Electrofishing, Theory

When undertaking electrofishing activities your ultimate goal is to generate an electric field in the water that will attract and temporarily immobilise the fish within it, but with minimum distress and no long-term harm to them.

To achieve this you will need control the properties of the electric-field by choosing suitable backpack fishing settings (such as voltage, duty-cycle and frequency) that are based on the environmental conditions you are operating in, the type of activity your are undertaking and the fish species you are likely to encounter.

With experience you will find that it becomes easier to understand the effect of these settings, and make suitable and effective selections for the control parameters, but initially the following guidelines are provided to help you understand some of the theory behind electrofishing.

Water Conductivity

Knowledge of water conductivity is a very useful prerequisite for successful and safe electrofishing. It is recommended that reliable portable conductivity meter is included as an integral part of the survey equipment list.

Before setting up your fishing system at a new site, measuring the conductance of the water you will be working in will help determine the initial fishing settings to use. The electrical conductivity is a measure of the waters ability to conduct an electrical current, and its value is usually expressed micro-siemens per centimetre (µS/cm).

Water Temperature

To minimise stress and injury in fish, avoid fishing in high water temperatures…

• 16°C to 18°C for salmonids.
• 22°C to 24°C for coarse fish especially when pike and perch are present.

As fish become dormant in colder water temperatures, fishing will not normally be as effective during the winter months.

Electric Field

When you are using the fishing system, an electric field will be generated around the anode and cathode, with the strength of the field diminishing as you get further away from the anode.

A voltage gradient is developed along the length of fish within the electric field, such that ‘galvanotaxis’ stimulates their nervous system and they are forced to swim towards anode (the source of the field).

At a point approaching the source of the field, the fish enters the hold-zone, where the field is then of sufficient strength to temporarily immobilise them and thus aid in their capture.

In most electric fishing situations it is desirable to create as large an effective capture field as possible. However in shallow and narrow streams there is no need to create a field that will attract fish from many metres away since any fish present will never be far from the operator, while in very turbid water there is equally no point in immobilising fish at a depth or distance from which they cannot be seen and retrieved.

The size and geometry of the anode is an important factor for field generation and fish health.

• Smaller rings (<250mm diameter) will produce more focused and intense fields using less power (and lower voltages), but the intensity of the field may be harmful to fish at closer ranges, and their use is not recommended under normal circumstances. If the physical nature of the stream necessitates the use of a very small anode (for instance fishing for Bullhead or young Salmonids, in shallow water conditions in a boulder-strewn stream) then the applied voltage must be reduced accordingly, perhaps to as low as 50 Volts
• Larger rings (>500mm diameter) will produce a more dispersed fields that require increased power (and higher voltages) to sustain them. In higher conductivity waters maximum usable anode size may be limited by the power available.

A 300mm anode ring is provided with the system as standard as this is considered a good balance between all the factors discussed above. Please contact E-Fish sales to discuss any other anode requirements.

Voltage

The fishing voltage is the primary parameter that controls the size and strength of the electric field, and in order to attract and immobilise fish (without causing harm), it will need to be varied according to…

• Ambient water conductivity.
• The duty-cycle mode used. Either smooth Direct Current (DC, 100% duty-cycle) or pulsed Direct Current (PDC, less than 50% duty cycle).
• Size of effective capture electric-field required, and anode ring size used.

The overall aim during any electric fishing operation should be to maximise the effective field of fish capture whilst minimising the extent of the zone of very high voltage gradient around the anodes in which fish can be damaged. Where very sensitive or valuable species are present, operators should consider further reducing the risk of damage to fish by reducing applied voltage even if this means some compromise of fishing efficiency.

The following are therefore recommended as a guide (duty-cycle mode is discussed in the following section)…

Duty-Cycle (Pulsed Versus Direct Current Operation)

The duty-cycle (ratio of on-to-off time) of the backpack output can be adjusted to vary the power output and achieve the best fishing results depending on the water conductivity, environmental conditions and type of fish being sampled.

When the duty cycle is set to 100% the output is always on, and is referred to as Direct Current (or DC).

When the duty cycle is than 100%, the output oscillates at the specified frequency, and referred to as Pulsed DC (PDC). Typically PDC is used with duty-cycles of 50% or less.

Use of DC for electric fishing potentially offers a number of advantages over other waveforms notably in respect of attraction properties and in terms of fish welfare, so DC should be used wherever and whenever it is practicable.

However, DC is a “power-hungry” waveform leading to reduced fishing time from a battery pack, but does prove particularly effective in low-conductivity waters where power demands are generally small. DC’s effectiveness is also more prone to disruption by local variations in the conductivity of the riverbed, and it has limited ability to actually immobilise fish compared to PDC.

Attraction of fish toward the anode can be achieved at voltage gradients of as little as 0.1 volts-per-centimetre (V/cm) when using DC. When using PDC (PDC – i.e. a duty cycle of 50% or less), gradients of 0.2 V/cm to 0.3 V/cm are needed.

Immobilisation of fish using DC can be achieved at voltage gradients of 1.0 volt/cm whilst with PDC this can occur at gradients as low as 0.5 V/cm to 0.6 V/cm.

In low conductivity waters the voltage gradients needed to elicit attraction and immobilisation will be slightly higher than those given above, whereas in higher conductivity water these values will be slightly lower.

You should make every attempt to prevent the fish coming closer to the anode than the distance at which voltage gradient is sufficient for immobilisation and you should never touch a fish with an energised anode.

In summary…

Frequency

With the Duty Cycle parameter set to 100%, the Frequency parameter has no effect. However, when using a Pulsed DC the choice of frequency will be influenced primarily by the species being sought, bearing in mind that under normal circumstances we wish to maximise the attractive properties of our electric field whilst reducing the immobilisation zone to a minimum.

• Salmonids – frequencies of 40Hz to 60Hz are as effective in attracting fish as the commonly used but potentially more damaging 100Hz. 40Hz will still immobilise salmonids but only within a much closer proximity to the anode. 10Hz will attract salmonids but not immobilise.
• Cyprinids – optimum frequencies may vary but for roach 30Hz has been shown to give both good attraction and good immobilisation. Switching to 10Hz reduces the zone of immobilisation whilst increasing attraction properties. However, there may be difficulties in capturing cyprinids in some circumstances if they are only immobilised in a very small zone around the anode. An added benefit of use of 10Hz is that salmonids will be only slightly influenced by the electric field and unlikely to be immobilised at all. Therefore where adult salmonids are present and coarse fish surveys are being undertaken it is recommended that if pulsed DC is used, it may be worth considering fishing with low frequencies (10Hz to 30Hz).
• Perch – are more similar to salmonids in their response to electric fields and 100 Hz has the best attraction and immobilisation properties. However, as fish damage (to perch and other species) is more likely at this frequency, 30Hz to 40 Hz is recommended for percids though where good immobilisation is also required then 10Hz is better
• Pike – little research or reference is available in scientific literature, but fishing at 30Hz to 50Hz has proved effective.
• Eels – most frequencies investigated were effective in both attracting and immobilising eels, so bearing in mind the potentially more harmful effects of higher frequencies on some other species, frequencies of 10Hz to 40Hz should be employed as standard.