W.G. Hartley
Seaford, Sussex
England
Electrical fishing is a general term covering a number of very different methods, which all have in common the use of an electric current flowing through the water to impress the on fish within the space affected a common pattern of reaction, leading to their capture. The methods have the advantage over other means of collecting fish that they do not require preliminary preparation of the site, with consequent delay and the disturbance of the fish to be investigated, and that the requirements in terms of manpower and physical exertion are small. They have the disadvantages of variability of effect when compared with the use of nets or traps, and the risk of physical danger to both fish and operators, though these disadvantages are reduced to inconsiderable levels by experienced management. Competently carried out, the method does not result in mortality or damage to the fish to any greater extent than does netting, and indeed there ought to be no casualties at all. Safety of the personnel has the pragmatic sanction of freedom from injury hitherto in spite of the use of some very unsafe equipment by inexperienced hands. Modern practice is to use much more lethal equipment, but properly constructed and far safer to handle. Various forms of electric fishing gear have been described by Hartley (1975) and Weiss (1976).
Electrical fishing lacks the possibility of a complete removal of all the fish, as is nominally possible with a fine-mesh seine in an ideal site. It is at its worst in pond conditions, where the fish can escape in three dimensions, and is rarely effective in water more than 2 metres deep, though special arrangements can be made for such sites. In flowing water, conditions are more favourable, and the results more consistent; in these circumstances the freedom of working with electrodes is far greater than with nets. In suitable cases, the electrode array may be made as a fixed installation taking continuous samples of migrating fish, and used with an automatic recording device to monitor the numbers passing and the pattern of migration.
The reaction of a fish to an electric current is the result of stimulation of the nervous system, which induces a series of involuntary muscular movements ranging from those producing a particular orientation of the body to an involuntary directed swimming, according to the intensity of the stimulation. If the stimulation is excessive, the fish is rendered immobile, and the effect may de fatal.
As the reaction depends on the degree of stimulation, it is necessary for the user to understand the laws governing it, and this requires an elementary appreciation of the behaviour of electrical currents in water, and experience of the behaviour of different species of fish when stimulated. Failure to realize that different species may react differently is a common cause of mistakes when assessing the results.
The nature of the stimulation varies according to the electrical regime employed, but this aspect must be considered later. As far as the individual fish is concerned, the degree of stimulation depends on the difference in electrical potential between its head and its tail. It follows that the stimulating power of a given type of current will depend on the size of the fish and its attitude and position in the electric field, which is never uniform. Near an electrode, there will be a powerful effect, but at a distance the effect may be negligeable. It is most important to appreciate that fish are only controlled in a zone close to an electrode, the radius of which seldom exceeds two metres and may be far less in certain water conditions. The effective area can be modified by using several electrodes if sufficient power is available.
Fish which are not brought under control are merely irritated by the electric field, and will in general avoid it. As the controlling field has a finite size, it follows that the space from which fish are repelled must always be greater than the catching space, and as salmonids at least remember the disagreeable sensation for a day, a closed population will become progressively harder to catch. In flowing water and among fish which do not form shoals, the effect is rarely serious, but in lacustrine conditions and with fish which react as a shoal, it may become virtually impossible to catch any, unless nets are set to restrict the movements of fish to the fishing diameter of the apparatus in use.
The extent of the fishing diameter varies with the power available, the water conductivity - which may change abruptly in a stream where a drain enters - the temperature, and the efficiency of the type of electric current as a stimulator. An inefficient type, such as smooth direct current, shows great variations of effectiveness for slight variations in physical factors; alternating current is less sensitive, and a properly selected pulsed current has an almost uniform action.
The practical implication of these variables in the use of electrical fishing to obtain population data may be simply summed up in the recommendation never to extrapolate. There is no way in which a fish population can be estimated from a single fishing, however thoroughly this is carried out; it is not possible to know the efficiency of an electrical fishing in advance, but only in retrospect. The fact that a given machine has fished at 70 percent efficiency in a particular site does not mean that it will not fish at 15 percent efficiency in the same site a week later, or in another the same afternoon. Not even an electric fish-screen, working in constant conditions among salmon smolts of uniform size, maintains a fixed efficiency; the behaviour and motivation of the fish vary with numbers and changing physiology to produce abrupt alterations in the results.
It follows that population assessment requires the use of the one of the recognized methods of treating samples, normally either the Petersen mark-and-recapture plan or else the de Lury catch-depletion system. Both methods are well-tried, and the statistics familiar when netting or trapping methods are employed, but with electrical fish-collection special consideration must be given to the sampling process to ensure that uniformity is obtained. The necessity for this has been shown repeatedly when users have combined the Petersen and de Lury methods in a single operation; the Petersen result is an over-estimate, and the de Lury an under-estimate. The discrepancy is due to failure to appreciate the effect of electrical fishing on fish behaviour.
In the Petersen system, a sample of the fish population is marked and returned to the water. After time has been allowed for the marked fish to become thoroughly mixed with the unmarked majority, a second fishing produces a mixed sample in which the proportion of unmarked fish represents the number of fish associated with the number originally marked. A third fishing gives data on the confidence limits of the postulated answer. The sources of error include failure of homogeneous mixing between marked and unmarked fish, and mortality, including predation, which usually acts selectively on the marked fish. Clearly these are affected in opposite directions by the delay between successive fishings; a short interval allows imperfect mixing, and a long one excessive mortality. Comparative results for different species can be used to monitor the influences of these factors.
The usual difficulty with the de Lury method lies in obtaining real equality between the successive fishing efforts; even with methods other than electric fishing.
When electrical fishing is used, there are different variables to be considered. It is easy to produce equality of effort between successive electric fishings, but the effectiveness of any single fishing varies with the size of the fish in the sample. As is well-known, larger fish are more readily affected than smaller, and this involves not only a greater certainty of capture in the controlled zone, but a wider extension of the surrounding zone of repulsion, and hence a greater number of sensitized large fish. Usually there is a mechanical limit to the size of the smallest fish which can be collected electrically, but this is common to all other methods, and electrical gear will manage fry less than 2 cm in length if necessary. Fishing in a particular manner will not produce mutually comparable proportions of a stock of fresh-swimming fish, bottom feeders and eels; the various categories have to be separately collected.
Vilbert et al. (1960) divided trout involved in an experimental fishing into three categories - those which are caught, those which escape by hiding, and those which swim completely out of the experimental area. It is this difference of behaviour which must be taken into account to provide agreement between the results of Petersen and of de Lury estimations.
The category of fish which escape by hiding ought not to be numerically large. It often includes those individuals which have been rendered immobile by too severe or too prolonged an exposure to electric current, and trapped in weeds or holes. Good electrode design can reduce this risk by limiting the peak value of the potential gradient - a matter of using a diffuse electrode of the largest convenient size - although in the case of cyprinids there appears to be a narrow margin between controlling the fish and stunning them, at least when pulsed currents are employed. This is a matter of experience.
The sampling error arises mainly from the fugitive fish. A batch of fish caught electrically, marked and subsequently fished over, will include some which will subsequently avoid an electric field at the first sensation. This sensitivity may decrease with time, but in the case of salmonids it persists for more than a day. The effect of this shyness on the part of some marked fish is to increase the numbers of unmarked fish associated with the marked ones re-caught, and so to inflate the estimate of population. Among shoaling fish there is some transmission of behaviour, and though this will not affect the Petersen estimate, it will do so with the de Lury.
One of the attractions of the de Lury method is that the successive fishings can be carried out in rapid succession, thus minimizing errors due to mortality or re-colonization. It has, however, long been known that a fish submitted to repeated electrical exposures becomes increasingly resistant to their effects for some hours afterwards, at least in laboratory conditions.
It follows that successive fishings which are equal in every controllable parameter are in fact not entirely equal in compulsive effect, and are moreover not carried out over the same progressively depleted stock, but over a stock diminished also by the escape of sensitized individuals. Thus a de Lury regression based on two fishings only gives a gross under-estimate.
A great deal of attention has been devoted to correcting the discrepancy between Petersen and de Lury estimates obtained by electrical fishing, and thus to the improvement of this fishing method for serious studies. Timmermans (1974) pointed out that at least with trout, the principal error occurs between the first fishing and the second. His correction involves plotting all the successive catches on the usual semi-logarithmic scale against the aggregate catch, but constructing the regression line on the basis of the second and subsequent catches; the reliability of the line is indicated by its straightness. A line parallel with the plotted regression line is then drawn through the first catch value, and the total population taken from the intercept of this with the base line. This gives close agreement with the Petersen estimate for a small stream, though there is still a discrepancy of almost 10 percent for a river. Naturally enough, agreement is worst among the larger fish, and as the de Lury value is still lower than the Petersen, it may be that the latter requires correction; a de Lury estimate greater than the Petersen indicates that something is wrong.
Cross and Stott (1975), working with pond fish, come to similar conclusions and a similar correction. They examine the factors involved in departures from expected results during sequential fishings, and this is an aspect deserving far greater attention than it normally receives. We have seen great attention given to far-reaching analyses of experiments which ignored the basic rule that before it is permissable to compare the relative effects of different fishing machines, it is essential that they should be shown to be capable of consistent behaviour.
Comparability is at the root of most of the ambiguities inherent in electrical fishing for census work. If a given fishing machine, working in a channel which it ought to be capable of clearing, does not yield a steady value for fishing effectiveness in successive runs over fresh fish introduced shortly beforehand, there is something fundamentally wrong with it. Normally, the second fishing would be more efficient than the first with the same operators, as they would have gained experience; in fact, the most significant factor in fishing efficiency is the experience of the user. A machine is either capable of catching fish or incapable of catching fish; it is the experience of the user which allows him to obtain the best catch possible with an inefficient apparatus, and if he is confronted with a new one, he has to learn its characteristics.
This applies particularly where various types of electrical regime are employed. The man familiar with a direct-current set initially obtains very disappointing results with a pulsed current because his previous experience leads him to stun the fish before they can be caught. Once he is familiar with the reactions of fish to the far more intense stimulation, he obtains much better results than with direct current, at a fraction of the power.
This introduces a subject mentioned initially and postponed until now. The electrical regime used for fishing plays an important part in obtaining results comparable between different waters and in different seasons. As has been stated, the reaction of a fish depends on the voltage drop between its head and tail. This value, at a set distance from the electrode, depends on the water conductivity and the electric current flowing. Using the same equipment, the voltage drop therefore changes with the water conductivity and the temperature, and the fishing power of the set varies accordingly.
Although it has a greater attractive effect than any other, pure direct current is inefficient as a stimulator, and is thus sensitive to variations in fishing conditions. The consequence is that the effective fishing radius of a direct current set varies in use, from place to place and from day to day. This does not matter for collecting fish, but it is incompatible with fishing by constant units of effort for purposes of population estimation.
An interrupted current designed to produce efficient stimulation of nerves provides all the fishing effect necessary at a low potential gradient, further from the electrode and thus more uniformly spread. It also requires only a small generating set to supply it, and is thus very portable. The chief handicap is that fish are not attracted as certainly as with smooth direct current, and may become immobilized before they reach the catching zone, though this can be corrected by electrode design. The most important point is that the fishing radius remains constant, because the electrode voltage does not vary with water conductivity - the sampling net is always the same size. It is important to distinguish between true pulsed current, derived from the rhythmic discharge of a capacitor at a constant voltage, and the use of interrupted alternating current, commonly produced by using a semi-conductor device in the circuit to suppress all but the descending part of the positive half-cycle. This latter form of current is sensitive to load, as the power is restricted to that instantaneously available, so that the electrode voltage varies. The pulse form also approximates to a half-cycle rectified pattern, and is not a sharp-fronted pulse. In very pure water such a system works well enough, but it rapidly loses its effect as the conductivity increases.
In tropical countries, particularly where high temperatures are combined with very low water conductivity, the effectiveness of various current forms differs from temperate experience. In such conditions it is of course necessary to establish a suitable type of electrical regime before attempting to use it for quantitative investigations.
Cross, F.G. and B. Stott, 1975 The effect of eelctric fishing on the subsequent capture of fish. J.Fish Biol., 7:349-57
Hartley, W.G., 1975 Electrical fishing apparatus and its safety. J.Inst.Fish Manage., 6:73-7
Timmermans, J.A., 1974 Etude d'une population de truites dans deux cours d'eau de l'Ardenne belge. Trav.Stn.Rech.Eaux For.Groenendaal-Hoeilaart(D), 43:37–45
Vibert, R., P. Lamarque and R. Cuinat, 1960 Tests et indices d'efficacité des appareils de pêche électrique. Ann.Stn.Cent.Hydrobiol.Appl.Paris, 8:53–89
Weiss, D.M., 1976 A high-power pulse generator for electric fishing. J.Fish Biol., 9:119–24
