Other fishing methods

The fish pump

This method of fishing attempts to capture fish by sucking them out of the sea through a wide hosepipe in a powerful stream of water pumped aboard the fishing vessel. This is not as simple as it may appear. The disturbance of the water caused by pumping frightens the fish away, and only those quite close to the orifice of the hosepipe are likely to be drawn in. The fish must therefore be attracted in some way towards the hose so as to concentrate them near the opening before the pump is switched on. The method has been applied commercially in the Caspian Sea where there are species of Clupeonella which can be attracted close to the pump by light, but many fish will not come near to a powerful lamp. Another possibility is to attract fish to the pump electrically (see below).

Electric fishing

When direct electric current is passed through water, a fish within the electrical field will turn and swim towards the anode. This is known as the anodic effect. The intensity of this effect depends upon the potential gradient to which the fish is exposed, and large fish are therefore influenced more than small ones. If the strength of the electric field is progressively increased, the fish eventually becomes paralysed and finally electrocuted.

These effects have been applied in various ways to the capture or enclosure of fish in fresh waters. In the sea, it is difficult to maintain electric fields of sufficient strength because of the high conductivity of the water. None the less, various experiments in marine electrical fishing have been conducted, and some commercial applications may ensue. For example, the anodic effect might be used to attract fish towards nets or fish pumps. As they approach the anode, the fish are likely to become paralysed by the increasing field strength and consequently unable to avoid capture.

Electrocution has been applied commercially to tuna fishing to cut short the struggles of fish caught by electrified hand lines, and electric harpoons have been used in whaling. Electrodes attached to trawls increase the catch of animals that burrow beneath the surface (shrimps, Norway lobsters, plaice, soles, etc.), pulses of current stimulating reflex muscle contractions which cause them to leap out of the sand or mud and become caught in the net.

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