Impact of flow regulation and habitat improvement on brown trout in Yorkshire rivers

Stone, Timothy James

Abstract

In the UK, reservoirs are a relatively common component of the landscape and provide a variety of functions, including the provision of water to meet industrial, agricultural and societal needs, as well as recreational activities such as sailing and fishing. However, reservoir function and operation impacts on downstream aquatic ecosystems by severely altering and degrading the flow regime. The importance of a natural flow regime in rivers is crucial not only to geo-morphological processes, but also to the life cycles and ecology of ichthyofauna present in these rivers. The introduction of the Water Framework Directive in 2000 (WFD: 2000/60/EEC) provided a framework by which the surface water in all 27 EU member states must achieve Good Ecological Status (GES) or Potential (GEP) by 2027. This thesis examines the drivers of population dynamics of brown trout (Salmo trutta L.) in rivers where the natural flow regime is severely degraded due to reservoir operation, as well as determining the effectiveness of introducing and amending reservoir release regimes and physical habitat restoration on the habitat quality and brown trout populations.

Using long term brown trout population data and flow, rainfall and temperature data the roles of density-dependent and density –independent regulation were investigated in three heavily impounded Yorkshire Rivers (Rivelin, Loxley and Holme). In each of the three rivers no relationship was found between monthly discharge rates and rainfall levels indicating that the natural flow regime was un-synchronised from that expected under un-impounded conditions. Investigations into density-dependent regulation of 0+ and ≥1+ brown trout, as well as length at age one, found that no meaningful density-dependent regulation was occurring in brown trout populations throughout the three study rivers; this was likely due to the low densities in which brown trout were found throughout the study sites. Using mixed effect linear models, it was found that the variability of flow regime in the summer period (June – September) was significantly correlated with 0+ brown trout densities, and that the flow variability during the emergence period (April –May) was also significantly correlated with the length of brown trout at age one. Temperature and flow during any other period of the year were not found to have any significant interaction with 0+ brown trout density or length at age one.

The introduction and amendment of compensation releases from impounding reservoirs is an established methodology for improving the ecological potential in downstream reaches of rivers. Long term effects on brown trout populations and estimates of habitat quality in rivers where new compensation release regimes were introduced were examined in three study rivers (Dibb, Dale Dike and Holme). In all three rivers the changes to densities of three age/size classes of brown trout (0+, ≥1+ <20cm and ≥1+ >20cm), length at capture of 0+ brown trout and habitat quality for the three age/size classes were examined to determine if the new compensation regime had any meaningful impact using a Before After Control Impact (BACI) study design. In the River Dibb, where a four stage seasonally varied release was introduced, there was no significant changes to the biological metrics but, there was a significant improvement in the habitat quality for ≥1+ (<20 cm) brown trout which could be attributed to the introduction of the compensation release. Operational constraints from Dale Dike reservoir were such that only an annual minimum flow could be established from the reservoir; in the study no significant changes to the biological metrics were detected, but there was a significant decrease to the 0+ brown trout habitat quality that was attributed to the introduction of the annual minimum compensation release. The findings suggest further amendments to the compensation release regime or alternative restoration methods (such as habitat restoration) should be considered to achieve GEP in Dale Dike. The compensation release from Brownhill and Digley reservoirs were revised to provide better flow conditions in the River Holme. There was no significant response from the biological metrics tested, however there was a significant improvement to the habitat quality of ≥1+ (> 20 cm) brown trout, suggesting that the revised compensation release provided better habitat conditions for larger, more fecund brown trout.

As it is not always possible to attain GEP in impounded rivers by amending reservoir release regimes due to operational circumstances the use of physical habitat restoration measures can be implemented to improve habitat conditions downstream of impounding reservoirs. In the River Washburn a BACI study was undertaken to detect if any meaningful change to brown trout populations and habitat quality had occurred following habitat improvement works in 2015. Immediately following the habitat improvement works the habitat quality for all brown trout age/size classes improved, but in late 2015 a 1-in-100-year flood event destroyed and damaged a majority of the instream works. There were significant improvements immediately following the habitat restoration work to the habitat quality for both ≥1+ size classes, but due low densities of brown trout following the habitat improvement there was a significant decline in ≥1+ (<20 cm) brown trout populations.

As it was found that there were significant changes to the habitat quality for brown trout across the four study rivers, further sampling of brown trout would be required to determine the biological response to the habitat changes. As the level of temporal and spatial variance was high in brown trout populations throughout this study, further monitoring would be required to draw robust conclusions as to the impact of Heavily Modified Water Body (HMWB) mitigation on brown trout populations. As habitat requirements for brown trout are different depending on the life stage, the complementary use of both habitat improvement and flow restoration techniques should be explored in future projects to mitigate HMWB status.

Citation

Stone, T. J. Impact of flow regulation and habitat improvement on brown trout in Yorkshire rivers. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4221403

• Thesis Type: Thesis
• Deposit Date: Apr 10, 2019
• Publicly Available Date: Feb 23, 2023
• Keywords: Fisheries
• Public URL: https://hull-repository.worktribe.com/output/4221403
• Additional Information: Hull International Fisheries Institute, The University of Hull
• Award Date: Feb 1, 2018