may not be closely related to the particle-size composition of 

 the substrate. Second, the actual numbers of YOY grayling using 

 the site and the numbers we sampled by electrof ishing may be 

 completely different (i.e., the electrof ishing results do not 

 accurately reflect the numbers present) . Third, the sites cored 

 for substrate analysis may not be completely representative of 

 the actual spawning site. Further substrate analysis and an 

 improved and more accurate method of estimating YOY grayling 

 numbers in a given area are needed to more accurately assess the 

 relationships between YOY grayling and substrate composition. 



Table 10. Coefficients of determination (R 2 ) for substrate 

 composition variables and YOY numbers in the Big Hole River, 

 Montana, June - August, 199 0. 



Variable Mean (SD) 



% 



Sand 14.04 (4.203) 0.0172 



Silt 0.036 (0.1603) 0.0201 



Gravel 81.54 (3.162) 0.2803 



Cobble 4.053 (3.799) 0.0910 



Use of Irrigation Ditches 



Three irrigation ditches were electrof ished: two on Cal 

 Erb's Ranch and one near the Wisdom Airport (Figure 1). The small 

 irrigation ditch (Table 1) near the airport was narrow and 

 weed-choked when sampled, and use by salmonids was minimal. No 



24 



