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February 1983 1\otes 121 Tnac?r 1.-Relation of size to sexual maturity for bluehead suckers from (he Yampa, Green, Colo- rado,and Gunnison rivers, 1975-1976. Colorado and G unnison rivers Yampa an d C;rcen ricers F emale Male Fema le Male Total length Number Percrm Number Percem Number percem Num her Prrcent (turn) of fish mature of (ish manne of fish mature of Gsh mature 301-310 3 33 311-320 3 66 3 67 321-330 2 100 5 80 2 100 331-340 6 67 7 57 l 100 341-350 11 82 15 80 1 100 4 100 351-360 5 100 5 80 2 50 5 80 361-370 12 92 6 83 6 83 4 100 371-380 7 86 20 95 5 100 3 100 381-390 18 100 8 100 2 100 8 I00 391-400 17 100 5 100 4 100 401-410 14 100 5 100 411-420 4 ]00 1 100 2 I00 421-430 4 100 2 100 431-440 3 I00 _ method was estimated by wta] counts of ova from three fish. Percentage error ranged Erom an underestimate of 3.9% to an overes[imate of 1.5 ~. Diameters of preserved, mature ova were measured to 0.01 n(m with an ocular micrometer. Ova from anterior, middle, and posterior sections of the ovary ~+'ere mixed, and 30 chosen from that group. These were measured and the average value was considered the diameter of ova Eor that Fish. The total length-Fecundity relation for bluehead suckers was best described by the logarithmic equation: Log F=br Log TL - bu where TL =total length in mm, F =fecundity (mature eggs per Female), and bo, b, =empirical constants. tie fitted regression cquatiotu by using 'ST:~T ° .~" programs available a[ Utah State University. In spring collections, ma[ure Females were recognized by presence of large, yellow eggs. Ova size was variable, and although egg diameter generaHq increased with increase in Eish length, the rela- tion was not significant. Diameters of preserved, mawre uya ranged Fro l.~- ....-.~~ •••~• ~ -- -. 1.87; n = 79). In the Colorado or Gunnison rivers, the smallest mature female collected was 313 mm TL and all females > 380 mm were mature (Table 1). The smallest mature Female collected from the Yampa or Greett rivers was 345 mm TL, and nearly all larger females were mature. The smallest mature males were 305 mm TL in the Colorado or Gunnison river collections and 323 mm in those from the Yampa and Green rivers; all males > 380 mm in the Colorado and Gunnison rivers and > 360 mm in the Yampa and Green rivers were mature. The relation between fecundity and length Eor bluehead suckers collected Erom the Yampa and Green rivers in 1975 was significantly different From that Eor Eish collected in 1976 (P < 0.05; Fig. 1). Data on fecundity Eor bluehead suckers from the Colorado river (1975) and the mouth of the Gunnison River (1976) were combined because there was no significant difference between them (Fig. 2). The difference between fecundity-length relations Eor Eish from the Colorado-Gunnison rivers and the Yampa-Green rivers was highly significant (P < 0.001)- Bluehead suckers from the Colorado and Gunnison rivers produced more eggs than fish of similar sizes from the Yampa and Green rivers. Size at maturity varies [hroughout the range o[ [he bluehead sucker. Fish Crom small tributaries do not reach the large sizes attained in the main rivers, and mature at a smaller size (Smith, pp. 125-171, in Fishes in North American Deserts, [7;'aiman and Sol[z, eds_~ John Wiley & Sons, New