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 Substrate and Cover 



All substrate types, except for boulders, were found at the 

 capture sites, although on the average no substrate type 

 predominated (Table 6) . All substrate types occupied between 15 

 and 26% of the surface area of the streambottora, and silt was 

 estimated to be made up of 21% organic material (primarily dead 

 algae) and 79% inorganic material. In many cases, large gravel 

 and cobble were plentiful, but were covered by a thin layer of 

 silt or sand, and were therefore not counted. The substrate at ^ 

 capture sites averaged 53% embedded, with a range of 39-67%. A 

 comparison of substrate types and levels of embeddedness in both 

 the utilization and availability databases suggests that the fish 

 exhibited a slight preference for larger and less embedded 

 ("cleaner") substrate (Table 6). This is consistent with the 

 preference shown for fast water and riffle/run habitats (Tables 5 

 and 6) , where larger and less embedded substrate predominates. 



Cover, on the average, occupied 24% of the surface area of 

 the streambottom within the 3.28 ft^ cell surrounding the capture 

 sites (Table 7) . Aquatic vegetation was by far the most abundant 

 type of cover found at the capture sites (84.6% of the 

 obseirvations) , and was primarily composed of Ranunculus aquatilis 

 and Elodea canadensis . The average distance to cover from 

 capture sites was 0.92 ft, and aquatic vegetation was the closest 

 cover type to 86.9% of the capture sites (Table 7). Information 

 from the availability database shows that the percentage of cover 

 available is roughly similar to that which was utilized, 

 suggesting that there may be no preference for cover within the 

 3.28 ft capture site. 



Temperature 



Measurements of water temperature were taken throughout the 

 duration of the study. Measurements were taken at irregular 

 intervals, and typically only from mid-morning to early 

 afternoon, so it was usually not possible to determine maximum 

 daily temperatures. Even so, temperatures over 70°F were 

 frequently recorded, and the maximum temperature was recorded on 

 July 21 when the Big Hole River reached 79.5°F (Table 8). Water 

 temperatures were also recorded at the Wisdom Bridge during 1988 

 by the United States Geological Survey (Shields et al. 1989) . 

 The period of record was May 1-July 24, August 12-27 and 

 September 21-30. The maximum temperature during this period was 

 76.1°F on July 22, and average daily maximum temperatures were 

 54.3°, 66.6°, 72.4°, 67.2°, and 56.0 , for the periods of record in 

 May, June, July, August and September, respectively. 



The fry stage of Arctic grayling is reported to be the least 

 sensitive to high temperatures (LaPerriere and Carlson 1973, 



