tion (table 9) and were reaching catchable size 

 by June of their second year. The mean total 

 calculated lengths, in inches, at the end of each 

 year's growth from 1953 to 1957 were: I, 4.7; 



II, 9.8; III, 13.8; IV, 16.0; V, 18.7; and VI, 

 21.4. The mean annual growth increments at- 

 tained by fish of different ages from 1948 to 

 1956 did not show any consistent changes by 

 calendar year of growth (table 10). 



Rainbow trout young-of -the -year were 

 growing more rapidly than the brown trout while 

 the older age classes were growing more slowly 

 (table 9). The growth rates did not fluctuate 

 greatly among different calendar years (table 10). 



Madison River 



Both the rainbow and brown trout in the 

 Madison River were growing at about the same 

 rate as in the Firehole (table 11). No definite 

 differences in growth rates were found among 

 various calendar years (table 12). 



A sample of 36 mountain whitefish, col- 

 lected during 1956 and 1957, showed that this 

 species was reaching the following mean lengths, 

 in inches, at each year of life: I, 5.1; II, 8.9; 



III, 12.0; IV, 13.7; V, 15.3; and VI, 16.9 

 (table 13). Sigler (1951) summarized the studies 

 on growth rates of mountain whitefish for this 

 region; the Madison River fish were growing 

 more rapidly than any of the stream populations 

 mentioned. 



AGE COMPOSITION OF CATCH 



The catch of brown trout in the Madison 

 River from 1953 to 1957 was composed princi- 

 pally of fish in their third and fourth years of 

 life in the stream (table 14). The catch of brown 

 trout in the Firehole generally included more 

 fish in their second year (Age Class I) than did 

 the Madison. Only 7.7 percent of the catch in 

 the Madison and 6.2 percent in the Firehole 

 were older than Age Class III. 



More rainbow trout were caught in their 

 second and third year (Age Class I and II) than 

 were brown trout in both the Madison and Fire- 

 hole Rivers. The oldest rainbows in the catch 

 were Age Class IV. The catch of whitefish in 

 the Madison in 1956 and 1957 was distributed 



among age classes as follows: II, 8; III, 11; 

 rV, 9; V, 5; and VI, 3. Whitefish did not reach 

 the creel until their third year and the catch was 

 made up of more fish of the older age classes 

 than was the case with brown or rainbow trout. 



RATE OF EXPLOITATION OF WILD FISH 



The rate of exploitation of brown trout by 

 angling has always been inherently low in streams 

 where detailed studies have been made (Schuck, 

 1948; Allen, 1951; Cooper, 1952). In the Madi- 

 son and Firehole Rivers, the catch of brown 

 trout has been influenced by changing fishing 

 pressure, but there has been no evidence of over- 

 fishing on the population. There have been some 

 complaints that the Madison does not now yield 

 as many large browns as it formerly did. We 

 cannot compare fishing pressure prior to 1953 

 with our present data, but it has undoubtedly in- 

 creased, since the number of visitors to Yellow- 

 stone National Park has increased from 581, 761 

 in 1941, the largest pre-war year, to 1,368,515 

 in 1955. This increased fishing pressure, pos- 

 sibly over 100 percent, may have reduced the 

 number of large trout. Another possible cause 

 for the small number of large browns is the fact 

 that the present fishing season (May 30 -Septem- 

 ber 30) closes too soon. Undoubtedly, many of 

 those large browns were caught in the late fall 

 on a spawning migration. Brown and Kamp (1942) 

 mentioned October 9, 1936, as being one month 

 prior to the spawning season in the Madison 

 River; thus, it is possible that if these large 

 brown trout still migrate up the Madison, they 

 may do so after the present fishing season is 

 closed. It is also possible that the "fly fishing 

 only" regulation that began in 1950 restricted 

 the catch of large brown trout. 



From our population estimates of the 

 Firehole in September, 1957, there was a mini- 

 mum of 7, 620 catchable -size brown trout left in 

 the population after the 1957 fishing season was 

 essentially completed. Since only 3,467 were 

 caught, and because most of the surviving 

 smaller trout grew large enough to enter the 

 fishery in 1958, we believe that more brown 

 trout could be caught without depleting the popu- 

 lation. Our calculations show a maximum rate 

 of 35 percent exploitation and it was probably 

 closer to 15 percent since our population figures 

 from the lower Firehole and the entire Madison 



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