Electric Fishing Fry Index Survey
River Lynher
January 2023
Hannah Winchester
Westcountry Rivers Trust Electric Fishing Survey Report - River Lynher 2022
Westcountry Rivers Trust is an environmental charity established in 1995 to restore, protect and improve the rivers, streams, and water environments in the region for the benefit of wildlife and people.
This report has been written and prepared by: Hannah Winchester
Published by: Westcountry Rivers Trust Rain Charm House, Kyl Cober Parc, Stoke Climsland, Callington, Cornwall, PL17 8PH.
Tel: 01579 372140
Email: info@wrt.org.uk
Web: www.wrt.org.uk
Charity no. 1135007
Company no. 06545646
Document history:
© Westcountry Rivers Trust: 2023. All rights reserved. This document may be reproduced with prior permission of the Westcountry Rivers Trust
Westcountry Rivers Trust Electric Fishing Survey Report - River Lynher 2022
Executive Summary
Westcountry Rivers Trust (WRT) undertook electric fishing (EF) surveys throughout the River Lynher catchment during the summer of 2022. This is the first year of WRT surveys in the Lynher catchment where 13 sites were selected to cover the entire span of the river, it is hoped to form a long-term data set for the catchment. Surveys were funded through a collaborative electric fishing survey grant funding from the Rivers Trust and Environment Agency. Salmon fry appeared to dominate and were most successful, producing excellent classifications on the main stem of the river, whereas trout were recorded in far lower numbers. Conservation strategies, such as the Defend/Repair/Attack approach, has been applied on a sub catchment level dependent on classifications received during the semi-quantitative surveys.
1. Introduction
Westcountry Rivers Trust (WRT) undertook semi-quantitative fry index electric fishing surveys of the River Lynher catchment in August 2022 which was funded by a partnership between the Environment Agency, the Rivers Trust and Westcountry Rivers Trust This was the first year of fish monitoring undertaken by WRT on this catchment, with the goal of these surveys to show the potential of regular electric fishing. Rivers, such as the Lynher, have historically received little survey effort and these initial results help establish a long-term dataset of semi-quantitative fry numbers. A total of 13 sites were surveyed during the 2022 season across the catchment, mainly on the main stem of the river, with one tributary site (Withy Brook) on the upper catchment. Sites were selected to cover the entire length of the river and targeted not to collide with surveys being undertaken by the Environment Agency.
2. Methodology
2.1 Electric Fishing Protocols
Electric fishing uses a controlled electric current to induce fish to swim toward an anode and into a hand net, and thereby be counted and assessed. When carried out correctly by experienced and qualified surveyors it is not harmful to fish and the fish are released
back to the same location they were caught. In upland streams and shallower sections of rivers, an electric fishing backpack is used and therefore this type of kit was used for all the Lynher surveys.
There are several approaches to electric fishing assessments in rivers; quantitative, area semi-quantitative and time semi-quantitative methodologies. All three methods have their advantages and disadvantages.
Quantitative electric fishing is a thorough methodology that has the highest degree of accuracy of all the methods. The main disadvantages of this approach are it is less mobile than backpack equipment and it takes longer to undertake surveys. It Is therefore more costly than other approaches. With this method, an area of river is netted off and the fish are removed from this defined stretch in multiple passes until sufficient fish are removed to form a very accurate assessment of species and numbers. It is not required to remove all the fish from the area but rather ensure a consistent fishing method that gives a linear decrease in the number of fish caught per pass. The overall catch decline gives an accurate estimate of the total number of fish in the location. This is known as the ‘depletion’ methodology.
An area-based semi-quantitative electric fishing methodology follows the same process as quantitative electric fishing but only a single pass is carried out. A lack of multiple passes renders the method only semi quantitative and therefore less accurate, but it has the advantage of being much quicker than the depletion method, and it is suitable for use on all waterbody types. It is able to detect multiple species and is reasonably accurate but is less time efficient and therefore costlier than a time-based methodology (described below).
A time-based, semi-quantitative electric fishing methodology differs from both the approaches described above. Instead of limiting the area fished (by use of nets) it limits the amount of time used to fish to assess fish numbers. As no nets are deployed, fish in deeper sections of large rivers can frequently avoid capture using this method. It is therefore only suitable to assess salmonid fry, who are restricted to a shallower section of upland streams and rivers. This method is extremely rapid and therefore costeffective, allowing for deployment across whole river catchments although its major drawback is its lower accuracy than netted approaches.
In weighing up the pros and cons of the various approaches it is worth considering what would be required for a truly reliable method. In scientific publications it is usually considered that if an approach is accurate 95% of the time then this is an acceptable standard. Such an approach would be said to have sufficient statistical power to answer the question asked, for example, ‘has this habitat improvement resulted in more fish in the area studied?’. For an electric fishing methodology to have sufficient statistical power it requires a large number of sites to be fished in a fully-quantitative depletion methodology over a number of years. As such an effort is rarely practicable and will cost more than the habitat improvements it attempts to measure, this approach is rarely applied in the UK. River managers in the UK have limited budgets and therefore it is the case that an
electric fishing programme of insufficient statistical power to achieve 95% confidence is usually accepted as a compromise between accuracy and cost.
Bearing in mind the limits of statistical power that these approaches usually have (as practically applied), it is important to consider the aim of a given electric fishing programme. In the case of WRT’s catchment scale electric fishing programme, the aim is to build up historical data on each catchment to provide information as to how to best take action to improve the fish stocks for salmon and trout. To achieve this, the largest number of sites possible for maximum catchment coverage must be fished over several consecutive years to i) guide current/future conservation strategies and ii) identify whether or not the actions taken on the catchment have had a positive effect on fish numbers. Most importantly the electric fishing programme is specified to be carried out at a catchment scale where salmon and trout spawning areas occur. As most rivers have many tributaries or main stems of considerable length, a relatively large number of sites are required for full coverage. This typically equates to between 20 to 100 sites on rivers in southwest England, depending on the river catchment geography. All things considered, a timed semi-quantitative approach was considered most appropriate for the WRT electric fishing programme. This method will indicate the main issues and areas that need addressing on a river catchment including:
• Upstream barriers to fish-passage
• Degraded habitat quality
• The upper limit of salmon spawning
• Successfully/Unsuccessfully enhanced habitat
• Catchment-scale fry migration due to river levels
• Point source and diffuse pollution
2.2 Life Cycle and Bottlenecks
Classifications for each sub catchment were analysed and given an average score based on the number of classifications received which were then used in the final classification and Defend Repair Attack (DRA) strategy. A higher number of sites surveyed allows for a more accurate score, so some caution is advised when low or singular sites were surveyed for some sub catchments. The aim of the semi-quantitative electric fishing program is to identify issues that prevent salmonids from effectively completing their lifecycle, and then proposing solutions that are proportionate to the issue at hand. It is useful to adopt certain conceptual frameworks to each of these aims, and in this report, we will use two of these frameworks; the ‘habitat bottlenecks’ that describe the causes of issues in salmonid ecology (Figure 2), and the ‘Defend/Repair/Restore’ conservation strategy framework which describes the appropriate habitat action depending on the ecological situation found at the site.
2.3 Catchment-Based Fisheries Conservation Strategy (Defend/Repair/Attack)
Classifications for each sub catchment were analysed and given an average score based on the number of classifications received which were then used in the final classification and Defend Repair Attack (DRA) strategy. A higher number of sites surveyed allowed for a more accurate score, so some caution is advised when low or singular sites were surveyed for some sub catchments. In using the fry index classification, catchment population abundance monitoring and determining the river reach density classification, a series of priority areas can be outlined, and management recommendations made tailored to the particular species. Recommendation actions broadly follow the Defend/Repair/Attack concept, developed by Ronald Campbell of the Tweed Foundation. For every river reach that is classified, a management action can be loosely determined. Whilst this provides a useful structuring framework, the reality of given situations can bring many complexities and lies on a continuum between these extremes. The goal is to move the river reaches of the Lynher up from the unstable point (i.e. poor fish stocks and habitat) to the broad top of a healthy, natural riverine ecosystem. Where the populations are in a very poor state, radical actions may be required to see a change. Conversely, where the stocks are already good, habitat re-engineering and stocking operations would be inappropriate. Actions to achieve these improvements can be divided between ‘fish stock actions’ such as fish translocations or bag limits for anglers and ‘fish habitat actions’ such as removing barriers to migration or coppicing. In many situations, both types of action will be required. This concept helps divide catchment scale management for fisheries into priorities and therefore can help to maximise multiple benefits through targeted work.
2.4 Site Selection
Survey sites on the Lynher were strategically placed to cover the entire span of the river and selected to provide representative samples from distinct river reaches, characterised by habitat type, proximity to barriers. This suite of sites will help to contribute to a longerterm electric fishing survey dataset where trends can be established, and positive or
negative patterns of salmonid recruitment inferred based on the findings within a sub catchment.
2.5 Field Sampling and Data Analysis
Permission for all sites was established prior to surveying and each site was surveyed by a two or three-person team. The voltage of the unit was set at each site depending on the water conductivity, measured using a handheld conductivity meter. The operatives fished continuously for a standard five minutes over suitable fry habitat without the use of stop nets. The fishing area was variable, and the length of fishing time was fixed. Fish were collected in a net and placed into an aerated holding bucket before processing.
All salmonids were identified to species and fork length was measured and recorded. Numbers or density estimates were recorded for all other species captured. Habitat features such as land use, substrate type and shading were recorded at each site. Any fry that were missed or escaped during electric fishing were assigned to either trout or salmon groups depending on the relative percentage of each species already recorded at the site.
The results of the electric fishing survey are classified according to the methodology of Crozier and Kennedy (1994), displayed in Table 1, with each site being given an equivalent density classification compared to quantitative monitoring. This semi-quantitative methodology was designed by Crozier and Kennedy for both salmon and trout. However, the results for trout need adjustments to consider the difference in the regression line for trout and salmon as described by Crozier and Kennedy.
Based on the lengths of fish captured during the survey, using a length frequency histogram, salmon fry were considered to be any individual that measured up to 85mm (Figure 3) and trout fry were considered to be any individual measuring up to 85mm (Figure 4).
Atlantic salmon frequency distribution, 2022
Brown trout frequency distribution, 2022
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,
Westcountry Rivers Trust Electric Fishing Survey Report - River Lynher 2022
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.
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.
Category Status
Defend
Repair
These areas have good fish stocks and habitat and need safeguarding actions to ensure no decline occurs.
These areas have moderate fish stocks, and fish habitat in a moderate condition; these areas need assisted habitat recovery to move them into the Defend category.
Attack
These areas have poor fish stocks, and the habitat is significantly degraded. These areas need drastic intervention such as habitat reengineering in order to improve their status.
Action
Maintain bag limits
Habitat Safeguarding
Catch and release
Assisted habitat recovery
Stock action
Habitat re-engineering
Average Fry Index Class & Conservation Strategy
Sub-catchment
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.
Westcountry Rivers Trust Electric Fishing Survey Report - River Lynher, 2022
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.
