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<br />TABLE 2. Comparison of mean within week and mean between week population <br />estimates for several streams in Wyoming. <br /> <br />... <br /> <br />Mean <br />within <br />Pop. Est. <br /> <br />Mean <br />between <br />Pop. Est. <br /> <br />Location <br /> <br />Date <br /> <br />Species <br /> <br />Rainbow <br />Rainbow <br /> <br />6409 <br />5181 <br /> <br />786 <br /> <br />6947 <br />5039 <br /> <br />1614 <br /> <br />Big Horn River <br />> 6" <br /> <br />1981 <br />1982 <br /> <br />1981 <br /> <br />Rainbow <br /> <br />Green River <br />> 6" <br /> <br />North Platte River <br />at Bessemer Bend <br />6 - 10" <br />10 - 14" <br />> 14" <br /> <br />1982 <br /> <br />Rainbow <br /> <br />3136 <br />4878 <br />948 <br /> <br />462 <br />665 <br /> <br />4496 <br />6347 <br />1112 <br /> <br />829 <br />958 <br />Mean for Rainbow <br /> <br />Wind River <br /> <br />1981 <br />1982 <br /> <br />Rainbow <br />Rainbow <br /> <br />Salt River <br />> 6" <br /> <br />Brown <br /> <br />2132 <br /> <br />3504 <br /> <br />1981 <br /> <br />North Platte River 1980 <br />at Pick Bridge <br />> 6" <br /> <br />Brown <br /> <br />1685 <br /> <br />3568 <br /> <br />Mean for Brown <br /> <br />54.0 <br /> <br />DISCUSSION <br /> <br />Percentage <br />difference <br />between <br />estimates <br /> <br />92.3 <br />102.8 <br /> <br />48.7 <br /> <br />69.8 <br />76.9 <br />85.3 <br /> <br />55.7 <br />69.4 <br />- <br />75.1 <br />60.8 <br /> <br />47.2 <br /> <br />Comparison of the within and between population estimates clearly indi- <br />cated differences between the 2 sets of population estimates. Obviously one <br />or more of the underlying assumptions was violated so that the fish did not be- <br />have like beans in an urn. Mortality of marked trout during the resting <br />period may be partially responsible for the difference in within and between <br />week estimates, however, any trout that is unable to swim away upstream after <br />marking is not released back to the stream. Although occasionally marked <br />trout are found dead the following day, mortality is not thought to influence <br />population estimates greatly. <br />Movement data collected on the Big Horn River from rainbow trout in May <br />and November during population sampling showed that, of the rainbow trout cap- <br />tured twice during 6 days of electrofishing, 81 and 85% (May and November, <br />respectively) had moved one half mile or less between capture and recapture <br />location. Electrofishing a 3 mile section immediately downstream from the <br />Big Horn River study section indicated little movement out of the study sec- <br />tion as, of 200 r~inbow trout collected only 2 had been tagged in the upstream <br />study section. Movement of marked trout out of the study section did not <br />appear responsible for the difference in within and between week population <br />estimates. <br /> <br />25 <br />