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40 Planning of electroflShing surveys in streams - Specify the object of the investigation with special reference to the target area and whether estimation of population size (density) or popu- lation change is the main aim. The target area is the total water area you wish to make state- ments about and in which quantitative electro- fishing is possible. - If the target area is so large that a total sampling is impractical, you have to decide upon the number and size of sampling areas in the target area, e.g. by following the recommendations below. - Map the target area with respect to biotope type of relevance for the population density and group the sampling areas into two or possibly more density classes (strata). If the biotope is homogeneous, grouping (stratifications) may not be required. - Divide each stratum into sampling areas of suitable size. The size of these can vary accord- ing to biotope variation, e.g. in small streams (Fig. 8b), or be of equal length (Fig. 8a). In waters with homogeneous biotope type, sampling areas of equal length may be the most practical (Fig. 9). - In order to optimize the size of the sampling area, use the following guidelines: - The final computations are usually more accurate if the total number N of the sampling areas is the target area is not too small (in the magnitude of some tens). It may therefore be convenient to chose small sampling areas if the target area is small. - The risk of fish displacement from a sampling area increases with the perimeter/surface ratio. In large streams and the litoral of lakes it might therefore be better to chose larger areas than in small streams. - The final calculations are usually simpler and the estimates more precise if the size of the sampling area is chosen so that its population is 50 or more. - Prom the aim of the investigation, state the approximate precision requirement, e.g. ac- cording to the precision classes suggested above. - For a preliminary determination of the number (n) of sampling areas required to reach this precision, use the N value obtained (the total number of sampling areas within the target area) and consult Table 4 if population size or density is the main aim, and Table 5 if popula- tion change is the main aim. As an example, if N = 100 and Class 2 required, a sample size of 20-50 sampling areas may be tried if popula- tion size (density) is the target, and 11-15 in the case of estimating population change. If field data are available, eq. (18) may be used to calculate the sample size required. - If you have stratified the target area, distribute the n sampling areas so that large, high density strata will get a larger fraction of these than small, low density strata. - From each stratum, select the sample required by simple random sampling (SRS) or propor- tional probability sampling (PPS). Performance of electro fishing in streams - The removal method is usually sufficient. Mark-recapture methods will in some cir- cumstances yield more accurate estimates. - If the removal method is used, at least 3 removals in each sampling area is recom- mended. If the estimated catchability is lower than about 0.5, make more than 3 removals. - For each removal, each species and size class is noted separately. - Make a break between the removals, at least 30 min. - Work in an up-stream direction. - Do not disturb the sampling area before fishing. - Generally, the use of block nets is not neces- sary. - If estimation of population change in time is the main aim, the fishing should be carried out during similar conditions (type of gear, flow, turbidity, temperature, time of the day etc.). - A void fishing during periods of high or rapidly changing flow, especially in large streams. - A void fishing when fish are inactive (e.g. in low temperature) or schooling. r ~ I . I ! I r I