A systematic review to assess if in-stream structures increase the abundance of European bullhead Cottus gobio 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? CEE Review 05-006 (SR12). CEE (Collaboration for Environmental Evidence) Systematic Reviews, 12


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:  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.

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