A systematic review to assess if engineered in-stream structures and woody debris increase the abundance of salmonids in streams and rivers

  • 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? Systematic Review No. 12. Centre for Evidence-Based Conservation


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:  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.

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