Results & Discussion

3. Results & Discussion

WRT surveyed eight semi-quantitative sites within the PRK area in September. Weather and general survey conditions were dry and very warm, which resulted in extremely low river levels and increased water temperatures. It should also be noted that summer 2022 was the driest since 1995, as well as the second hottest year on record. Surveyors kept a close eye on river water temperatures to ensure surveying did not proceed if water temperatures exceeded 18°C, in accordance with Environment Agency guidance.

Figure 4 Trout fry classifications, PRK 2022

Figure 5 European eel presence and absence map, PRK 2022

The 2022 surveys demonstrate that the small streams within the PRK project area have the potential to support salmonid spawning and the importance of these small streams to salmonids. DS Whitleigh Wood Bridge, DS Tamerton Foliot Road and US Seven Stars all achieved good classifications for trout with 21, 15 and 13 fry being recorded respectively, although all were lower in trout abundance than the 2021 surveys. Salmon were absent from all eight sites. All five remaining sites were absent of trout fry including Thilmere Gardens that achieved a good classification for trout in 2021. It is possible that due to the nature of the streams in the PRK area being small, the lower river levels have displaced fish into deeper water or downstream into different river sections or natural variation dictates that it is a less productive year for fry recruitment. Eel were only present at two of the previous year’s five sites and although this may appear discouraging eel can inhabit many different habitat types, so they are not only present in juvenile salmonid habitat which are the primary focus of these surveys. The nature of urban streams, such as those within the PRK area, do suffer from point source pollution incidents which have been an issue on these streams in previous years which may impact long term survival for trout in certain areas of these streams. In addition, all reaches have numerous instream barriers present which all have a varying degree of impacting free migration upstream and downstream, which could potentially isolate areas of river which inhibit natural spawning to occur. From this year’s data, Reach 2 is still the most productive reach for brown trout recruitment, however, due to the lack of sites being able to be surveyed on Reach 3 this may not be the case so would benefit from additional sites being added in the mid and upper reaches of Reach 3.

4. Recommendations

The WRT EF surveys are undertaken to gain an understanding of year-on-year recruitment in salmon and trout, but they have slightly different habitat preferences, and one species will often dominate over the other where the other has limited or no presence. There will be natural annual variations in populations, and this is to be expected depending on the success of recruitment of a species. Despite good trout fry populations in some areas, it is important that defence strategies are put in place to maintain good recruitment and to restore numbers elsewhere in the PRK project area. In all areas, salmon are absent, then the attack strategy and appropriate actions should be implemented where funding allows. Therefore, management strategies need to be considered for each species, hence a conservation strategy for both salmon and trout.

The strategy for restoration and conservation of sites suggested here broadly follows the “Defend, Repair, Attack” (DRA) concept (Table 6) developed by Ronald Campbell of the Tweed Foundation, and has, in the past, been applied locally in the Exe catchment by the River Exe and Tributaries Association project. The fry productivity of the rivers is assessed by a combination of historic semi-quantitative electric fishing results. These results are then applied in context of existing plans (e.g. Salmon Action Plan, habitat walkover surveys and genetic data) to produce assessments and recommendations for each subcatchment of the river. These sub-catchments are classified according to three levels: Defend, Repair, and Attack.

Despite the DRA strategy being a useful tool to identify and prioritise works in catchments, the requirements of waterbodies can rarely be quite so clear cut. The coloured arrow in Table 6 represents the continuum of the three strategies and the goal for each waterbody; to move all the PRK sites from their current position to somewhere in the Defend category, or to ensure they remain in this status if fish stocks are already good.

Recommended works:

Fencing: Riparian zones identified as receiving significant livestock access, with apparent habitat degradation, should be fenced to limit trampling and bank side poaching. Precautions should be taken to ensure livestock can access drinking water supply. Effective buffer strips dependant on site characteristics is advised.

Coppicing: Targeted selective coppicing of woodland and abandoned riparian coppice adjacent to juvenile habitat riffles should be undertaken. This will increase primary productivity and food source for juvenile fish Shade should be maintained on deeper pools and runs for water temperature and adult fish habitat cover.

Erosion Control: Fencing and effective marginal habitat management will reduce erosion. However, where specific areas of high pressure and vulnerability are identified, erosion protection measures such as woody debris installation, environmentally sensitive revetments, and strategic tree planting would be advantageous.

Fish Passage Assessment: Assessment of potential fish migration barriers using the Coarse Resolution Rapid Assessment technique developed by the Scottish and Northern Irish Forum For Environmental Research (SNIFFER). A standardised survey technique to assess porosity of in-channel structures.

In-Channel Habitat Restoration: Installation and construction of habitat enhancing features, including woody debris introduction, flow manipulation with groins and kickers, bank reprofiling for marginal zonation, strategic tree planting, gravel introduction and riffle creation, and historic channel restoration. Advanced management usually applied post success of other recommended actions.

Modify flow regime: Where flows are impacted by storage reservoirs, liaise with service provider to discuss and inform of findings of EF surveys to determine management of flow regime from impoundments at appropriate times of years for fisheries benefit.

Walkover Surveys: Recording of habitat availability relating to ontogenetic stages of fish, including observed local land use and factors negatively impacting habitat quality. Often the starting point for work in an area following from poor fry index survey results. An important component of catchment management. Walkovers essentially build wide scale understanding of a catchment and allow forging of relationships with local landowners.

Interpretation: Sensitive spawning sites can be exposed to disturbance at key times of year. Interpretation can be used to inform of salmonid presence and advise on in river site avoidance.

Assess monitoring effort: By adapting survey effort, appropriate information can be gained according to strategic plans for the catchment based on current and historic data. It is recommended that redd observations be encouraged annually to coincide with fry index survey data. Continue with volunteer invertebrate monitoring e.g. Riverfly, and WRT recommended the Citizen Science Investigation (CSI) programmes run by WRT.

These works should be tailored to each sub catchment and prioritised according to the status of the river bodies. Table 8 displays the recommended works required in each waterbody to restore habitat and increase fry populations.

5. Acknowledgements

Thanks to all landowners including Plymouth City Council and the Woodland Trust involved for their kind permission to undertake the surveys and to the Environment Agency for providing consent for the surveys.