Effects of Electricity on Fish
|A recent fish survey undertaken on a southern section of the grand union
canal gave the perfect opportunity to gain some photographic evidence of
fish reaction to electrical fields. I shall not delve to deeply into the
rather confusing areas of theoretical reactions of fish as this is long-winded,
slightly mundane and truly unnecessary for this page.
As discussed in the electrofishing page three types of electrical current are available to fisheries scientists - uninterrupted direct current; pulsed direct current; alternating current.
Three keys stages are witnessed when fish are exposed to electrical fields. The initial reaction is commonly known as 'excitation' and all current types result in similar responses. Fish show restlessness and begin to shudder as they come into line with, and then lie along, the lines of the force.
The second stage is known as 'electrotaxis' and does tend to vary slightly between current types. Uninterrupted and pulsed DC are, however, similar. The fish begin to move towards the anode and away from the cathode. When the anode is reached the fish becomes immobile with their head facing the anode.
The slight difference with pulsed DC is that if the pulse is set to frequent some larger fish can succumb to muscular cramps. The reaction of fish when using DC in the headed section of 'electrotaxis' is sub-headed to galvanotaxis . Conversely, in the same section, but applying AC, is known as oscillotaxis. This area of fish reaction varies from that of DC. The fish become oppressed into occupying a position along the euipotential (central between positive and negative).
The final section of fish reaction is 'electronarcosis'. With DC this sub-section is known as galvanonarcosis. Fish become dazed and immobile and show little or no response to alternative stimuli. At this point the fish will cease to breathe for short periods. Depending on species, some will sink (eels, bullheads, initially carp and zander) and others will float (bream, pike, carp after a short period and zander after a couple of minutes but float for a long period there after). This is a critical point as long periods in the field can cause death. Conversely, shorter periods are ideal as fish will recover and return to normal.
The effects of AC are known as oscillonarcosis and are very similar to galvonarcosis, however, the physiological effects are much greater thus the reason why DC is now more commonly used.
The difficulty in gaining accurate photographs on a working boat means that the following images only fall between the categories of the final stages of galvanotaxis through to complete galvanonarcosis.
|These three images are good examples of multiple fish reaction to direct
current (left click image for a larger clearer version). The centre image
shows a large crucian carp clearly under the state of electrotaxis. By looking
at the water pattern this shows the fish being pulled towards the anode
(positive) headfirst. The right hand picture shows some bream in a state
of electronarcosis - visual state of immobility and beginning to float to
the surface. The image on the left shows the result when counteracting a
shoal of fish. All responses are shown in this picture. All pictures show
the right arm of the electrofishing boom boat and a fisheries technician
working an extra hand held anode to increase the fishable width and efficiency.
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