fishery at Yellowstone (Cope, 1953). These 

 length data were analyzed without regard for 

 ages of the fish. If we can assume that year- 

 class composition remains fairly constant in 

 the spawning runs from stream to stream, we 

 see that som e streams have populations which 

 differ significantly from those of other streams. 

 The 1952 spawning runs into Grouse Creek 

 averaged 374.7 millimeters in mean total length, 

 which is significantly different from the spawn - 

 ers of the Yellowstone River, measuring 367. 7 

 millimeters, those of Pelican Creek, measur- 

 ing 355 9 and of Arnica Creek, measuring 353 .5 . 

 Chipmunk Creek characteristically supports the 

 largest spawners of all these streams, and in 

 the years when sampling was good the size 

 superiority was statistically significant. 



If racial differences in sizes cannot be 

 claimed for these spawning populations because 

 lengths of fish of like year classes were not com- 

 pared, then a claim may be made on the basis 

 of differences in age composition of the spawn- 

 ing runs. The fact that the size differences 

 appear from year to year, with the different 

 streams retaining their same relative ranks, 

 suggests that variation in year-class composi- 

 tion may be fairly small from year to year, and 

 that the differences noted above may be related 

 to racial differences in this drainage . Figure 3 

 illustrates the extent of differences in length 

 for three streams throughout the 1945 season. 

 The same general relationship between these 

 three streams was repeated in 1951, 1952, 

 1953, and 1954. 



The dispersion about the points which con- 

 trol the curves in figure 3 can be visualized 

 from the following coefficients of variation: 

 May 28-29; Chipmunk Creek, 4.09; Grouse 

 Creek, 5.23; Pelican Creek, 7.78. July 2-4; 

 Chipmunk Creek, 5.79; Grouse Creek, 6.47; 

 Pelican Creek, 8.61. July 11-14; Chipmunk 

 Creek, 5.37; Grouse Creek, 5.56; Pelican 

 Creek, 6.90. The sizes of fish at the beginning 

 of the spawning runs are significantly different, 

 from stream to stream, but the significance is 

 lost as the season progresses. The mean 

 lengths of fish in all streams becomes smaller 

 with the progress of the season . 



The influence of differential fishing pres- 

 sures on age and size composition of spawning 



runs must be considered here . If trout from 

 certain streams are subjected to unusually heavy 

 fishing pressures which take high tolls of the 

 larger and older fish, subsequent spawning runs 

 will probably contain smaller fish. This has 

 been suggested (Cope, 1953) for Yellowstone 

 Lake, where heavy fishing pressures in the north 

 end of the lake appear to have affected the sizes 

 of spawners in Pelican Creek and of fish taken in 

 the Fishing Bridge area fishery. Chipmunk and 

 Grouse Creeks do not seem to have been affected 

 to such an extent. Fish size still appears to 

 relate to racial differences, however, because 

 size differences were measured in the years be- 

 fore fishing pressure became so heavy. Also, 

 compare Chipmunk Creek fish with those of 

 Grouse Creek. The two streams enter the South 

 Arm not far from each other, the postspawners 

 occupy the same parts of the lake and are subject 

 to the same fishing pressure, the populations 

 are usually about the same size, and yet Chipmunk 

 Creek spawners are consistently larger than 

 those of Grouse Creek. 



Size differences have also been demon- 

 strated in fish taken from two parts of the lake 

 fishery, the West Thumb and the Fishing Bridge 

 areas. Here we are dealing not with individual 

 races, but with mixtures of races. Fish taken 

 in the Fishing Bridge area fishery (in the north- 

 east part of the lake) are consistently larger 

 than those caught in West Thumb, at the being- 

 ning of the season and thereafter . Tagging has 

 shown that fish from certain streams commonly 

 move to certain parts of the lake after spawning, 

 even though the preferred lake habitat may be 

 some distance from the spawning stream. Des- 

 pite the fact that straying into other portions of 

 the lake occurs to a minor extent, the lake 

 populations in each part of the lake are dominated 

 by fish from the same streams each year. There 

 are, then, certain races of fish associated with 

 particular lake areas, as well as with particular 

 spawning streams, and this holds true from year 

 to year . 



Size s of eggs- -"Tiere is not a great deal 

 of morphometric information available from cut- 

 throat in Yellowstone Lake, but there are records 

 pertaining to egg size and numbers that seem to 

 show that fish from different streams bear eggs 

 of different sizes and in different numbers. 

 Keeping in mind the general principle that the 



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