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Disinfection . In order to prevent spread of diseases (i.e. virus and bacteria) and parasites (e.g. Gyrodactylus and the crayfish plague) all the equipment (electrodes, waders, dip nets, holding nets, buckets, boats etc.) must be adequately disinfected before fishing in new areas (up-stream areas or in new catch- ment areas). Disinfectants approved by the national authorities should be used. Formalin (2 % solution) iodophor solution (50-100 ppm free iodine) and complete drying are normally sufficient. Conclusions and recommendations concerning the use of electrofishing for studies of fish populations Equipment - Do not use alternating current. - Use high output voltage (> 500 V) and weak generators (< 1 kW) when fishing in low con- ductivity waters (< 100 11 S) and low voltage ( < 300 V) and strong generators (> 2 k W) in high conductivity waters (conductivity > 500 11 S; cfr. Fig. 6). - Use gasoline generators in high conductivity waters. Batteries are sufficient in low conduc- tivity waters ( < 75 11 S). - Use non pulsed direct current when the target animals are hidden in densc vegctation, under stones or in the mud, and when fishing in turbid waters, or at low temperatures. - Use pulsed direct current when fishing in slow flowing streams and open water areas of lakes. - Use condenser discharge pulses when the effect of the power unit is restricted (e.g. because of high water conductivity or transportation problems ). - Whcn using pulsed direct current, use fre- quencies over 50 Hz and a pulse length of more than 1 ms. If the attraction radius is too small, increase frequency and/or pulse length. - If possible, the radius of the ring shaped anode should be 40 cm or larger. In high conductivity 39 watcrs, use non-corrosive alloys for the anode, e.g. aluminium. - For stationary equipment, cathods made of iron netting are recommended. Wire cathods are suitable for portable equipment. - The area of the cathode should be at least three times as large as that of the anode. - For recommendations concerning security, see the previous section. Quantitative electro fishing for different fish species - Realize that the behaviour of the fish species and the biotope type in their habitat determine their catchability and thereby the possibility for estimating the population density with accuracy and precision. As there are only a few studies in which population estimates based on elec- trofishing have been checked against popu- lations of known size, little is known about which fish species are successfully quantified by electrofishing. As a rule, the following can be stated: - For territorial salmonids, e.g. Atlantic salmon, brown trout and stream dwelling arctic charr, electrofishing is often successfully applied. - Bottom dwellers, e.g. eel, sculpins and burbot, generally have a lower catchability, probably resulting in a larger but unknown underesti- mation than in the preceding species. - Schooling species, e.g. most cyprinids, often havc a low and unpredictable catchability. Absolute population estimates of these species are therefore often of doubtful value. - Some pelagic or semipelagic species have a tendency to avoid the electric field and might therefore often be hard to quantify, e.g. grayling. - For other species, the estimated catchability may be large, but critical studies on the relation between estimated and real population size are lacking. I i r t