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cited in Hubert et al 1985) . Bioassays showed a median tolerance 

 limit of 76.1°F for fry; for juvenile fish (20 cm fork 

 length — approximately the length of age I fish) , 100% survival 

 was seen at 72.5°F, but 0% survival at 76.1°F. Maximum daily 

 temperatures on the Big Hole River were frequently between 

 72-76°F in 1988, suggesting that temperature- caused mortality 

 could have occurred, particularly for age I fish. Studies 

 showing thermal tolerance limits for adult fish were not 

 available for this report, but Wojcik (1955, cited in Hubert et 

 al. 1985) found adult grayling to be stressed at 63.0°F, and to 

 activley avoid 68°F water. 



Reliability and applicability of microhabitat measurments 



One basic assumption made in this study with regard to 

 habitat- use measurements was that sites at which YOY Arctic 

 grayling were captured were in fact sites at which the fish had 

 chosen to reside, and that the fish had not been displaced by the 

 electrof ishing activity. An attempt was made to test this 

 assumption by taking advantage of the fact that the fish seemed 

 to orient to the base of riffles. If fish were displaced or 

 herded away from the electrof ishing crew, then it would be 

 expected that, when working upstream, the fish would be pushed 

 toward the bases of the riffles. Conversely, if the crew was 

 working downstream, the fish would be expected to be pushed away 

 from the riffles, and into the pools. A comparison of the 

 distance from the capture site to the base of riffles while 

 working upstream versus downstream was inconclusive, due to the 

 large variations in individual measurements and the small sample 

 size while working downstream (Table 9) . While the mean distance 

 from capture site to the base of upstream riffle was somewhat 

 greater when working downstream than when working upstream (35.1 

 ft and 23.2 ft, respectively), the reverse was true with the 

 median distance (6.9 ft and 9.5 ft, respectively). It appears 

 that if displacement or herding up- or downstream did occur, it 

 was probably only a short distance. Lateral displacement in the 

 stream — a displacement toward or away from the center of the 

 stream — could not be determined by this comparison, and remains 

 an unknown. 



Habitat measurments were taken from 1 day to one month after 

 fish had been captured. During this time interval, the discharge 

 on Swamp Creek and the Big Hole River dropped about 64% and 22%, 

 respectively. Water levels dropped as well. Based on 

 measurements taken at water- level reference stakes and the 

 discharge measurement stations, water levels on Swamp Creek and 

 the Big Hole River were estimated to have dropped 0.12 ft and 

 0.10 ft, respectively. The depth measurements used in the 

 microhabitat analysis therefore have an error up to 0.12 ft. The 

 only other habitat variable this error would have an effect on is 

 velocity, and the error could be expected to be greater for 



