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insufficient current. This argument is supported by the 

 preference that has apparently been shown for flowing water — 85 

 of 88 capture sites were in areas where there was a measurable 

 current. 



Fish may have been selecting habitat on the basis of an 

 interaction between depth and velocity. Correlation analysis 

 between the two variables in the utilization and availability 

 databases was used to determine if such a relationship existed. 

 Both databases were modified for the analysis by using the range 

 of depth and velocity measurements found to be utilized by 95% of 

 the fish (the highest 2.5% and the lowest 2.5% depth and velocity 

 measurements were eliminated) ; in this way, measurements that 

 were atypical of grayling fry usage were eliminated. The ranges 

 for depth and velocity used in the analyses were therefore 0.30 

 to 1.70 ft and 0.02 to 1.33 ft/s, respectively. In the 

 utilization database, a significant negative correlation existed 

 between the two variables (r = -0.53, P = <0.01, d.f. = 80), with 

 velocity increasing as depth decreased. A similar negative 

 correlation existed between depth and velocity in the 

 availability database (r = -0.24, P = <0.01, d.f. = 373). 

 Because both databases show similar negative correlations, it 

 seems likely that the phenomenon is due to physical properties of 

 the stream (i.e., the way depth and velocity are affected by 

 gradient, discharge and channel shape) , and not due to active 

 selection by fish. This is consistent with the observation that 

 low-flows (under 4 cfs) on both streams resulted in conditions 

 where high velocity water existed only in shallow riffles or 

 rapids. This does not rule out the possibility that YOY grayling 

 select habitat on the basis of an interraction of the two 

 variables, but it suggests that a proper test for such a 

 relationship would have to be conducted in these streams with 

 greater discharge or in a different stream where similar 

 correlations do not exist in both databases. 



Isopleth contour maps of water depth and mean velocity were 

 made for one intensive-use area on Swamp Creek and three such 

 areas on the Big Hole River, (Figures 9 through 16; the locations 

 of the intensive- use areas are shown in Figure 23). Proceeding 

 on the assumption that water depth and velocity are chosen 

 independently, the amount of "usable" area in the intensive-use 

 areas was determined by delimiting those portions of the stream 

 that encompassed the water depths and mean velocities found to be 

 utilized by 95% of the YOY grayling (0.30 to 1.70 ft for depth 

 and 0.02 to 1.33 ft/s for velocity). The area determined in this 

 way to be usable was primarily described as runs and portions of 

 riffles (Figures 17 through 20) . Usable habitat was also 

 predominantly mid-stream, with very few cases where appropriate 

 depth-velocity combinations were against banks. 



