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. 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. edelkrone backpackThe 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).backpack bicycle by chang ting jen Water conductivities can vary considerably between locations. cordura backpack xlargePure spring water will have a low conductivity, while increasing amounts of minerals or impurities will cause higher conductivities.backpack oxy acetylene cutting torch

To minimise stress and injury in fish, avoid fishing in high water temperatures... As fish become dormant in colder water temperatures, fishing will not normally be as effective during the winter months.rondreis costa rica backpack 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.eve 28l backpack 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).targus crave 16 laptop backpack 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. 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. In general, you should... Never keep fish in the electric field for longer than necessary. Avoid getting too close to fish with an energised anode. Never touch a fish with an energised anode, as this may cause electrical burns to the fish.

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... Low conductivity waters (less than 150 µS/cm) will generally require higher applied voltages for fish capture than higher conductivity waters (i.e. at least 300V). At medium and high conductivities, progressively lower voltages will be effective in fish capture because a lower voltage gradient is needed to elicit a response from fish at a given point in the electric field. 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.

As a general approach, electric fishing under any field conditions should be started at the lower end of the range of voltages recommended for those conditions. If you do not know the conductivity of the water, it is recommended to start at around 150V, assess the effect of fishing and keep making small adjustments until the best results are obtained. Larger fish are generally susceptible to lower voltage gradients than smaller fish in any given situation. The following are therefore recommended as a guide (duty-cycle mode is discussed in the following section)... Less than 150 µS/cm select 300 to 400 Volts 150 to 500 µS/cm select 200 to 300 Volts Pulsed DC or DC 500 to 800 µS/cm select 150 to 200 Volts 800 to 1000 µS/cm select 120 to 180 Volts Greater than 1000 µS/cm select 100 to 150 Volts 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. Better fish attraction and welfare properties compared to PDC operation. Effective in low conductivity water. Better immobilisation properties than DC. Effective in higher conductivity water. Better power consumption (longer battery life) than DC. Limited immobilisation properties compared to PDC.

Effectiveness is more prone to disruption by local variations in the conductivity of the riverbed. Limited attraction properties compared to DC. Less prone to disruption by variations in riverbed conductivity. Achieved at voltage gradients of as little as 0.1 V/cm Gradients of 0.2 V/cm to 0.3 V/cm are needed. Achieved at voltage gradients of 1.0 V/cm Gradients as low as 0.5 V/cm to 0.6 V/cm are needed. 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. Research has shown that whilst medium to high frequencies are more effective in capturing fish of some species groups, particularly salmonids, these are also more harmful. E-Fish (UK) Limited gratefully acknowledge the information supplied by the UK Environment Agency