Direct and indirect effects of riparian canopy on juvenile stream-dwelling salmonids in south-west Ireland David McCormick & Simon Harrison School of Biological,

Direct and indirect effects of riparian canopy on juvenile stream-dwelling

salmonids in south-west Ireland

David McCormick & Simon HarrisonSchool 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

Direct effects:

Cover from avian predatorsDecrease in primary productivity but increase in organic matter inputEffects on water temperature

Indirect effects:

Impact on macroinvertebrate communitiesDistribution and abundance of aquatic macrophytes

Effects of riparian canopy on salmonid habitat

• 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 broad-scale management.

Best riparian management strategy?

Role of instream macrophytes

• Many Irish streams have abundant submerged or floating-leaved 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

• 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.

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.

• 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.

(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

Mean abundance of dominant prey items in the stomachs of 0+ salmon in the open (O) and shaded (S) stretches

Salmonid diet

■=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

• Gut contents were significantly less equitable for salmon in the open, macrophyte-dominated stream sections: a few salmon ate a lot more!

Rank abundance curves for total gut contents of salmonids for all rivers combined.

Open stretches Shaded stretches

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…