of time. This may in turn promote longer periods of drifting behaviour in mayflies in a bid to avoid biological damage, also making them more available to feeding salmonids. We think that fewer chironomids are eaten at the more heavily impacted balsam sites because they are less easy to target — the opportunistic and visual nature of salmonid feeding means that they are more likely to switch to more easily accessible prey items in this case. 

Overall, preliminary analyses of these data suggests that the dietary choices made by juvenile salmonids are influenced more by relative abundances of invertebrates in the drift and on the streambed, and also by competition between and amongst trout and salmon, than by the presence of INNP. 

Full model predicted values (shaded polygon shows + 95% confidence intervals) from a statistical analysis of Manly-Chesson selectivity for Ephemeroptera (mayflies) plotted against Himalayan balsam cover. Individual points represent raw Manly-Chesson selectivity values. 

Broadly, it seems that salmonids are able to persist and obtain adequate volumes of prey, even in dynamic and disturbed habitats colonised by riparian INNP. Given their adaptable nature, perhaps this is to be expected, and is an indication of the potential for banks dominated by INNP to still supply an adequate subsidy of terrestrial invertebrate prey. Salmonids have been the focus of a wide range of studies examining the impacts of in-stream and riparian restoration efforts on fish populations. The research we are carrying out at Stirling seems to indicate that riparian restoration solely aimed at removing INNP and restoring native vegetation is unlikely to directly affect the dietary selections of trout and salmon. Improvements are more likely to be observed in response to interventions such as introduction of large woody debris, riparian buffers and mitigation of harmful pollution and sediment ingress (which is linked to INNP). However, future studies should incorporate riparian INNP when evaluating variation in juvenile salmonid populations, in a bid to further describe the impact of INNP in relation to other environmental and hydrological stressors.

In the near future, I hope to reanalyse these feeding data to incorporate some of the relationships we have demonstrated between INNP and riparian invertebrate communities. With a year of lab time behind me and roughly 140,000 invertebrates identified from aquatic and terrestrial samples, we have a wealth of data with which to continue the investigation into the impacts of INNP on riparian systems.