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Electrofishing in lakes In open water areas of depths greater than the attraction radius (appr. 1 m) electro fishing is generally unsuccessful irrespective of the abun- dance of fish. This is probably due to flight reactions. Electrofishing is ineffective when the transparency of the water is less than the attrac- tion radius, and is almost impossible when the transparency is less than the immobilisation radius (app. 0.3-0.5 m). In shallow water and/or in vegetated areas electrofishing is possible and might, depending on fish species and habitat, be very efficient. In suit- able cover from vegetation or bottom substrates most fish remain immobile even when boats carrying noisy and vibrating equipment are close. Fish generally do not show flight reactions until in contact with the repelling zone of the anodic field. Electrofishing in lakes is also useful for popula- tion estimates by mark-recapture methods. To reduce the inevitable effects of selectivity (with respect to habitat and fish size), recapture should be performed by other methods (e.g. gill nets, fyke nets or by trawling). It is advisable to check whether more or less closed subpopulations exist in e.g. litoral and pelagic areas. When calculating the population size, different size classes should be treated separately. By comparing the catch and the abundance of fish in different size classes the selectivity of the fishing method can be estimated. The abundance of fish in size classes with few or no recaptures can then be estimated by inter- polation of the selctivity (catch per effort/popula- tion size) of adjacent size classes. For some species and in some habitats the catchability is high and the removal method can be applied for population estimates. It is often useful to subdivide the litoral zone with respect to habitat (e.g. sand, gravel, stones, submerged vege- tation, emerged vegetation). In small lakes ( < 20 ha) the totallitoral zone can often be fished in one day. In larger lakes the litoral zone should be thorougly subdivided into different relevant habitats to facilitate subsampling. Movements of fish between habitats is often neglible during day- time. 37 All species occurring in the litoral zone can be caught by electrofishing. Bottom dwelling species like cottids, ruffe, Acerina cernua, gudgeon, Gobio gobio, and crayfishes are generally vulnerable to electrofishing. This is also true for species like perch, Percafluviatilis, and for some cyprinids like e.g. Phoxinus phoxinus and rudd, Scardinius erythrophthalmus. Pike, Esox lucius, and large cyprinids like tench, Tinca tinca and crucian carp, Carassius carassius can be found litorally during all seasons, but especially larger individuals are most easily caught during spawning. Pelagic species like roach, Rutilus rutilus, and bream, Abramis brama, can be caught in very high num- bers during schooling in litoral areas in late autumn and winter. Litoral fishing in shallow water can be per- formed by walking along the shore line. This method is often the only one possible for central and inner parts of vegetation. In most cases, however, it is more efficient and less strenuous to fish by boat. It is then also possible to use heavier and more efficient equipment. When using gasoline engines it is in most cases, both for practical and security reasons, advisable to place the engine in a small, separate boat tied to a larger one. The latter is then used by the working team and for the equipment necessary for handling the fish. If the fish are to be marked, an operational team should consist of three persons; one managing the boat(s), one fishing and one measuring and marking the fish. In this way the fish can be measured and marked before the effects of the electronarcosis have disappeared, which faciliates handling. Most fish can then be replaced in approximately the same habitat where they were caught. In cases where the whole popu- lation is simultaneously vulnerable to electrofish- ing the mark-recapture method can be substituted by multiple mark-recapture. The validity of the assumption of random sampling on a day to day basis inherent in this method must however be proven.