Direct and indirect effects of riparian
canopy on juvenile stream-dwelling
salmonids in south-west Ireland
David McCormick & Simon Harrison
School of Biological, Earth and Environmental
Sciences, University College Cork
Introduction
Potential multiple benefits of protected riparian zones:
-Trap nutrient and sediment runoff
- Enhance bank stability
- Prevent excessive sediment input from grazing stock
- Increase aquatic and terrestrial biodiversity
Increase in riparian tree growth
within protected zones is
assumed/hoped to be beneficial to
stream biota
Effects of riparian canopy on salmonid habitat
Direct effects:
Cover from avian predators
Decrease in primary productivity but increase in organic matter input
Effects on water temperature
Indirect effects:
Impact on macroinvertebrate communities
Distribution and abundance of aquatic macrophytes
Best riparian management strategy?
• Majority of research conducted in other ecoregions.
• Considerable variation reported in riparian canopy effects in
Ireland.
- Positive, neutral and negative effect on density
- Positive effects as cover
- Complex effects on diet and behaviour
• It may be difficult therefore to predict the local effect of broadscale management.
Role of instream macrophytes
• Many Irish streams have abundant submerged or floatingleaved vegetation.
• Significant contributor to habitat heterogeneity and
system productivity.
• Influenced by water chemistry, benthic substrate, flow
conditions and riparian shade.
Study design
• Paired multi-stream study with
open and shaded stretches on
each stream.
• 30-50m stretches electro-fished
3 times (May, August and
September).
• Gut contents of captured fish
taken for diet analysis.
• Macroinvertebrate samples were
taken from gravel and
macrophytes (where present).
Results
Mean (±SE)
estimated densities
of salmon and trout
in open (□) and
shaded (■) stretches
on the seven study
rivers. Streams are
arranged in
ascending order of
macrophyte density.
• Significantly greater numbers of 0+ salmon were found in
open, unshaded, streams stretches.
• 0+ salmon found in the open stretches were also
significantly longer than those found in the shade.
Macroinvertebrate density
(A) Comparison between the gravel surbers in the open (□) and shaded stretch (■) of
each pair.
(B) Comparison between the gravel surbers (□) and the macrophyte surbers (■) within
the open stretches.
• Macroinvertebrate density was significantly greater in the open stretches.
• Within the open stretches, macroinvertebrate density was significantly
greater in the macrophyte patches than in the gravel benthos.
Salmonid diet
Mean abundance of dominant prey items in the stomachs of 0+ salmon in the open (O)
and shaded (S) stretches
■=Chironomidae,
=Simuliidae,
■=Baetidae,
□=Other.
▒
•Trend towards more food in the stomachs of 0+ salmon in the open stretches
• Significant positive correlation between total gut contents of 0+ salmon and
macrophyte density in the stream
• Composition of the diet of 0+ salmon showed a greater similarity to the
macroinvertebrate community of the macrophytes than of the gravel
Rank abundance curves for total gut contents of salmonids for all
rivers combined.
Open stretches
Shaded stretches
• Gut contents were significantly less equitable for salmon in
the open, macrophyte-dominated stream sections: a few
salmon ate a lot more!
Conclusions
Variation in the effect of riparian canopy on juvenile salmon densities
and growth may be mediated by macrophytes.
Macrophytes are an important source of potential invertebrate prey
(Simuliidae, Chironomidae and Baetidae) for juvenile salmon.
Macrophytes also provide highly suitable foraging habitat for juvenile
salmon.
High macrophyte densities likely benefits populations of juvenile
salmon, providing they are not accompanied by deleterious habitat
quality.
Implications for riparian management…