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. . It '.. ~ ~ ,., i ~ 1 ~#• , ~`f ~ 1 ! •4 I ~~ \~~~] ~~ ~ `~~` 2000] COLORADO PIKEMINNOW IN WHITE RIVER a t i r :~,r- This study showed 5 of 7 Yampa River fish were within the spawning area in 1993 and 1994. In 1994 one of the 2 fish found just below the site in 1993 was found on the site and the other just above the site. One of 5 Green River fish was at the site in 1993 and 3 were there in 1994.Of the 4 fish located above the site in 1993, two were at the site in 1994 and the remaining 2 fish above the site in both years. The only way to confirm that there aze more spawning sites in the upper Green River drainage is to tag more fish and follow their migration patterns more closely over succes- sive years. Presumably, 100% of the radio-tagged Colo- rado pikeminnow survived over the 2-yr study. This rate is higher than the 85% annual sur- vival rate reported by Osmundson et al. (1997) for adult Colorado pikeminnow in the upper Colorado River. Our study confirms and expands the find- ings of other researchers. From 1980 through 1990, radio telemetry detected 37 Colorado pikeminnow moving in and out of the White River (Tyus et al. 1981, Tyus 1990, Trammell et al. 1993). Twenty-three of these pikeminnow remained in the White River, or at least were there on all monitoring dates. However, some were not contacted each time, and several months sometimes elapsed between tracking dates. It is thus possible that individuals mi- grated from the White River to spawning sites in the Green or Yampa rivers and returned undetected. Of the remaining 14 telemetered pikeminnow, 9 moved to the Yampa River spawning area and 5 to the Gray/Desolation Canyon spawning area in the Green River. Only 2 of the 14 are known to have returned to the White River. All 12 of the Colorado pikeminnow we stud- ied exhibited migratory behavior in 1993. This is in contrast with previous studies in the Green River where about 50% were nonmigratory. This suggests that White River stocks are composed entirely of reproductively active individuals that utilize the White River exclu- sively for adult habitat. The long-distance migrations of these fish to spawning areas in the Green and Yampa rivers suggest that adult habitats may be limiting for such an energeti- cally costly mechanism to have evolved. In addition, if natal imprinting is important in homing to spawning destinations (Ti~us 1990), the White River population of Colorado pike- ! :.1 ~ . ~ `, 23 minnow represents a mixed stock recruited from both upstream and downstream spawn- ing populations. Tyus (1985, 1989, 1990) noted that pikeminnow in the middle Green and Yampa rivers tended to spawn at the Yampa River spawning site, and fish from the lower Green River (i.e., Desolation and Gray canyons) tended to migrate to the Gray Canyon spawn- ing site. Although the White River confluence is located approximately equidistant from both spawning sites (confluence-Yampa River site 159 km, confluence-Green River site 140 km), it is much nearer to known nursery areas up- stream of the White River (Irving and Burdick 1995, McAda et al. 1998). Thus, it is surprising that about half the fish studied by us and oth- ers utilized the Gray Canyon site. This suggests that occupation by subadult/adult fish moving upstream, not the presence of nursery areas for juvenile fish, is the mechanism for coloniz- ing the habitat in the White River. Presumably, juvenile fish may move upstream in the main- stem Green River for some time before explor- ing tributary streams. Spawning migration dates of this study also match well with Colorado pikeminnow larval drift dates calculated by Bestgen et al. (1998). He found that larvae drifted downstream of both spawning sites between late June and mid-August 1993 and between mid-June and late July 1994 (Fig. 2). This study found that most tagged Colorado pikeminnow in the White River began their migration to the Yampa and Green river sites by mid-May or mid- June and then migrated back to the White River by mid- to late August (Table 1). Eight fish relocated upstream of Kenney Reservoir remained there 2~ months before migrating downstream. Two fish relocated just above the dam in Kenney Reservoir in Sep- tember 1993 moved through the reservoir to historic riverine habitats above the reservoir where they overwintered before migrating downstream to spawning areas the following summer. Adult Colorado pikeminnow we studied re- sponded differently from hatchery-reazed juve- nile and adult pikeminnow previously stocked into Kenney Reservoir. Trammell et al. (1993) stocked 96,597 juvenile Colorado pikeminnow into the reservoir between April 1988 and September 1990, but none remained in the impoundment following stocking. Results from 4 hatchery-reared adult fish implanted with