A10. Electric Fishing

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1. Applicability and Purpose

Electric fishing surveys can be used for several purposes, including:

  • Monitoring fish communities at stream or catchment level, either quantitatively or qualitatively
  • Assessing the impact on fish communities of potential harmful impacts or interventions/improvements
  • Assessing habitat occupancy
  • Evaluating stocking efficiency
  • Rescue and relocation operations

The aim should always be to collect data that are as representative as possible of the fish community in the river reach or catchment at the time, so that the information can be compared with that from other locations and other times.

2. Method Description

Electric fishing is a process by which an electric field is generated in water to attract and stun fish in order to capture them. An electric fishing system consists essentially of:

  1. An electrical power source – typically a generator or batteries;
  2. A control box which is used to modify the output from the power source;
  3. One or more electrodes which when immersed in water create an electrical circuit of which the water between the electrodes becomes a part;
  4. Safety mechanisms which enable any electric fishing team member to immediately break the electrical circuit in the event of danger.

Three broad categories of electrical wave-form can be used:

  • Alternating Current (AC) creates an electrical field in the water which changes direction between the two electrodes typically 50 – 100 times per second (expressed as 50 – 100 Hz).
  • Direct current (DC) where the current flows unidirectionally from anodes to cathodes.
  • Pulsed direct current (PDC) in which a series of pulses of direct current flowing from anode to cathode is generated at varying frequency and amplitude.

Alternating current is extremely effective for stunning fish but also the least benign, therefore we strongly recommend that it is not used. Direct current is effective in drawing fish toward the anodes but is less effective in immobilising fish. Pulsed direct current has characteristics between the two and can attract and immobilise fish when they come close to the anodes. By far the most commonly used waveform nowadays is PDC – power is delivered from the source via a control box which can be used to vary voltage, frequency and width (duration) of electrical pulse in order to optimise fish capture and minimise harm to fish.

3. Survey Planning

Electric fishing is a highly-technical fishing method and surveys require a high degree of preparation and organisation in addition to highly-specialised equipment.

3.1 Material and People

Electric fishing can be dangerous to operators and others and requires careful preparation and adherence to strict working protocols. According to Beaumont et al. (2002) to “ensure the proper level of consistency, effectiveness, optimisation, and accountability, survey leader proficiency should be demonstrated periodically and survey crew members should be instructed in correct techniques”. All equipment must be checked and tested for safety by suitably qualified electricians at regular intervals, and calibration of current form and intensity may also be needed to ensure consistency. Gear should also be visually inspected before each sampling trip. The number of people required to carry out electric fishing depends on several factors including width and depth of the river being fished, number of anodes deployed, size and numbers of fish expected, size and type of boat used, and health and safety considerations. For instance, two persons can safely undertake a backpack electric fishing survey in a small stream where there are few hazards and help can be summoned in case of any emergency, whereas fishing a larger river with a wading electric fishing crew and multiple anodes, where large numbers of fish with some large individuals are expected, may require up to eight personnel. Note that under English electric fishing health and safety regulations it is not permissible to use multiple, independent power sources in fishing operations.

3.2 Additional Health and Safety

As electric fishing activity sometimes attracts members of the public, an electric fishing working zone (scale, weight, tagging zone) must be delimited using a bright ribbon and/or signs advising people to keep their distance. When gear is based on the river bank, a team member should always be in attendance to ensure that people or animals do not interfere with the equipment.

3.3 Deployment

Electric fishing can be carried out from a boat, or by wading in the stream, the latter being the most commonly used method of survey. Backpack surveys are mainly used when wading in streams of >5 m wide and up to 0.7 m deep. Wading with generator-based electric fishing enables wider streams to be fished effectively and can be done either with the generator and control box mounted in a small boat towed behind the people fishing, or with the generator on the bank and the anodes deployed on long cables. In boat-based electric fishing - in channels deeper than 0.7 m - anodes are usually hand-held by the people fishing, or can be mounted on a boom carried on the front of a boat; in both boat and bank based fishing the cathode can be either a metal grid or a braided metal strap or series of straps. The survey can be done across the entire width of the channel, part of the channel or at individual sampling spots, depending on the objectives of the study. Four approaches can be applied:

  • A semi-quantitative survey with a single electric fishing run or pass, where a pre-determined estimate of capture efficiency may be used to generate an estimated total fish population in the area fished.
  • Catch per unit effort methods – a fixed river length or area is fished, or fishing takes place for a recorded time interval.
  • A depletion survey where two or more runs are used to generate a series of progressively smaller catches, from which it is possible to make a *Point-abundance sampling – where a series of randomly-chosen small areas, perhaps 1 – 2 m2 are fished, and the numbers of fish caught in each area are used to generate a whole river reach estimate.

Occasionally in some situations we may use a habitat-targeted survey where only certain species and specific habitat types are fished. On capture, fish are transferred to buckets or larger holding tanks, measured, weighed, (sometimes anesthetised, tagged, and scales taken for aging) and put in a recovery bath before being released back to the areas from which they were caught. Fisheries surveys for specific purposes, for instance Site Condition Assessment for protected sites, may have very closely defined protocols.

3.4 Timing of Electric Fishing Survey'

Electric fishing is normally carried out during daylight hours to aid visibility, however with the right equipment and illumination, electric fishing at night can be effective as fish are generally closer to the surface during twilight and darkness. Optimum time of year for surveys is dictated by the species and life stages being targeted. Surveys should clearly not be undertaken when it would not be expected that the target species were present, nor should they be done when the fish are spawning. This is in order to minimise damage to individual fish or their gametes at a time when they are vulnerable and to minimise the effect of spawning aggregations on the survey results. Potential damage to non-target species and especially redds and spawning grounds has also to be considered. Electric fishing during extremes of temperature should be avoided. Not only does the method itself work less well, but fish may be tightly aggregated in certain areas which can distort interpretation of the results, and probability of harm to fish is greater at high temperatures. Electric fishing should not normally take place during spate/flood events since efficiency of electric fishing will be impaired by increased turbidity, depth and current velocity and the river is likely to be more dangerous to operators.

3.5 Length of Individual Survey Sites

The length of survey site depends on the aim of the survey itself and the size and characteristics of the river being fished. Guidance is provided in Table 1.

Length of Survey Site
Survey Type Monitoring Programme Element Min Site Length (m) Max Site Length (m) Time (Minutes)
Quantitative electric fishing by wading Coarse Temporal 75 150
Eel Index 75 150
Quantitative electric fishing by wading Salmonif Temporal and spatial 30 100
Eel Index 75 150
Catch per unit effort (time) electric fishing Salmonid Fry surveys in rivers > 10 m wide ≥ 5
Quantitative electric fishing by boat Coarse Temporal 100 300
Qualitative electric fishing by boat (including boom boat single run and boom boat catch per unit effort (time) Coarse Spatial 100 300
Coarse Large river temporal 250 unlimited
Quantitative netting Coarse Temporal 100 up to 250
Qualitative netting Coarse Spatial 100 up to 120
Micromesh seining Coarse Fry surveys 50 m2 area 150 m2 area
Hydroacoustic Coarse Temporal 5 km Unlimited
Table 1. Length of Individual Survey Sites

3.6 Numbers of Individual Survey Sites and Frequency of Survey

Numbers of sites to be fished, and frequency, depends on the aims of the survey programme. However in order to make objective judgements as to the current state of the fishery, or any changes in the fish population after restoration, or longer term trends, there are some key principles: firstly the number and location of sites fished must be sufficiently representative of the river reach being studied; secondly, be mindful of the natural variation in fish populations through time; thirdly, a Before-After-Control-Impact (BACI) approach is required when we wish to investigate an impact or intervention.

3.7 Administrative Requirements

Before an electric fishing survey can be carried out, permission / approval must be sought from a range of other organisations.

  1. In England, electric fishing is only permitted for scientific and fishery management / husbandry reasons. Formal consent is required from the Environment Agency https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/612044/FR2.pdf before an electric fishing survey can take place.
  2. For sites protected under conservation legislation such as Sites of Special Scientific Interest, Special Areas of Conservation and Natura 2000 sites there may be additional site-specific stipulations relating to the protection of key species and habitats. Consult Natural England: https://www.gov.uk/guidance/freshwater-and-migratory-fish-protection-surveys-and-licences in order to establish what special regulations relate to the survey area and to obtain necessary permissions. Special permits for handling protected species may be required.
  3. Permission is also required from owners and lessees of the land bordering the river, and owners and lessees of fishing rights, before fishing at a site can take place; this includes not only the site being fished but access to it across other private property.

4. Advantages

  • Can provide a robust estimate of fish abundance and reliable data comparable in space and time (Lake, 2013).
  • Relatively unselective for fish species and size.
  • Basic data (numbers, species, sizes and condition of fish) are directly available on fish capture
  • Individual fish can be examined, and additional data gathered.
  • Allows for standardisation of catch per unit of effort.
  • Adverse effects on fish are minimised, if properly used.
  • Suitable in a variety of habitats.

5. Disadvantages

  • Effective field of capture for electric fishing is confined to a relatively small volume around the electrodes, hence electric fishing has limited applicability in larger, deeper rivers.
  • Electric fishing has limited effectiveness in very high and very low conductivity waters or in turbid waters where visibility is low.
  • Electric fishing is generally more efficient in capturing larger fish and some species are more or less susceptible due to behavioural characteristics (Reynolds 1983), for instance relatively territorial species with a preference for river margins, e.g. pike, are particularly susceptible whereas open water shoaling species like bream are more difficult to sample consistently by electric fishing.
  • Electric fishing can only be undertaken by personnel who are trained in the specific techniques of electric fishing, health and safety considerations and fish welfare (see above).
  • Electric fishing can damage sensitive fish species and life-stages if best practice in respect of choice of gear, equipment setting, and fish handling is not followed.

6. Costs

Cost of electric fishing surveys depends on the scope of the survey itself, numbers and locations of sites, width and depth of sites, which in turn dictate numbers of personnel and type of equipment. A generator and control box and set of electrodes for fishing a medium to large river may cost in the region of £50,000 – £60,000, whereas a backpack fishing kit costs in the region of £1,000 - £1,500. Manpower costs will vary according to scope of the survey. A four- or five-person team will probably only be able to survey one quantitative electric fishing survey site per day on a large river, whereas up to ten sites per day would be possible for a backpack electric fishing survey undertaking single pass fishing on short sites in a small stream where all sites are geographically close.

7. Data Analysis

Data analysis on electric fishing data does not differ fundamentally from that for any other fish-capture-based method, but interpretation of results requires an awareness of the limitations of the method and the survey design employed. Issues such as fishing efficiency, species selectivity, seasonality of individual species and site representativeness need to be considered when evaluating results.

8. Further Reading

  1. Barbour, M. T. (1999). Rapid bioassessment protocols for use in wadeable streams and rivers: periphyton, benthic macroinvertebrates and fish. US Environmental Protection Agency, Office of Water.
  2. Beaumont, W.R.C. (2016). Electricity in fish research and management: Theory and practice. Second Edition. John Wiley & Sons Ltd. ISBN 9781118935583
  3. BS EN 14011:2003 BS 6068-5.32:2003 Water quality -Sampling of fish with electricity
  4. Cowx, I.G. (Ed) (1990) Developments in Electric Fishing Fishing News Books, Blackwell, New Jersey. ISBN 0-85238-166-2.
  5. Environment Agency (1999) ‘Semi-Quantitative Methods for Fisheries Classification’, Technical Report W167. Environment Agency, Bristol.
  6. Environment Agency (2002) ‘Guidelines for Electric Fishing Best Practice. R & D Technical Report W2-054’, Environment Agency. Available at: http://cdn.environment-agency.gov.uk/sw2-054-tr-e-e.pdf.
  7. Environment Agency (2004) Manual of Best Practice for Fisheries Impact Assessments Sceince Report SC020025/SR. Environment Agency, Bristol. ISBN 1844323285
  8. Environmental Protection (Water) Policy (2018) Sampling fish communities using electrofishers - Monitoring and sampling manual.
  9. JNCC (2015) Common standards Monitoring Guidance for Freshwater fauna. Joint Nature Conservation Committee, ISSN no:1743 - 8160 (online)
  10. Scottish Fisheries Co-ordination Centre (2007) ‘Fisheries Management Svq Level 3: Manage Electric fishing Operations’, Electric fishing Team Leader Training Manual, (June), pp. 4–13.

9. Gallery