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through and out of the attraction and immo- bilization zones. This risk is positively correlated with the size of the fish and will therefore tend to reduce the positive size selectivity discussed above. It has been shown that fishes which originally are directed towards the anode are attracted more efficiently than fishes orientated in other directions (Sternin et al., 1972). The at- traction was especially fast with pulsating direct current. With alternating current the orientation of the fish is of little importance as the attraction zone is lacking. The fish will be caught only when in the immobilization zone, within which the orientation of the fish is unimportant (Vibert, 1967; Sternin etal., 1972). When attracted the fish will follow the current lines. Fish close to the anode will then move directly towards it, while fish further away will move towards the anode in a curve like manner. Due to irregularities in the electric field close to phase boundaries (sediment/water or water/air) they might then swim into the sediment, often giving rize to bubble up-welling, or break the surface. The same reactions will occur for fish being repelled by the cathode and leaving it along the current lines. Physical factors lJ Because of its effect on the voltage and the currcnt linc dcnsity thc spccific conductivity of the water is the most important physical-chemical factor. Using the same equipment, higher voltage will be achieved in waters with low opposed to high conductivity due to higher resistance in the former case. To compensate for the lower voltage obtained at high conductivities a more powerful generator is required, which will increase its size and weight. The need for increased power is somewhat reduced by the lower current line den- sity needed for attraction at low conductivities (50% reduction in oligotrophic compared to marine waters; Lamarque, 1967). The optimal effect with direct current is achieved at a voltage of approximately 300 V at 17 500 JI S (Fig. 6). This value increases with de- creasing conductivity to approximately 700 V at 25 JIS, To achieve this voltage an effect of2,5 kW will be needed in the first and 0.7 kW in the second case. At very low conductivities ( < 20 JIS) the fishing efficiency is strongly reduced (Sternin et al., 1972). The efficiency reduction reaches a mini- mum at approximately 51lS (the value being dependent upon the inner conductivity of the fish). The conductivity of the water varies with tem- perature. At DoC the conductivity of the water is reduced by 40 % compared to 20 0 C. The fishing efficiency will therefore increase with decreasing temperature (cfr. Bruschek, 1967). Moreover, for physiological reasons the fish will be less attracted by electricity at high temperatures (Vibert, 1967). The existence of phase boundaries causes irregularites in the electric field. At the water/air boundary a compression of the current lines will occur, whereby the fishing efficiency is increased. At the water/sediment boundary the current lines will in most cases be attracted by the sediment, because of its generally higher conductivity (Sternin et al., 1972). The fishing efficiency will therefore be reduced near the bottom. Electric fishing with ordinary equipment might be impos- sible and a short circuit could occur if the sedi- ments are very rich in electrolytes, e.g. marine deposits or iron hydroxide. In addition to the specific conductivity a num- bcr of othcr factors are important for electric fishing efficiency e.g. the transparency. The immo- bilisation radius is in most cases equal to or less than 0.5 m and adequate visibility within this distance corresponds to a secchi disc trans- parency of approximately 1 m (equal to 100 mg Pt/ 1). In humic and eutrophic waters the trans- parency rather than the' attraction radius will therefore be the limiting factor. Under such cir- cumstances there is less need to use stronger and heavier power units. Limited transparency will also cause a more pronounced size selectivity as the probability of seeing the fish within the immo- bilisation zone will be positively related with its sIze.