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Park, bluehead were rare under 28 cm and the 2001 histogram (Figure A3) was similar to 2000, even though sample size was less in 2001 (Table 8). On the Colorado River, juvenile size bluehead sucker were fairly common and there were no breaks in the histogram between 10 and 44 cm (Figure AS and A6). The Dolores River bluehead sucker size structure in 2001 was noticeably different from all other sites. In both 2000 and 2001 no bluehead sucker over 28 cm were collected. In 2001 a very high number of yearling (325) bluehead were caught (Figure A4), but no fish in that size-group were collected in 2000. About the same number offish between 19 cm and 28 cm were collected in both years (11 in 2000 and 18 in 2001). Table 8. Mean lengths of Blue head sucker captured during the study period (1998 to 2001), Yampa, Colorado and Dolores Rivers. 1998,a 1999, b 2000,c 2001,d Mean lenoth of bluehead sucker Incm Sevens 33.5,cd 33.6,cd 31.3,abd 30.0,abc Duffy 36.5 37.9 37.6 36.2 Lily Park 34.4, d 33.5,c Corn Lake 36.5,cd 33.3,bd 34.8,bc Clifton 31.8,d 33.5,c Dolores 23.6 12.1 Total number in sample - Number less than 15 cm Sevens 314 - 0 187 - 0 180- 0 89-0 Duffy 56-0 102- 0 45 -0 41 -3 Lily Park 347 - 3 212 - 0 Corn Lake 1212 -3 1010-16 1283 -31 Clifton 1374-51 1228 - 35 Dolores 11-0 343-308 *a, b, c, d following a mean length indicates significant difference (2 tail test) at @=0.5 for those years. The mean length of blue head sucker at Sevens in 2001 was smaller. Fewer large bluehead sucker is consistent with reduced riffle habitat availability due to low flows in 2000 and 2001. The smaller bluehead mean length (30 cm) at Sevens could be an indicator ofa change in habitat availability. In contrast to Sevens, Duffy had the highest mean lengths for bluehead sucker consistently for the four years. The high Duffy mean length (36 to 38 cm, Table 8) is due to lack offish under 35 cm, indicating Duffy had steady and heavy predation pressure throughout the study period. At Lily Park, bluehead sucker mean length was near 34 cm in both years even with fewer fish less than 28 cm in 2000 (Table 8). The mean length of bluehead in the Colorado River sites was 32 to 34 cm (Table 8) and appears to represent a healthy bluehead sucker size structure, since habitat is abundant and predation or competition is not a factor. The Dolores River had the smallest bluehead sucker mean length (12.1 cm) (Table 8) due to a very high number of yearling fish and a very low number of adults. The length frequency histograms were very similar for flannel mouth sucker for the first three years at Sevens, but percentage offish between 30 and 42 cm increased in 2001 (Figure A7) compared to prior years and mean length (43.8 cm) was significantly less (Table 9). 28 I I I I I I I I I I I I I I I I I I I