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. J the removal method, the following should be con- sidered: (1) A large estimated catchability is never a proof that the real catchability is large and that the amount of underestimation is small. (2) A low estimated catch ability indicates that the underestimation may be large and the preCISIon poor. (3) In order to evaluate the magnitude of the underestimation, the method should be checked against a population of known size under realistic conditions. 1 I Implicite in the trapping theory on which the removal estimator is based is the assumption that the catchability of one individual is independent on the catch ability of other individuals. This assumption may be seriously violated for schooling species for which individual schools rather than individual fishes may follow the theory, resulting in a breakdown of the estimator. As there are few studies in which estimates have been checked against known populations, the accuracy of the removal method for various species is largely unknown. From the behaviour and habitat, however, it might be possible to make some inferences of the catchability distribution and hence state some general guidelines: 1. For schooling species, the catches may not be binomially distributed as assumed, and the estimates may be totally misleading. 2. For species which are stunned rather than attracted by the voltage gradient, a population fraction may not be detected. This would lead to increased underestimation. Some cyprinids, e.g. the minnow, may belong to both categories . (1) and (2). 3. Species living in dense vegetation or in the bottom substrate will be underestimated to a larger extent than species that live in open water, especially if it is combined with (2). 4. Some species living in open water will tend to avoid the voltage gradient, e.g. the grayling. The catchability is therefore low and estimates may be of doubtful value. 5. For territorial salmonids in shallow, clearwater streams with little vegetation, the catch ability . 25 seems to approach the assumptions on which the removal method is based to a larger extent than the foregoing categories. Even so, how- ever, Bohlin and Sundstr5m (1977) demon- strated unequal catchability in populations of juvenile trout and also the effect of this - a general underestimation of about 15 % for yearling trout. Generally, thus, the removal method will probably yield an underestimation of population size. The magnitude of the bias is affected both by species specific behaviour and by habitat conditions affecting the possibility oflocating or catching fish in the voltage gradient. The main goal of electrofishing studies is often to monitor population changes rather than to obtain absolute figures of fish density. If the bias is proportional to population density - which does not seem unlikely - the relative change can be obtained without serious bias. In absolute terms, however, a certain bias will remain. Comparisons between the removal method and the Petersen estimator For p values of ordinary magnitudes the 3-catch method and the Petersen estimator have similar precision. On the assumption that fishing/mark- ing does not affect the fish, both methods are about equally sensitive to deviations from the assumption of equal catchability (Bohlin and Sundstr5m, 1977). As electric shocking may lower the subsequent catchability, this will lead to a further underestimation of the removal method, but a reduced underestimation if the Petersen method is used. The Petersen method will there- fore yield larger and probably more accurate esti- mates that the removal method (Cross and Stott, 1975; Heggberget and Hesthagen, 1979; Petersen and Cederholm, 1984) and might be the best choice if accuracy is important. Normally, the removal method is still valid, provided a time lapse, possibly in the magnitude of at least half an hour, between the removals.