San Juan River Recovery Implementation Program Summary Report 1991-97

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� I water temperatures tend to be cooler than the main channel, while in the summer the situation is reversed with backwater temperatures warmer than the main channel. Consequently, preference for backwater habitats during summer and avoidance of the same areas during winter may be primarily a response to temperature differences. Ongoing research and further analysis of data on habitat use of stocked YOY Colorado squawfish should provide valuable insights into habitat preferences of the ' early life stages of this species in the San Juan River. Juvenile and subadult Colorado squawfish (yearlings and older) are less frequently collected than YOY or adults and have been collected from a variety of habitats in the Upper Basin —from backwaters to more riverine habitats. It appears that as young Colorado squawfish grow, they use more of the main channel and have the ability to move upstream and into tributaries. Until 1997, only a few juvenile Colorado squawfish had been collected in the San Juan River, including two yearlings collected in a backwater in 1994 and two subadults (300 -400 mm TL) collected in the main channel in 1996. The stocking of YOY Colorado squawfish in 1996 resulted in the capture of ' numerous yearlings in late 1997. As noted above, the habitats used by the yearlings tended to be low velocity but fit the general pattern seen in other portions of the Upper Basin in that they were collected in more mainchannel habitats with current. Razorback Sucker Razorback sucker in the Upper Colorado River Basin have 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 in the spawning bars is cobble, gravel, and coarse sand. The razorback sucker is one of the first native fishes to spawn during the year, with spawning occurring during spring runoff, typically in May. Some authors have suggested that spawning may also occur in backwater habitats based on the occurrence of ripe fish in those habitats. Successful spawning also occurs along reservoir shorelines in the Lower Colorado M River (Minckley et al. 1991). Adult razorback sucker 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. Razorback sucker are often caught in association with Colorado squawfish, which also use these habitats. Following spawning and as the rivers recede, razorback sucker are much more difficult to locate. Tyus (1987) tracked six radio - tagged razorback sucker in the Green River and was surprised to find they primarily used midchannel sand bars in the summer, habitats not easily sampled by collectors. During the springtime, near -shore runs were utilized by radio - tagged razorback sucker, although previous sampling typically yielded most razorback sucker 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 radio - tagging studies in the upper Green River (Modde and Irving 1995) found that razorback sucker 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. Only one wild razorback sucker has been found since 1987 in the study area. This adult was collected from a mainchannel run near Bluff, Utah, in April San Juan River Recovery Implementation Program December 1997 Summary Report 20 PR -602 -1