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A strategic programme for NERC Lowland catchment research
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Off-river fish habitats : influence of flow regimes & management implications


Utilisation of off-river habitats by lowland river fishes: influences of flow regime and implications for river management.

Overview of the project

Many freshwater fishes utilise different habitats during their lifecycles and in response to changes in environmental conditions. Maintenance of habitat diversity and access to habitats is strongly influenced by the degree of connectivity within lowland river catchments. Lateral connectivity to floodplain habitats is increasingly at risk from changes in land management and hydrology.


This project had three main aims, to:

  • test the hypothesis that switching between main channel and off-river habitats is a fundamental ecological characteristic for many lowland river fishes;
  • elucidate, by experimental means, the influence of key environmental factors such as discharge and turbidity on lowland river fish behaviour and habitat use; and to
  • collect data and develop models of how fish use lateral habitats in relation to habitat characteristics, including those associated with changes to the hydraulic regime and land-use in lowland catchments, in order to inform the sensitive management of floodplain systems for fish communities.

Main findings

Flow, and associated variables such as aquatic vegetation and substrate, was found to be the main factor distinguishing side channel habitats and their fish communities. However, even within each side channel category (ditches, natural streams, millstream) their ecological function for the fish community of the river Frome was considerable. Ditches were mainly used in spring for spawning by limnophilic species such as pike and in winter most likely for refuge by many species. Low summer-time oxygen levels in ditches, due to their high organic loads, ameliorated in winter and water levels increased, enhancing their use by fishes. Passive integrated transponder (PIT) telemetry at entrances of side channels, has demonstrated seasonal patterns of fish activity. This has subsequently been modelled fro different fish species and environmental conditions.

The results highlight the ecological variability of off-river side channel habitats, their utilisation for a variety of fish species and age groups and a high degree of flux in the fish populations between the main river and side channel habitats, including ditches. This supports the hypothesis that switching between main channel and off-river habitats is a fundamental ecological characteristic for many lowland river fishes.

Three side channel types were distinguished:

  1. Floodplain ditches fed by groundwater and influenced by backwater ponding due to downstream connectivity with the main channel at high flows. These are characterised by low, relatively uniform velocities, but high spatial variability in depth, bed texture and vegetation. Downstream connections to the main channel are also associated with large-scale flow recirculation features in ditch mouths. Flow recirculation occurs on the outer bank in high curvature bends.
  2. Side channels containing substantial flow due to upstream connections with the main channel. These are characterised by substantial spatial and temporal heterogeneity in hydraulic conditions (equivalent to that within the main channel) associated with pool-riffle morphology, width variation and spatial/seasonal variability in vegetation density.
  3. Tributary streams that receive substantial runoff during peak flow events, but are dominated by backwater effects at other times due to connectivity with the main channel at their downstream ends. These features are characterised by seasonal variations in mean depth, but generally low mean velocity and hydraulic heterogeneity except following periods of high rainfall.

Researchers observed a variation in strategies for using habitats amongst individuals. For example small, mature male pike entered spawning ditches earlier and spent more time there than larger males and females. Much of the adult dace (small cyprinid fish) population appears to spend long periods of time in the lower river Frome, often near the tidal reaches, but we have shown from PIT- and radio-telemetry, that particular components of this population use different side channels in the study reach at different times of the year. For both species, there is clear subpopulation structuring of space use, which is not wholly related to physical environmental features. This has not previously been appreciated. For the first time, we have identified some factors responsible for sub-population structuring of habitat use and although the adaptive significance is not yet fully understood, its existence should be appreciated in the context of lowland river management.

Description of activities

The activities undertaken included:

  • Quarterly sampling of seven side channels over a three year period, including drainage ditches, natural streams and a millstream, together with repeated habitat measurements.
  • Water levels within each side channel were continuously monitored.
  • Ppassive Integrated Transponder (PIT) telemetry was used at entrances of side channels to determine fish activity.
  • Environmental properties, e.g. water temperature, were quantified using a combination of field monitoring and numerical modelling.
  • Radiotelemetry of fishes, especially of pike, was used to gather home range and habitat use data.

Areas of application

The outcomes of this research will mainly be used by policy makers and regulators, especially the Environment Agency and Defra in the context of ecologically sensitive management of river catchments.

Related and future work

Aspects of the work are relevant to work being undertaken by Durham University as part of the new Rural Economy and Land Use Programme (RELU) project "Angling in the Rural Environment".

Researchers' details

Principal Investigator:


Other researchers:

  • C Knight, PhD student
  • M Bone, Durham University


All publications from LOCAR projects are listed in the publications database.

Several papers describing the results of this project have been submitted to journals and are under review.

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