The influence of flow management and habitat improvement works on fish communities in Yorkshire rivers

Taylor, Marie Jane

Biological sciences
May 2017

Thesis or dissertation

© 2017 Marie Jane Taylor. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.

Many of the rivers in the UK are heavily modified by channelisation, impoundment (dams and weirs and off-river storages), land drainage and flood defence. These modifications have reduced the natural variability of flow and habitat diversity and in turn rivers are failing to meet Water Framework Directive (WFD: 2000/60/EEC) targets. Mitigation measures such as modifying reservoir flow releases and habitat improvement works are carried out to remediate the potential impacts of river development. This thesis examines the effectiveness of modified reservoir flow releases and habitat improvement works in Yorkshire rivers using brown trout (Salmo trutta L.) as the indicator of change.

The importance of natural flow regimes and how reservoirs and flood defence works have had negative impacts on fish populations was reviewed. The current UK guidance around managing reservoir releases and reducing flood risk was reviewed with regards to what measures are in place to mitigate their impacts and what biological responses are expected. One of the main conclusion was that to meet WFD targets, monitoring is required to investigate the effectiveness of activities aimed at improving rivers to inform management decisions and ensure activities are efficient and cost effective.

The long term effects of introducing seasonal compensation flows and a single freshet were examined by comparing differences in the hydrological regime and monitoring brown trout populations downstream of water storage reservoirs in Yorkshire. Hydrological parameters were not significantly different following the introduction of the revised reservoir release programme and brown trout populations were found to be variable throughout the years studied, and any changes in population characteristics could not be attributed to the new regime and further changes to the reservoir releases maybe required.

Manual radio tracking was used to obtain a detailed knowledge of the movements and distribution of adult brown trout downstream of two water storage reservoirs in Yorkshire following the introduction of single freshet releases (November 2012) to stimulate upstream migration. Brown trout occupied small home ranges and a single freshet release did not result in long distance upstream migration possibly because the releases were not performed at the appropriate time of year or the magnitude was inadequate to promote migration. The number of releases was increased to one each in the months of October, November, and December 2013 but still did not result in long distance upstream migration. It was suggested that the freshet releases which lasted only 8 hr, provided brown trout with little opportunity to move a reasonable distance. Further changes to the reservoir releases may be made to meet the flow profile recommended by UKTAG for autumn and winter flow elevations to support spawning migrations.

A monitoring programme was designed to detect changes in brown trout population following habitat improvement works. Baseline surveys carried out as part of this programme found brown trout to be present at low densities and exhibit slow growth rates, which was attributed to lack of suitable habitat, particularly spawning and juvenile riffle habitats, lack of deeper pooled areas for larger brown trout and lack of available cover. It was recommended any habitat improvement works should therefore improve flow, habitat and sediment issues.

A further study compared brown trout population and habitat parameters at Malin Bridge on the River Don pre and post flood defence and subsequent habitat improvement works, the latter designed to mitigate adverse effects of flood defence works. The flood defence works provided very little habitat diversity and cover for larger brown trout, instream channel features were added to improve habitat. Following the improvement works brown trout populations returned to densities and composition found prior to flood defence works, indicating impacts associated with flood defence works can be reduced when incorporating habitat improvement works into flood risk management.

Department of Biological Sciences, The University of Hull
Harvey, Jon P.; Cowx, I. G. (Ian G.)
Sponsor (Organisation)
University of Hull
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