Direct and indirect effects of riparian canopy on juvenile - PowerPoint Presentation

Slide1

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 CorkSlide2

IntroductionPotential

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 biotaSlide3

Direct effects:Cover from avian

predatorsDecrease in primary productivity but increase in organic matter input

Effects on water temperatureIndirect

effects:

Impact on

macroinvertebrate communitiesDistribution and abundance of aquatic macrophytes

Effects of riparian

canopy on

salmonid

habitatSlide4

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?Slide5

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

.Slide6

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).Slide7

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.Slide8

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) C

omparison

between the gravel

surbers in the open (□) and shaded stretch (■) of each pair.

(B) C

omparison

between the gravel surbers (□) and the

macrophyte

surbers

(■) within the open stretches

.

Macroinvertebrate

densitySlide9

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 gravelSlide10

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 stretchesSlide11

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…