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r The sampling area If the target area is too large for a total survey, the next step is to define the sampling area, which in this context is electrofishing scctions within the target area. There are two main ways of defining these: of equal size (usually length) or of unequal size. In Fig. 8a, the target area is divided into 43 sampling areas (units) of equal length. In Fig. 8b, the target area is divided into 14 sampling areas (units) of a size which is allowed to vary according to natural variations in biotope. The following considerations may give some guidance in the choice of approach. N is the total number of units within the target area. Size of the sampling area To obtain sampling areas of suitable size, it is generally better to divide the target area into many small sections than few very large ones. The reason is that the methods of stock assessment proposed below may require an N value not too small. There may, however, be problems if the units are very small. Regardless whether blocking nets are used or not, the displacement offish from the area due to disturbance is likely to increase with a decreased section area, especially in large streams where edge effects will be more pronounced. Further, the large sample theory on which the Petersen method and the removal method are based may not apply if the population in a section is small. The minimum section size is therefore dependent both on the type of stream and the population density. Equal or unequal size of sampling areas Hankin (1984) recommended a design based on sampling areas with a size varying according to the biotope variations (Fig. 8b). This design is especially suitable if the biotope units (pools, rimes etc.) are of a practical size (see above). In a large stream they may be far too large. Sometimes, especially in small streams, sections of equal length are used, mainly because the com- . putation of total stock in this case only requires knowledge of total stream length. There are reasons to be flexible; in a stream with long uni- 27 form rimes, occasionally interrupted by pools of varying area, a suitable design may be to let the 'pool sections' size vary with the actual area of the pools and to choose 'rime sections' of approxi- mately equal length. The number of sampling areas We have now defined the sampling universe, which in this case is the target area divided into a number N of sampling areas. The next question is how many of these should be sampled and how to select them. The number of sampling sections required depends on (1) the precision level required (e.g. Class 1,2 or 3, (2) the variation of the fish popula- tion between the units, and (3) the size of the target area, expressed as N (total number of units). For a specific study, the precision level is chosen and N known, so to get an idea of the sample size needed we must have some additional information on the spatial variation of the popula- tion. For salmonids in streams it appears that the spatial variation, expressed as the population coefficient of variation Cp = Standard Deviation s/mean y, often is of similar magnitude despite large differences between populations and streams. In Table 3 we have compiled some data, ranging from large Northern streams (Alta) to small southern streams (Norum). It is therefore surprising that Cp seems to be relatively constant (mean about 0.8, approximate range 0.5-1.0). This can be utilized in the following way. First, choose an appropriate level of precision, expressed as C (for example 0.1 if Class 2 is chosen). Then, using either your own data or consulting Table 3, find a preliminary Cp value. A crude magnitude of the number n of sections that has to be sampled to reach the precision level chosen is then n = C~N/(C2N + C~) (18) If Class 2 is chosen (C = 0.1) and Cp = 0.8 is taken from Table 3, and if the stream in Fig. 8a is