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I" j to be known. If so, use eqs. (21-22), with y replaced by d = x - y, to estimate absolute change and eqs. (27-28) to estimate relative changes. To summarize, it may be possible to estimate both population density and population changes with reasonable precision, even in large streams, if the sample size is large enough, but the precision of the population density estimate may be hard to obtain if M is difficult to measure. Population density estimates from different streams should, however, be compared with care. In small streams there may be a possibility to chose between all the three methods proposed above. As stated above, a design with varying section size is especially valuable if the 'natural' biotope units are of a practicable size. Hankin (1984) made the following general recommenda- tions: (1) If N is small (say 20 or less), neither the SRS ratio estimation or the PPS with replacement can be recommended. This leaves us to either the SRS without use of auxiliary variable or to the PPS without replacement. (2) IfN is large (say 50 or more), the choice is less clearcut. If N is very large (say 100 or more), the SRS ratio estimation may be recom- mended ifn is not too small (say 12 or more), and the PPS with replacement if n is smaller. As the sample size required usually is more than 12 and N often moderate or large, the SRS ratio method appears to be useful, especially as the result always can be directly compared with SRS without auxiliary variable. As proposed above concerning the sampling design of large streams, a 'mixed' strategy may be useful even in small streams, using the SRS ratio estimation for den- sity or total stock and SRS without auxiliary variable for population changes. Electrofishing for other purposes ., When applied properly, longterm effects of elec- trofishing on fish are small. The method is there- fore suitable for catching fish to be kept alive for 35 further experimentation. Non pulsed direct cur- rent should then be used. Electrofishing may be used in. situations other than quantitative population estimates. In order to ensure inclusion of a given fish species, it is however, necessary to have certain degree of knowledge of that species' habitat preferences and selectivity when carrying out electrofishing. When documenting the occurrence of different fish species in a watercourse, it is essential that all types of habitat (water velocities, depths, sub- strates, etc.), are covered, while at the same time making sure that those species are catchable with an electrofishing apparatus. As Thymallus thymal- Ius often is difficult to catch by electrofishing, it may be necessary to include other equipment such as sport fishing gear or nets. In such cases elec- trofishing is carried out when water levels are lowest. In most studies it is necessary that specimens are preserved for later examination. Experience from Norway (as well as other places) has shown that growth of pres molt salmon varies considera- bly in different parts of the same river. It is therefore essential that material for growth analy- sis is collected from more than one location in the river. The distance between collection stations should never exceed ten kilometers. Repeated electrofishing at short intervals should be avoided because frequent electro shocking (< 3 months) are likely to underestimate growth rates (Gatz etal.,1986). Dcnse populations of small, stationary trout or other fish species are often found in streams and small rivers. Fish required for release in other rivers may be obtained by using electrofishing equipment. Experiments conducted in Norway have demonstrated that it is possible to catch 100- 200 trout per hour of a size between 5 and 20 centimeters in streams with dense fish popu- lations. The advantage of this kind of fishing is that large fish, capable of surviving, are caught at the same time that an overcrowded trout popula- tion is reduced. Fishing should be carried out at low temperatures, in order to ensure that fish are transported without high mortality. In order to avoid spreading of undesirable fish species, dis-