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1 ,I ,} m r---:'l\ r- ~; . :~ ~ :.;~ f'- ~ 'l C"J 0 ; OJ . :'~ :~ ; r.":, ",,:.:-- .:'.~~ :; . ~ '..: .1. '"f, ':,.,1 ;. :.1 .. , " .: velocity warm habitats prior to spawning, move to selected cobble bar areas for spawning, and then disperse and use a variety of habitats during the remainder of the year. Use of run habitat appears to be slightly higher in the San Juan River and may be related to prevalence of this habitat type in the system. Young Colorado squawfish primarily utilize low velocity habitats, particularly backwaters, which provide warmer temperatures and other early life history requirements. Razorback Sucker Razorback sucker in the Upper Colorado River Basin has been less studied than Colorado squawfish, so habitat preferences are not as well understood. In the Upper Green River System spawning occurs at two primary "spawning bars" in the mouth of the Yampa River and in the mainstem Green River (McAda and Wydoski 1980, Tyus 1987). Substrate is cobble, gravel, and course sand. Spawning occurs during spring runoff, typically in May when the razorback is one of the first native fishes to spawn. Some authors have suggested that spawning may also occur in backwater habitats due to the occurrence of ripe fish in those habitats, and successful spawning also occurs along reservoir shorelines in the Lower Colorado River (Minckley et al. 1991). Because relatively few young razorbacks have been captured, habitat requirements are difficult to judge. Recent theory is that larval razorbacks utilize warm, flooded bottomlands during spring runoff. Loss of spring flooding due to dams has reduced the availability of this needed habitat and ' is the reason so few young have been found in recent years when collecting has been intensive. The few juveniles that have been collected have generally been found in backwaters (Minckley et al. 1991). Adult razorbacks have been collected in a variety of habitats but have been primarily found in the spring in backwaters, eddies, and the flooded mouths of tributary streams. All of these areas are typically warmer than the mainstem rivers. Razorbacks are 'often caught in association with Colorado squawfish, which also use these habitats. Following spawning and as the rivers recede, razorbacks are much more difficult to locate. Tyus (1987) tracked six radiotagged razorbacks iil the Green River and was surprised to find they primarily used midchannel sand bars in the' sUnlmer, habitats not easily 'sampled by collectors. During the springtime, near-shore runs were utilized by radiotagged razorback sucker, although previous sampling typically yielded mostrazorbacks from backwater habitats during spring. This difference in spring habitat use may have been caused by the effects of radio tagging procedures on spawning behavior of the fish. More recent radiotagging studies in the upper Green River (Modde and Irving 1995) found that razorbacks used deep runs and eddies during the summer, and that movement to the spawning areas appeared to be triggered by increased flow. Very little is known about razorback sucker habitat requirements in the San Juan River since few of these fish have been collected in recent times. Oilly one wild razorback sucker has been found since 1987 in the riverine study area. This adult was collected from a mainchannel run near Bluff, UT, in April 1988 (Platania 1990). Adult razorbacks have been collected from shoreline habitats in the San Juan Arm of Lake Powell. To study razorback habitat use in the San Juan River, on 1\1arch 22, 1994, the SJRRIP stocked five radiotagged (6-month duration tags) sub adult (mean total length [TL] = 277 rom) razorback suckers (reared from San Juan Arm of Lake Powell broodstock) into the river at Bluff(RM 79.6), five at the San Juan River Recovery Implementation Program Summary Report 25 17 January 1997 PR-576-2