Now a day, electro fishing is being successfully used in the many countries of the world. Commercial electric fishing is common in these countries both for river and sea fishing. Electro fishing is an unconventional method of fish collection and used to catch fishes through saving time, labor, money and man-power. Also, this method does not affect the growth, the viability and the reproductive capacity of the fishes; moreover, food of fishes present in the water does not influenced by the electric field created during fishing by this method.
Electro fishing and its mechanism:
In brief, electro fishing is a quick method of harvesting large number of fish by affecting them through creating electric field in the water. During fishing, an electric field is created in water between two electrodes. These electrodes are called anode and cathode. When fishes became affected by the electricity produced by the field, they are collected by netting or fish pumping. Generally 3 types of electric current are used in this process, i.e. direct current (D.C.), alternative current (A.C.) and interrupted current (I.C.). Fishes show different types of reaction depending on types of electricity produced in the field.
Reaction of fishes to direct current (D.C.):
- At first, when electric field starts to produce electricity and if the voltage is low, the target fishes are will not be affected or influenced by the electric field and they may escape from the field area.
- When voltage increases and crosses a certain threshold value, electrotaxis reaction will be shown by the affected fishes. During this time, if the fish will remain in transverse position to the current direction, it will turn parallel to it and the fish-head will be directed towards the anode. The body of fishes will be vibrated if the fishes remain in a parallel position to the current direction.
- Then, with a fresh pulse application, all affected fishes will turn parallel to the current direction and begin swimming towards the anode which is called galvanotaxis.
- Before reaching anode by swimming, the fishes will be stupefied and will loose locomotion ability and may turn upside down which is called galvanonarcosis.
- It the electric current is switched off at this level, the narcotized fish will be able to recover completely.
Reaction of fishes to alternative current (A.C.):
- If certain threshold value of voltage is produced by the field, the affected fish will show oszilloiaxis reaction. In this reaction, all fishes between anode and cathode will take up a position transverse to the direction of the current. Fishes will cease to swim and will narcotized with fading of body color as a consequence of concentration of the pigments.
- If the current source will turn off, the fish do not recover immediately and for several minutes, they remain in a state of hypnosis, unable to recover. During this period, the fish body stays in lateral or dorsal position.
- After hypnosis period, the affected fish will return to their normal swimming posture and may be escaped from the field.
Reaction of fishes to the interrupted current (I.C.):
- When fishes affects by a pulse threshold voltage and show electrotaxis, they undergo heavy vibrations and starts swimming towards the anode.
- If the electric voltage increased gradually and then decreased abruptly this time, the fish will turn about and swim towards the cathode.
- Galvanonarcosis will follow in the same way as it occurs in case of direct current.
When the affected fishes in an electric field receive electric current, the free nerve endings in the fish skin become stimulated which produces excitation in the central nervous system and this excitation leads to tetanic contractions of the muscles resulting in the stoppage of locomotion of fishes. During this time it becomes easy to catch the affected fishes.
In commercial fishing both A.C. and D.C. are used in freshwater habitats whereas I.C. is used to harvest fish in sea water. The attracting anodic effect and the blocking effects are used to advantage of fishery.
Attracting anodic effects:
Attracting anodic effects is generally produced to capture fish on commercial scale in both freshwater and sea water environments. In freshwater, two electrodes (anode and cathode) are immersed in water and 2:3 (anode:cathode) size ratio of electrodes are maintained. They electrodes are operated on batteries and anode carries a hoop net at its base. The narcotized fish assembled at the anode are removed by the hoop net and the entire operation is conducted from a boat.
In this method, affected fishes are frightened or forced to move towards a particular zone or area. The effects when produced in freshwater shows following facts-
- Fishes are forced to enter new river systems, bays catching areas for easy fishing.
- Unwanted fishes are frightened so that they leave the specific water area for harvesting target species only.
- Fishes are prevents from entering the turbines or pumps of hydro-electric plants and dams.
Disadvantages of electro fishing:
Electro fishing is becoming something like an art and skilled fishermen or fish catcher can caught large amount of fishes but this method has some disadvantages too. These are as follows-
- Extensive electricity current may damage the fish population.
- Turbid water and excessive vegetation restrict visibility and reduce the value of immobilizing capability of A.C. electro fishing.
- High conductivity bottom materials tend to “short circuit” the current out of the water into the bottom material reducing the electro fishing effectiveness.
- There is always a great risk of scaring away the fishes at the hands of untrained catchers.
- Other fish population (except target fish species) may also be damaged by the electric field.
- Comparatively high-tech for the fishermen of developing countries like Bangladesh.
Though electro fishing is being practicing successfully in foreign countries, but in Bangladesh, this practice is completely unknown.
- Biswas, K.P., 1996. Harvesting Aquatic Resources, Daya Publishing House, Delhi-110035, pp. 188-199.
- Khan, I., 1998. International Encyclopaedia of Fishes and Fishery Culture, Vol. 7, Anmol Publications Pvt. Ltd., New Delhi-110 002 (India), pp. 2005-2009.