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\. , I .. The population estimate (eq. (I)) is negatively biased even if all assumptions are met; the bias, however, is negligible if mc/y is larger than about 4 (Youngs and Robson, 1978). In practice, there- fore, the bias introduced by deviations from basic assumptions are probably of greater importance and is discussed below. As the catch ability of electrofishing in streams is often in the magnitude of 0.50, the confidence interval (3) is approximately valid if c is larger than about 30 (or the population size larger than about 60). If the catch ability is lower, larger popu- lations are required for valid confidence limits (Cochran, 1963; p. 57). Robson and Regier (1964) provided graphs for determining sample size when using the Petersen estimator. These are highly useful when planning the study of large, mobile populations, e.g. fish populations in lakes. When applying the Petersen method to closed sections in streams or in the litoral zone of lakes using electrofishing both for capture and recapture, the situation is somewhat simpler. Assuming that the whole area is suitable for electrofishing and that both fishings are equally efficient, the coefficient of variation C = SE(y)/y is, from (2), ~ C = (I - p)/(pj9) (5) From this expression we have calculated the expected C values for some various population sizes y and catchabilities p. These are given in Table 1 and may be compared with the precision classes suggested above. If p is 0.6 or larger the precision obtained usually corresponds to Class 1. Note, however, the combined effect of small populations and low catchability. j Table J. Coefficient of variation C % for the Petersen estima- ~ tor for various values of population size y and catchability p. The assumption is that both captures have the same p. Relative precision of the Petersen estimator. y = 50 y = 100 y = 200 y=400 P = 0.4 21 15 11 7.5 P = 0.6 9.4 6.7 4.7 3.3 P = 0.8 3.5 2.5 1.8 I.3 21 Of the assumptions on which the Peterson estimator is based (see Seber, 1973; p. 59), the following are probably the most critical ones in connection with electrofishing: 1. Equal catchability of all individuals. Electro- fishing is selective: large individuals have on the average and for many populations a larger catchability than small ones, and individuals in certain biotopes (territories) may be harder to catch than an average individual. The result is a general underestimation of the population size. The effect of size selection can be ruled out if separate estimation for each size class is carried out (which, however, may result in small populations and hence reduced preci- sion). Unequal catch ability depending on biotope heterogenity within the area will remam. 2. Catch ability is the same for marked and non- marked members. Though electrofishing is one of the most harmless sampling methods ifused properly, the experience is that an electric shock will reduce the catch ability for some time. This time may vary widely depending on species, size, temperature, electric field strength, skill of the fisherman etc. Peterson and Cederholm (1984), working with juvenile coho salmon, found that catchability returned to acceptable levels after a 1 hour time lapse, but that it did not quite equalize even after 24 hours. If the catchability is reduced as a result of handling, r will be below expectation and the population size y therefore overestimated. In practice, deviations from assumptions (1) and (2) will therefore tend to offset each other. 3. A random sample is obtained in the second fishing. For stationary fish spread over an area possible to cover with electrofishing, this assumption is probably well met. For schooling species, where there is a possibility of missing the school in one of the fishings, the estimates may become quite misleading. The Petersen method should therefore be applied with care to schooling species in large bodies of water. A practical limitation of mark-recapture methods is that the fish individuals often are