River rehabilitation and fish populations: assessing the benefit of instream structures

Published source details Pretty J.L., Harrison S.S.C, Shepherd D. J., Smith C., Hildrew A.G. & Hey R.D. (2003) River rehabilitation and fish populations: assessing the benefit of instream structures. Journal of Applied Ecology, 40, 251-265.
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Summary

River rehabilitation schemes are widespread in the UK, but there have been few systematic assessments of their ecological effect, particularly on target organisms such as fish. Fish populations were therefore assessed in 13 lowland rivers using point abundance measures and electrofishing. Each river was sampled in two reaches, containing a small-scale rehabilitation scheme (artificial riffles or flow deflectors) and an unrehabilitated control reach. Geomorphological surveys were undertaken in each river to assess the physical and hydraulic effect of rehabilitation.

Study sites: Thirteen sites along eleven small/medium sized rivers of lowland England were chosen for study (see Table 1, attached, for locations, rehabilitation schemes and physical characteristics). An effort was made to choose structurally similar rivers and similar rehabilitation structures. All sites were low gradient, channelized and predominantly surrounded by agricultural land with little riparian buffer zone. Two rehabilitation schemes were investigated, riffles and deflectors.

Sampling: Two reaches in each river were sampled, one containing the rehabilitation scheme ('manipulated reach') and the second as a control reach (100–500 m up or downstream) Control reaches resembled the manipulated reaches except for the rehabilitation measure.

Fish populations were assessed using point-abundance electrofishing. Between 20 and 50 haphazardly chosen points were sampled in each manipulated reach, and each control reach. The fish caught at each point were identified, counted and released. Sampling was conducted at all sites in July/August 2000 and repeated in July/August 2001, although five sites could not be visited in 2001 due to access restrictions.

Repeated depletion sampling electrofishing was used to compliment the point abundance sampling. This was carried out by the Environment Agency in summer 2000. Due to resource constraints, only six rivers were fished in both control and manipulated reaches, and a further two in the manipulated reach only.

Both electrofishing sampling techniques indicated that overall, total fish abundance, species richness, diversity and equitability were not significantly different between rehabilitated and control reaches. Bullhead Cottus gobio and stone loach Barbatula barbatula (both species of UK conservation concern) tended to be more abundant in rehabilitated reaches but only for artificial riffles, not deflectors. There was a significant between-year difference in fish abundance.

In general, rehabilitation increased depth and flow variation, and fish species richness and diversity appeared to respond positively to increased flow velocity. However, there were few significant relationships between the fish fauna and physical variables, indicating that increasing physical (habitat) heterogeneity does not necessarily lead to higher diversity.

Conclusions: From this sample of lowland rivers, there is little evidence of any general benefit to fish of small-scale instream riffle or deflector structures. The weak response of fishes to these rehabilitation schemes may have been because the schemes were inappropriate in design and scale for low-gradient rivers. Furthermore, fish assemblages may have lacked the potential for recovery because of poor water quality and/or because the schemes were isolated within longer sections of degraded river.

See also: Case 484 'A systematic review to assess if engineered in-stream structures & woody debris increase the abundance of salmonids in streams & rivers'; and Case 485 'A systematic review to assess if in-stream structures increase the abundance of European bullhead Cottus gobio in streams & rivers'.

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