Electric Fishing Fry Index Survey - Plymouth River Keepers (PRK)

Electric Fishing Fry Index Survey

Plymouth River Keepers (PRK)

January 2023

Hannah Winchester

Westcountry Rivers Trust Electric Fishing Survey Report – PRK 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

Executive Summary

Westcountry Rivers Trust (WRT) undertook electric fishing (EF) surveys throughout the small streams of the Plymouth River Keepers project area, within Northwest Plymouth during the summer of 2022. The surveys are the second year of a three-year monitoring effort funded as part of the wider Plymouth River Keepers project (PRK). Salmon were absent at all survey sites, along with complete absence of any fish species at some of the survey sites. The only species that were present at some sites were brown trout and European eel. Overall, brown trout abundance was lower and European eel were absent at more sites than the 2021 surveys. It should be noted that summer 2022 was the driest since 1995 as well as the second hottest year on record. Conservation strategies, such as the Defend/Repair/Attack approach, can be applied on a sub catchment level with recommended fisheries management actions suggested to either improve or maintain salmonid habitat and numbers.

1. Introduction

The Westcountry Rivers Trust (WRT) undertook eight semi-quantitative fry index electric fishing surveys on the small streams within the PRK project area within the Tamar catchment in September 2022 (see Figure 1). This was three less sites than in 2021, due to such low flows of a dry summer with the headwaters of these streams becoming very shallow and small. Severe low flow increases risk of damage to fish health if surveys were to go ahead. The three streams have been named Reach 1, 2, and 3 within the wider PRK project. This was the second year of fish monitoring undertaken by WRT and forms part of establishing a long-term dataset of semi-quantitative fry numbers.

Sites were selected based on potential fisheries improvement works that are scheduled be carried out within the project area. Monitoring will aid in understanding if improvements have been effective in increasing fish numbers and/or presence of already absent species. It is hoped that these sites will continue to be surveyed every year to build a strong baseline dataset for the catchment.

Figure 1 Site name and location reach names

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 PRK 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

The aim of the semi-quantitative electric fishing program is to identify issues that prevent salmonids from effectively completing their life-cycle, 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 PRK area 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.

2.4Site Selection

Survey sites in the PRK area were selected to provide representative samples from distinct river reaches, characterised by habitat type, proximity to barriers and proximity

Figure 2 Diagrams defining salmonid habitat bottlenecks (Summers et al, 1996)

Westcountry Rivers Trust Electric Fishing Survey Report

to targeted restoration works under project. Sites are proposed by WRT and will help to contribute to a longer-term 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 before electric fishing surveys took place and each site was electric fished by a two-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, trout fry were considered to be any individual measuring up to 105mm (Figure 3). Salmon fry were absent from all sites so no frequency distribution was made.

Table 1 Semi-quantitative abundance categories for salmon fry (Crozier & Kennedy, 1994)

Brown trout frequency distribution, 2022

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 3 Trout fry length frequency distribution, PRK 2022

Table 2 PRK salmon and trout fry classifications including parr and eel presence absence for 2022 (ordered from upstream to downstream)

Figure 4 Trout fry classifications, PRK 2022

Figure 5 European eel presence and absence map, PRK 2022

Table 3 PRK salmon fry historic classifications 2021-2022

Table 4 PRK trout fry historic classifications 2021-2022

Table 5 PRK eel historic classification 2021-2022

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.

These areas have good fish stocks and habitat and need safeguarding actions to ensure no decline occurs.

Repair

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.

Maintain bag limits

Habitat Safeguarding

Catch and release

Assisted habitat recovery

Stock action

Habitat re-engineering

Table 6 Defend/Repair/Attack strategy

Table 7 PRK sub catchment classification and DRA strategy

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.

Table 8 Recommended DRA actions for PRK sub catchments

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.