YCT Multi-state Assessment February 10, 2003 



occupied within the historical geographical boundary (Figure 1). Of our total, Wyoming 

 contained the most historical habitat (6,886 miles; 40%), Idaho was believed to have 6,267 miles 

 (36%), Montana contained just over 4,040 miles (23%) , Nevada and Utah had 97 and 103 miles, 

 respectively (0.5% each). While the total number of historically occupied miles between our 

 status update and the status review completed in the mid-1 990's were very similar there was a 

 marked difference in the mileages associated with Wyoming, Idaho and Montana. TTie 

 difference between our estimate and that developed in the previous assessment (May 1996) was 

 likely due to two significant factois. First, our assessment used a set 1 : 100,000 hydrography 

 base layer that was attributed and queried within an ARCView platform and a standardized 

 database. The status assessment of the mid-1990's relied on area maps having differing scales 

 and stream designations. Notations were made on each map and stream distances were 

 calculation using a mileage function on a planimeter. Secondly, the mileage estimate for 

 Wyoming in the earlier assessment was suspect due to the nature of Wyoming's stream database 

 at the time (May 1996). 



Varley and Gresswell (1988) estimated the historically occupied habitat at 24,000 km (15,000 

 miles). Since Varley and Gresswell (1988) did not provide a rationale of how they derived the 

 number of occupied stream miles, it was difficult to compare with our estimate. Their estimate 

 of about 15,000 miles did not include mileages covered by large lakes, particularly Yellowstone 

 Lake, and thus their total is reasonably close to the historical occupancy estimates in our status 

 update and the status review completed by May ( 1 996). 



Current Distribution 



Based on our assessment, YCT currently occupy about 7,530 miles of habitat (43%) of 

 historically occupied habitat) within the historical range (Figure 2). YCT currently occupy 3,861 

 miles (51%)) in Wyoming, 2,174 miles (29%) in Idaho, 1,417 miles (19%) in Montana and 

 Nevada and Utah have 44 and 42 miles, respectively (0.5% each). In contrast, the assessment 

 reported in May 1996 reported that approximately 6,817 miles of currently occupied habitat. Of 

 that total 1,622 (including 3 1 miles in the Goose Creek and Raft River drainages) were in Idaho, 

 4,624 were in Wyoming and 625 were in Montana. The reason for the significantly large 

 difference in currently occupied habitat within Montana was because only those YCT that had 

 been genetically tested were counted in the earlier assessment (May 1996). In addition, 

 differences could be attributed to mapping and hydrography scales, differences in the individuals 

 participating the assessments and fact that more YCT had been documented in the six year 

 period between assessments. Varley and Gresswell ( 1 988) estimated that YCT in their pure form 

 occupied 38,500 ha of lake environment and 2,400 km (1,500 miles) of stream habitat. A reason 

 for this estimate being significantly lower could be linked to the notafion that only "pure" YCT 

 were considered. Varley and Gresswell (1988) did not provide a discussion of how purity was 

 determined to support their esdmate of current distribution. 



Hanzel's (1959) work on YCT distribution in Montana was updated by Hadley (1984). Both of 

 these efforts relied on visual observation to identify YCT. As referenced in Hadley (1984), 

 Hanzel made YCT collections within the Yellowstone River basin in the section between 

 Yellowstone National Park and the mouth of the Bighorn River. These collections indicated that 

 YCT were present in the Yellowstone River from the park boundary downstream for a distance 

 of 145 km (90 miles). Menfion was also made that remnant YCT were confined to small 



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