To overcome the problems usually met as a consequence of small numbers of fish collected for trout stock size evaluation in small rivers, a selectivity function was developed and tested in two small rivers in western Switzerland. The function takes the form In E = bL-k whereEis the fishing efficiency, L is the fish length in cm, k is a constant and b is the slope, estimated by regression analysis.A good fit was obtained f o r k = 1 (R. Flon de Carrouge) and k = 2 (R. Cerjaule). Individual selectivity functions have to be computed for each survey, because they depend on the operators of the fishing gear and on the physical and biological conditions in the rivers.The results are discussed in the light of the Petersen mark-recapture method and the De Lury removal method for stock evaluation. The mark-recapture method gives better results when time intervals between successive fish removals are short.

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