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- Only trained personnel should perform quanti- tative electrofishing. Performance of electro fishing in lakes .. - Use electrofishing in shallow and vegetated areas at transparencies higher than 1 m. - Fish by boat and place gasoline engines in a separate boat. - Pursue to handle the fish before it has recovered from the electronarocosis. - For population estimates use mark-recapture methods and recapture the fish by methods other than electrofishing. 4 Calculation of population estimates - In the case of the removal method, use the appropriate maximum likelihood estimator. - Separate estimates for each species and size class are made. - If the population in a sampling area is smaller than about 50, or if fewer than 3 removals have been carried out, use the 'pooling' method (cfr. examples 3 and 4). - The smallest size class may be subjected to large underestimation. For salmonids, special attention is called for when a large population fraction is smaller than about 50 mm. - In large rivers, where the area in which quanti- tative electrofishing only is possible in small relative to the total stream area, estimation of total population size is not possible by this method. Evaluation of results , - Generally, estimated population size or density is lower than true population size (density). - A low estimated catchability is a warning sign, as the underestimation in this case probably is large. Observe, however, that a large estimated catchability is no guarantee that the under- estimation is low. 41 r i r f f - The degree of underestimation may vary with fish species, fish size, fishing gear and fishing conditions, e.g. biotope, temperature, water flow, turbidity. - Comparison of population densities can there- fore only be relevant in specific situation (same or similar fish species, biotope, gear etc.). - In large rivers, population change in selected areas over time should be estimated rather than assessment of total population. Recommendations concerning safety - Electrofishing equipment should be approved according to national standards. - Only DC systems should be used. - The anode should be provided with a water- proof safety control (dead-man switch) operat- ing through a low voltage secondary circuit. - Only trained personel should perform electro- fishing. - This training should be provided by a course, approved by the national fisheries authority. - Permission for electrofishing should be perso- nal on specific conditions. - Equipment should be checked at least annually. - A void fishing in rainy weather. - For both safety and efficiency, a fishing team of two persons is the minimum requirement. - Disinfection of equipment should be made adequate. l ~ f r- l Acknowledgements The study group has held tree meetings, and travel and living expenses were provided by NOS-N (Joint Committee of the Nordic Natural Science Research Councils) as well as the respective institutions of individual members. The participa- tion of T. Bohlin was made possible by grants from the Swedish Environmental Protection Board, project No. 53130126-6 and 5313013-4. R. Lynn McComas is thanked for linguistic and editorial advice and I. Bremseth is thanked for typewriting of several versions of the manuscript. , i" f I I"