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an urn to use the hypergrometric model to estimate population size. And if they do not, can experimental design constraints be applied to "force" the animals to behave similar to beans so the hypergeometric model is appro- priate. The intent of this study was to determine whether the length of time between mark and recapture runs affected population estimates, and, if it did, to determine which model assumption(s) were violated. METHODS On streams with flows greater than 500 ft.3/sec., two positive elec- trode, boom mounted electrofishing boats were used to collect trout, with each boat collecting trout from either bank to the center of the stream. Two sets of electrofishing runs were made on 3 or 4 consecutive day periods separated by a rest period of 7 to 10 days. On streams with flows less than 500 ft.3/sec., one electrofishing boat with a throwable electrode attached to an extension cord was used to collect trout. Streams were narrow enough such that by keeping the boat approximately in the center of the stream, all available trout habitat could be shocked via the throwable positive electrode. Two sets of electro- fishing runs were made on 2 or 3 consecutive days separated by a 7 to 10 day resting period. All trout captured were marked so they could be identified as having been caught on a specific day. This was done on some streams by notchinq a different fin on each day of electrofishing. On other streams, all trout captured greater than 6" were fitted with uniquely numbered monel metal jaw tags. By marking in this manner, the recapture history of each trout could be determined. Population estimates were made using the Chapman modification of the Petersen estimator (1) (Chapman, 1951). N = (M + 1) (C + 1) - 1 (1) R + 1 N Estimated population size M Number captured in 1st sample C Number captured in 2nd sample R Number in C that were also captured in M Using the various marks, population estimates were made for all possible combinations of pairs of days. The population estimates for each stream section were divided into two groups. The first group, within week estimates, was composed of population estimates made where both the mark and recapture runs were made within the same week. If four consecutive days were electrofished, the maximum amount of time between mark and recapture efforts would be 2 days, i.e., the first (lay would b(' the' mark nm ilnrl loll<' fOllrlll <lilY w011ld hI' 111,. ("(',q""r" ('1111:; ('I',-1Idf1 I). 'I'll" l~'\""lld '1""11', 1'ld"I/'" II 'rJf~' I- f'~~1 illl.ll"ll, \iJ;I' ""IIlIJlI,:,.d.d 1"'1'1,1.1 I i,," ""II'"nl"" ~"I"'I'c 111" 111<1,1; <11101 Itct"'I" lilt' IlJll~; W.'1." :';l'I),1tdl(~d l,y 1.11(' "/ I,) 10 day restinq ueriod ('l'ab1e 1). 'l'he Ilonparametric sign-rank test was used 23