One of the key tools available to fisheries management is that of electrofishing. Electrofishing is a reasonably simple concept to explain practically, but theoretically it can be rather confusing. Simply, a field of electricity is passed through the water that causes a muscle response reaction from the fish forcing them towards the netsman. A full explanation can be found later on this page. The main purposes for electrofishing are stock assessment, sampling/health surveys, tagging, catching spawners, anaesthetising or eliminating species

Electrofishing is an effective tool for fishery scientists because most aquatic organisms become motionless when the body voltage exceeds a certain value from nose to tail. Early methods were only applied to freshwater streams and small pools; this was due to limited knowledge of fish reactions.

The inherent nature of the process allows shallow water to be fished successfully; also there are limits on the total area that can be worked. Taking these two principals into consideration the ideal waters would be small shallow rivers, commonly chalk streams, drained canals or full navigational pathways with a maximum depth of 2.5 metres; the width of the canal is the main restriction. Conversely, the littoral zones of lakes and reservoirs lend their self to efficient electrofishing especially if there are many bank side features such as overhanging bushes, trees or reed beds. The aforementioned site descriptions tend to be very difficult to seine net thus electrofishing is the ideal replacement. On occasions fish traps can work well as an alternative if instant results are not required. Electrofishing can be undertaken in many forms with the most common applications being classical wading, classical boat fishing, trawling and screening/guiding

Gear requirements and reactions
Commonly the gear requirements for electrofishing consist of three main pieces of apparatus. Firstly the power unit, this can produce a 2 or 3-phase alternating current; the energy produced by this device will ultimately be determined by the conductivity of the water. It is usual to convert this to direct current through a transformer and this can be at different voltages plus the shape, length and frequency of the pulse will also be determined at this stage. The final section consists of the electrodes, and the shape of these will influence the field strength.

The electrodes consist of two types, the anode (positive) and the cathode (negative). The choice of material and size of electrode can influence fishing efficiency; ultimately this will determine the field strength (voltage gradient). Essentially an increase in electrode size will increase the electrical field size, decrease the resistance of the electrodes and most importantly reduce harm to the fish. .

When applying direct current the cathode should ideally be at least three times the size of the anode. With direct current the cathode tends to corrode due to electrolysis when in medium to high conductivity water, but this does not occur with alternating current.

Two types of current can be used when fishing with electricity, these incur different physiology and behavioural occurrences upon the fish. Direct current is the most common; this utilises a permanent anode and cathode. The fish are attracted to the anode, but repelled from the cathode. Alternatively, alternating current can be applied, the difference being that the electrodes continually alternate between anode and cathode. Fish are not attracted to either electrode, but instead are orientated in line with the field. Although alternating current has a larger immobilisation zone direct current is commonly the first choice as the attraction zone is larger, and thus more efficient. Constant direct current does not incur the same amount of injuries to fish as does pulsed (when keeping energy inputs the same), but the total area of fishing is always larger with pulsed direct current.

This image shows basic electrofishing being undertaken on a shallow river. The red generator can be seen in the centre of the boat with the control box sitting on top. The anodes are visible and are held with the end just under the water surface. The cathode, although not visible, is trailing behind the boat. This is not an ideal set up as electrofishing has it's dangers thus space is of utmost consideration. A spacious, tidy, well organized operation is not only safer, but much more efficient.

Factors affecting efficiency
Even with the correct electrofishing set up the efficiency of capture will be influenced by certain physical and biological factors. Firstly, the total fishing efficiency increases exponentially with the fish length. This makes electrofishing a size selective method, if the capture of smaller fish is a requirement then an increase in the field strength will decrease selectivity, but in turn this will increase mortality due to the higher voltage gradient. The second factor affecting the fishing efficiency is the specific conductivity of the water. For example, freshwater has a low conductivity, therefore a high field strength (volts per cm) can be achieved due to the reduction created in current flow (amps) caused by the increased resistance (ohms). Conversely, saline water is a better conductor of electricity, it has a lower resistance hence a better current flow. With everything staying equal, more power is required in saltwater to achieve the same voltage as in freshwater. Another important parameter is the temperature. There is a 40% reduction in conductivity when the water temperature is reduced from 20oc to 0oc therefore colder water will increase the fishing efficiency.

Fish Reaction
Fish show certain characteristics when in the presence of electricity. The reaction distance of fish to electricity is dependent upon the field strength, the pulse type, the fish length (and on occasions species) and the position of the fish in relation to the anode. Taking direct current as an example, both the anode and cathode are placed into the water body at some distance apart and fish in the vicinity will swim towards the anode. On approaching the anode swimming will cease, the fish will commonly turn over and then go into a state of narcosis. This process involves three key reactions: attraction (electrotaxis, this can in fact be split into positive and negative as fish do show initial movements away from the anode), stunning (electronarcosis) and electrocution The value of the field strength at which these three reactions occur is determined by the corresponding body voltage. The body voltage is dependent upon the field strength in the surrounding water, the fish length and the relation of the specific resistance of the water body to the resistance of the fish body.

More interesting is the visual behaviour patterns displayed by fish when encountering an electrical field. Carp charge from the electricity until they feel it is the correct time to take the field on and then they charge straight at the boat. Pike charge side wards towards the canal's bank, but when in the field continually follow the boat until totally stunned. Zander are extremely odd. They appear and then sink later to be found coming back to the surface after the boat has travelled a further 20 yards. Roach and rudd leap from the water surface and bream just roll over and give up. Each species, irrelevant of the size, react in different ways which makes it important that the equipment is set correctly.

The Hi-tech Boom Boat
MEM Fisheries Ltd. has built quite an extraordinary electrofishing boat. The design of the boat is aimed at for the use on the UK's canal network, when full, for carp or zander removal. The pictures below show the large yellow booms that are air operated, each has two anodes attached. Additionally two probe men handle long fibreglass poles with anodes attached. These are used to access the underneath of bankside vegetation, a common area for larger canal fish to reside. It would be fair to say it's the most technical electrofishing boom boat currently in action in the UK. Recently we have been undertaking the annual zander removal scheme carried out on the Oxford canal. This is the fifth year this boat has been utilized and the total percentage of zander caught is decreasing noticeably each winter.

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