Study

Does the use of in-stream structures and woody debris increase the abundance of salmonids? CEE Review 05-006 (SR12)

  • Published source details Stewart G.B., Bayliss H.R., Showler D.A., Pullin A.S. & Sutherland W.J. (2006) Does the use of in-stream structures and woody debris increase the abundance of salmonids? CEE Review 05-006 (SR12). CEE (Collaboration for Environmental Evidence) Systematic Reviews, 12.

Summary

Study 1

In-stream structures (such as flow deflectors, weirs and woody debris) have been in widespread use for many decades, being incorporated into streams and rivers in an attempt to increase fish stocks, primarily salmonids, but also species of conservation concern such as European bullhead Cottus gobio. A large number of studies have been undertaken to assess the effectiveness of in-stream structures, often with conflicting results. It has therefore been hard to develop a consensus regarding the efficacy of in-stream structures despite their continued use.

A systematic review (see: www.cebc.bham.ac.uk  for methodology) was undertaken to synthesises empirical evidence regarding the effectiveness of in-stream structures in terms of impact on abundance of salmonid fish. 

A total of 38 studies provided quantitative data regarding the impact of in-stream structures on salmonids (or C.gobio) suitable for meta-analysis. Fifty four independent data points provided evidence regarding the impact of engineered in-stream devices on salmonids, with a further 30 data points regarding woody debris.

Engineered structures: Meta-analysis indicates a weakly significant positive impact of engineered in-stream habitat structures on salmonid populations. No ecologically significant impact on salmonid population size or habitat preference was evident. There are no significant relationships between the effectiveness of engineered instream structures and hydrological or ecological variables at a population level, although there is limited evidence that in-stream structures provide preferential habitat at higher discharges.

Woody debris: Woody debris has a significant impact on salmonids resulting in increased population abundance. This is especially pronounced for brook trout Salvelinus fontinalis. There is a lesser, but still significant, positive impact on microhabitat preference. Woody debris provides more preferential habitat at longer timescales and higher discharges, but appears to be less effective for coho salmon Oncorhynchus kisutch than other salmonid species.

Conclusions: The available evidence does not demonstrate an ecologically significant impact of engineered in-stream structures on salmonid populations, although they may provide preferential habitat where discharge is high (>6m³/s). However, evidence suggests that woody debris does increase salmonid abundance, especially  brook trout Salvelinus fontinalis. It may also provide more preferential habitat over time (>4years) where discharge is high (>1m³/s) but does not appear to provide preferential habitat for coho salmon.

Further long term work and monitoring is required to corroborate the evidence presented in this review. Hydrological and ecological factors such as stream gradient, proportion of cobbles in the substrate, degree of existing modification, water quality and canopy cover are insufficiently reported and studied, although they are known to impact fish populations.

Study 2

In-stream structures (such as flow deflectors, weirs and woody debris) have been in widespread use for many decades, having been incorporated into streams and rivers in an attempt to increase fish stocks, primarily salmonids, but also species of conservation concern such as European bullhead Cottus gobio.

A systematic review (see: www.cebc.bham.ac.uk  for methodology) was undertaken to assess the impact of in-stream structures on European bullhead Cottus gobio and salmonid fish. Those relevant to C.gobio are summarised here.

Bullhead studies: Only nine relevant data points from two studies concerning Cottus gobio were available. Small sample sizes precluded comparison of types of in-stream structure except for the impact of riffles and deflectors. Other hydrological and ecological factors such as stream gradient, proportion of cobbles in the substrate, degree of existing modification, water quality and canopy cover were insufficiently reported for analysis, although they are known to impact fish populations.

One study conducted along 13 rivers in lowland Britain (7 with artificial riffles; 6 with flow deflectors) found that bullhead tended to be more abundant in rehabilitated reaches, but this was only significant for riffles. The second study along a regulated river in northern Belgium, found that bullhead were mainly found near bridges characterised by the presence of artificial stones and relatively high water velocity.

Meta-analysis: Data only allowed comparison of the effects of deflectors and riffles. Deflectors resulted in no significant change in bullhead abundance. Riffles resulted in a significant increase in local bullhead abundance (significant heterogeneity between data points due to one showing a significant positive impact and the other four showing no significant impact).

Conclusions: Further work is required to back-up the findings of the evidence presented in this review as sample sizes are very small, with only two studies providing four data points for deflectors and five for riffles. This limited evidence suggests that C.gobio populations are not increased by deflectors but riffles increase local abundance and may provide preferential habitat. However, these conclusions are open to question, given the limited work available.


Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.cebc.bham.ac.uk/Documents/CEBC%20SR12%20Instream%20devices.pdf

 


 

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