8 minute read
Results & Discussion
3. Results & Discussion
WRT surveyed 13 semi-quantitative sites on the River Lynher and Withy Brook during August 2022. Weather and general survey conditions were dry and very warm which resulted in extremely low river levels and increased river temperatures. It should also be noted that summer 2022 was the driest since 1995, as well as the second hottest year on record. As with previous years, 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.
The 2022 catchment survey demonstrates that the Lynher catchment can support excellent salmon spawning and the importance of large main river stem spawning for salmon reproduction. Trout spawning was less productive at most sites, with mostly fair and poor classifications being achieved. There were eight excellent classification sites,
along with two good sites for salmon fry, with salmon fry absent from one of the 13 sites. A total of 502 salmon fry and parr, and 70 trout fry and parr were caught across all sites in 2022.
Figure 5 Total catch for salmon and trout, River Lynher, 2022
Figure 6 Salmon fry classifications, River Lynher 2022
Figure 7 Trout fry classifications, River Lynher 2022
The highest excellent classification achieved was DS Clapper Bridge, with 89 fry being recorded and the lowest being Trebertha with 35 fry recorded. Although this was the lowest of excellent classifications, it was still much higher than the 23 individuals required to achieve an excellent classification. Tregirls was the furthest upstream site in the catchment and was absent of salmon fry. This site is near the headwaters of the river, with in river substrate dominated by finer sands, lacking larger fractions of gravel suitable for salmonid spawning. This was further exacerbated by the low river levels which likely displaced fish into larger deeper areas of water. One of the main tributaries on the Lynher is the Withy Brook, located higher in the catchment, this site achieved a poor classification with one fry being recorded. The result was poor it is still, however, encouraging that salmon are utilising these tributaries to spawn. It was noted that the Withy Brook site was dominated by larger cobbles and immobile boulders, lacking some of the smaller fractions required to optimise spawning, therefore, it would be advantageous to move the site to an area of better suited habitat. In contrast, trout fry recruitment was far lower, this is not likely due to a lack of habitat but rather when one species (trout or salmon) is in lower abundance the other tends to dominate as demonstrated at several sites. However, where some sites have lower classifications for each may indicate that there is limited quality habitat available for either species or further investigation may be required to direct future works to improve the limiting factors.
Overall, for the first year of surveys, the Lynher performed excellent results for salmon spawning with most sites, as well as including healthy numbers of parr present when habitat suited both life stages. Observations from surveyors report that the habitat of survey sites were healthy and most held healthy gravel assemblages with very little sediment smothering valuable spawning beds, this is due to very little agriculture operating in the area and where there is, it is very low impact. In addition to this good riparian zones had a varied tree canopy which allowed light to penetrate important juvenile habitat, in turn, increasing primary productivity and the number of juvenile salmonids. There was also a good amount of instream woody material present at most sites, although some could benefit from the addition of more woody material from areas where abandoned coppice has been left to grow. This may help create additional preferred habitat for brown trout when competition is high from large abundance of salmon. As there is a large presence of salmon on the river, it would be worth investigating some of the smaller tributaries to identify if brown trout are utilising these in favour of larger main river habitat. There are very few barriers on the main river, and where some are present, salmon are clearly able to move freely over these which is supported by the results of salmon being present throughout most of the main river. Although the Lynher has an operating hatchery, these surveys were planned to take place before 2022 stocking. so although it can be stated that the fry in these results were not stocked fish, it would be beneficial to determine the success of the hatchery and how many parr or returning adults were from the stocking effort.
4. Recommendations
The WRT electric fishing surveys are undertaken to gain an understanding year on year recruitment of salmon and trout The two species 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. 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 3) 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 3 represents the continuum of the three strategies and the goal for each waterbody; to move all the Lynher 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 5 displays the recommended works required in each waterbody to restore habitat and increase fry populations.
5. Acknowledgements
Westcountry Rivers Trust would like to thank Nick Lintott for all of his hard work and help showing us around the catchment and helping with landowner permission. We would also like to thank all of the landowners allowing access and allowing us to conduct the surveys and also thanks to the Rivers Trust and Environment Agency.