Fish Habitat Assessment & Enhancement Study – Botley Bypass

Project Brief

Carcinus Ltd was commissioned by Hampshire County Council (HCC) to deliver fish habitat assessment surveys and habitat enhancement consultancy services in support of the high-profile Botley Bypass highways project. The project has been permitted after passing through the Environmental Impact Assessment (EIA) process and planning phase. The scheme will involve the crossing of two watercourses, the first being the River Hamble by a new free-span bridge and the second located at Woodhouse Gully / Pudbrook Lake, where an existing culvert will be replaced.

Planning permission for the scheme incorporates six conditions set by the Environment Agency (EA). Condition II calls for fisheries habitat creation within a ‘Habitat Management Plan’ to be implemented as a component of the approved scheme. The scheme was therefore considered to provide an opportunity to gain a better understanding of the aquatic environment and to create additional fisheries habitat and enhancement opportunities.

Our Involvement – Ecological Surveys and Consultancy Services

Fish Habitat Surveys

Comprehensive fisheries ecology surveys were undertaken to identify, map and characterise existing fish habitats. Surveys comprised of a detailed walkover assessment of the proposed river crossing locations, including up and down stream. Following all relevant land-owner consent and having undertaken detailed risk assessments, the waterways were mapped by our team of experienced aquatic ecologists and fisheries scientists. Mapping was undertaken with sufficient accuracy and detail to provide a quantitative assessment of the in-channel and riparian habitats as well as geomorphological features of interest relating to fish habitat requirements. The habitat mapping surveys utilised our in-house habitat mapping and field data acquisition software which recorded all features characteristic of salmonid, lamprey and coarse fish habitats in a GIS geodatabase. Following completion of the survey, all data was thoroughly checked and validated as part of our quality assurance procedures and georeferenced mosaics of the various habitats along the selected sections of waterway and total areas of available habitat were then calculated and presented in a series of high quality visual plots.

Fish Ecology Baseline Assessment

In parallel to the fish ecology surveys, a detailed ecological baseline study was undertaken, drawing on existing data sources and direct consultation with statutory and non-statutory stakeholders. The study assessed the fish assemblage present within the study area, their ecological status and identified environmental pressures. A relatively diverse fish assemblage was identified with notable annual returns of sea trout Salmo trutta in the River Hamble following the successful installation Botley Mill bypass channel (fish pass) in 2009. However, several environmental pressures were identified and were validated during the walkover surveys. These related mainly to barriers to fish migration caused by obstructions on the Woodhouse Gully and impoundment effects on the main River Hamble.

Fish Habitat Enhancement and Improvement Options

Habitat improvement and enhancement options were identified and evaluated based on the findings of the fish ecology surveys, assessment and desk-based review. A series of pragmatic recommendations were made with priority being given to those with the greatest perceived ecological benefit. The assessment provided commentary on the feasibility of each recommendation, with acknowledgment given to options that were likely to be technically difficult to achieve and / or uneconomical. Recommendations included measures such as the creation of additional off-channel fish refuge habitat, a reduction of overhanging tree canopy in some areas, improvements in relation to migratory fish passage and bank stabilisation using suitable bioengineering techniques.


The assessment was well received by HCC. The Council is currently reviewing the available options before project construction starts in 2019/20. Carcinus remains committed to supporting the Council and is currently contracted to undertake on-going migratory fish monitoring in support of the project.

More information about our fish ecology services

Find out more about our services on our ‘Freshwater Ecology Surveys‘ page or download our ‘Freshwater Ecology Capability Statement’ below:

If you require more information regarding our freshwater fish ecology survey capabilities and fisheries consultancy services, please contact us.

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