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. Age Analysis Age results, even grouped into YOY, juvenile, and adult age classes, show an interesting shift from predominantly adults in 2001, 2002, and 2003 to predominantly juveniles in 2004 and an even number of both in 2005 (Figure 8). The inability to follow an age class through time suggests that in 2003 and 2004, removal efforts disturbed the population to the extent that most individuals are now removed each year and new individuals from other parts of the basin replace them. We do have definitive evidence of four individuals moving down from the Yampa River into critical habitat for razorback sucker in the middle Green River over the study period (Table 7). These fish moved between 192.1 and 306.5 river kilometers (119.4 and 190.5 river miles) downstream to get to the middle Green River near Jensen. We therefore know that downstream movement occurs; we can only assume it occurs to the extent that the majority of pike in the middle Green River were spawned in the Yampa River. Beginning in 2006, northern pike captured in the middle Green River were sent to researchers at Colorado State University for analysis of stable isotopic signatures. If pike sources in the Yampa River drainage have a strong isotopic signature such that this signature imprints itself on the resident fish, we will eventually know the source of middle Green River pike. The cleithra results suggest that northern pike may have successfully spawned in 2001 and 2002; however, because numbers of these younger fish in the river remained low (we detected only one YOY in both 2001 and 2002), it is more likely that these fish migrated down from the Yampa River, thus allowing us to keep this removal effort at a preventative level. There are some issues to consider with the cleithra results. The researchers at the time certainly believed they were accurate in their age determinations; however, Quist et al. (2006) point to difficulties with aging catostomids of the upper Colorado River basin based on cleithra . 41 . . . . . . . 26 .