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STATUS OF GREEN RIVER COLORADO PIKEMINNOW 1367 0,18 ~ .16 ;::l ,14 15.. G ,12 t: 0.1 g ,08 ~ .06 -g .04 0: .02 o ~ o 200 400 600 8013 1000 1200 Colorado pikeminnow TL FIGURE 5.-Probability of capture of Colorado pikeminnow as a function of length for all 12 sampling passes (only years are identified) conducted in the Yampa River, Colorado, 2000-2003, Dashed lines with solid squares represent passes in 2000, solid lines passes in 2001, dashed lines passes in 2002 (the lowermost two passes overlap completely), and solid lines with open circles passes in 2003. onstrated a strong quadratic effect for fish size. The logit parameter values are as follows: TLN = 0.2392 (SE = 0.0845) and TLN2 = -0.1904 (0.0526); the maximum transformed p's for the indlividual reaches, years, and passes are given in Table 3. This result stems from the fact that small and large fish had relatively low capture probabilities and fish from about 500-600 mm TL had the highest ones (see Figure 5, in which Yampa River fish are used as a representative example of the variation observed). It should be noted that the estimated probabilities of capture may be functions of factors other than abundance (e.g., behavior or habitat use) that make fish in a certain size-class more or less prone to capture. Probabilities of capture were relatively low and varied widely (0.01-0.20) among sampling passes, reaches, and years (Table 3). Average probabilities of capture were slightly higher for the Vv'hite River and the lower Green River than for other reaches. Average annual values were lower for most river reaches in 2002 and 2003, which were drought years character- ized by short runoff periods and low maximum discharge levels. Average probabilities of capture tended to remain the same or to increase from passes 1 to 3 in some reaches, even when fourth-pass data were not combined with third-pass data. Probabilities of Transition The low frequency (mean much less than 1 %) of transitions between sampling passes conducted in a single year did not justify our sampling pass-specific I/1's. Transition probabilities that characterized the annual likelihood that tagged Colorado pikeminnow would move among the five different Green River TABLE 6.-Annual transition probabilities (I/t, i.e., proba- bilities of movement to a different reach between years) for average-size Colorado pikeminnow (500.4 mm TL) captured in the Green River basin, 2000-2003. River reaches are as follows: Yampa River (rlan 192 to rlan 74), White River (rlan 167.4 to rlan 0), middle Green River (rlan 539.4 to rkm 396,1), Desolation-Gray Canyon (Green River; rkm 395.9 to rlan 206.1), and lower Green River (rlan 193.2 to rkm 0), Reach shift 1\1 95% CI White to Yampa 0.007 O,OOI-(J.055 White to middle Green 0,048 0,022-0,102 White to Desolation-Gray 0.080 0,043-0,144 White to lower Green 0,003 0,000-0,067 Yampa to White 0 Yampa to middle Green 0.036 0,009-0,135 Yampa 10 Desolation-Gray 0,018 0,003-0,122 Yampa to lower Green 0 Middle Green to White 0,022 O,Oll-O,045 Middle Green to Yampa 0,018 0,007-0,045 Middle Green to Desolation-Gray 0,045 0,024--0,081 Middle Green to lower Green 0,001 O,OOO-O,1l5 Desolation-Gray to White 0,032 0,010-0,099 Desolation-Gray to Yampa 0 Desolation-Gray to middle Green 0,078 0.031-0,186 Desolation-Gray to lower Green 0,041 0,012-0.131 Lower Green to White 0 Lower Green to Yampa 0 Lower Green to middle Green 0 Lower Green to Desolation-Gray 0,121 0,048-0,274 basin river reaches were low during the sampling period, especially those for nonadjacent reaches (e.g., the lower Green River to the White or Yampa River; Table 6). Reaches in the middle of the study area had the most fish moving in. For example, the Desolation- Gray Canyon reach was the only reach to have fish moving in from every other reach. The negative slope for covariate TL reflected the fact that transition probabilities among reaches were higher for smaller fish (Figure 6). Transitions may reflect relatively short- --<>- LG-DG - DG-LG - -. 'DG-MG ~MG-YR g 0.4 :B '" ..c 0,3 0 <:;., '" 0,2 .g ';;) '" 0.1 '" ~ - .. .. .. .. .. 0 300 400 500 600 700 800 900 Colorado pikeminnow TL FIGURE 6,-Annual transition probabilities for Colorado pikeminnow of different lengths for various reach combina- tions in the Green River basin, 2000-2003. Abbreviations are as follows: LG = the lower Green River reach, DG = the Desolation-Gray Canyon reach, MG = the middle Green River reach, and YR = the Yampa River.