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Although data are not conclusive, it is suggested that repeated exposure to electroshock can change the probability of capture of trout and affect population estimates. Evidence elsewhere (Cross and Stott, 1975) indicates the effect in population estimates is caused by a learned avoidance behavior of shocked but not captured trout. Whether this effect is caused by decreas- ed catchability of shocked but not captured fish or an increased catchabil- ity of shocked and captured fish, the result is population estimates that are biased low. The 7 to 10 day resting period used here is proposed as an ade- quate amount of time for a fish after being shocked to return to its normal preshock capture probability. More work needs to be done to more accurately determine the minimum resting period length necessary for equal capture prob- ability of marked and unmarked fish. Obtaining unbiased population estimates is the ultimate goal, however, in those studies where mark and recapture runs were made with an inadequate resting time, population estimates, although biased, will still be comparable if no change in methods has occurred in population size and age structure. If estimates are biased and methods have changed differences in population estimates may be more related to changes in methods than actual changes in population size and great care should be taken when comparing estimates. in addition to the mark/recapture estimates, bias caused by a change in catchability after exposure to electroshock in any of the estimators assuming constant catchability (e.g., DeLury, Seber-LeCren, Zippin) should be investi- gated. SUMMARY Length of time between electrofishing mark and recapture runs can have considerable effect on resulting population estimates. Population estimates made with 0 to 3 days between mark and recapture runs (within week estimates) averaged only 75.1 and 54.0% of population estimates made with 7 to 10 days between mark and recapture (between week estimates). Although several of the underlying assumptions may have been violated, violation of the equal catchability assumption best explained the difference between the within week and between week estimates. ACKNOWLEDGMENTS I thank Mark Fowden, Jim Sherman, Game and Fish Department personnel who ing of trout collected in this study. would have been possible. Jim Shelley, and all the other Wyoming aided in the electrofishing and mark- Without their help none of this work LITERATURE CITED Chapman, D. G. 1951. Some properties of the hypergeometric distribution with applications to zoological sample censuses, University of Cali- fornia Publ. Stat. 1(7). 131-160 pp. Cormack, R. M. 1979. Models for capture-recapture, in R. M. Cormack, G. P. Patil, Co-operative Publishing House, Fairland, MD. 217-255 pp. 27