West Thumb area), while those pertaining to standard and total length came 

 from data taken by Dr„ Wright and his associates at urouse>, Chipmunk, and 

 Pelican Creeks. The differences in factor values between these pcpalations 

 of fish appear to be so minute as to be of little practical importance „ The 

 use of values for the combined sexes when applied to fish of either sex will 

 usually give converted measurements that ai-e within one millimeter of the 

 actual values. 



SPAWNING TROUr 



Spawners from five streams tributary to Yellowstone Lake have been 

 measured during the past several seasons. The Pelican Creek measurements, 

 extending over seven seasons, is the longest series for which data are 

 available. Unfortunately, data from some years are not suitable for compari- 

 son because of unbalanced sampling distribution within individual seasons. 

 In some cases too many small fish were measured tc yield a mean total length 

 measurement truly representative of the population. 



The criteria for the acceptance of a set of data were based on sampling 

 distribution and total number measured. The requirement for acceptance was 

 that the sample must have been taken at two or three day intejrvals through- 

 out the entire season and that the total number measured be at least in t he 

 neighborhood of 350 fish. Table 3 shows that only four season's measurements 

 on Pelican Creek were deemed satisfactory for comparisono Chipmunk Creek, 

 which enters the South Arm of Yellowstone Lake, provided data for six seasons, 

 and three of the six are satisfactory. Grouse Creek provided three good 

 seasons out of fivej Arnica Creek yielded three excellent seasons of data| 

 Yellowstone River, at the Cascades, five miles downstream from Fishing Bridge 

 gave one good season out of two. Cascade fish, unlike those of other groups 

 treated here, were not caught in permanent traps but were taken in dip nets 

 as they ascended the steep rapids during the spawning run. 



Table 3 summarizes statistical comparisons between mean total lengths 

 of spawners from year to year. These comparisons indicate the direction and 

 extent of trends in sizes of spawners and in some cases show that trends 

 have been moderately continuous. Pelican Creek shows three changes that 

 are statistically significant— 19U5 fish averaged 9 millimeters longer than 

 the 1952 group, 195o fish were 8.U millimeters longer than the 19^2 fish, 

 and 195l fish were 7.2 millimeters longer than the 1952 fish. In Arnica 

 Creek, a similar decrease in fish length was seen from 1950 to 1952. The 

 1950 fish averaged 367.6 millimeters (lii.5 inches), 1951 spawners averaged 

 362.8 millimeters (II4.3 inches) and the 1952 average was 353.5 millimeters 

 (13.9 inches) . 



Chipmunk Creek fish, on the other hand^ showed increases in mean total 

 length from year to year. Lengths in 19U5 averaged 361^.9 millimeters (lli.ii 

 inches) 3 in 1950, 378.6 millimeters (lU.9 inches), and in 1951, 385.7 milli- 



