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262 300 zoo E U ,oo C. A. KART AND H. M. TYUS ~r~ nur aurE ~uv MONiH Fig. 2. Relationship between average distribution hy- drograph and spawning period for humpback and round- tail chubs, Yampa River, 1987-1989. Dashed vertical lines delineate first and last capture of ripe humpback chub; solid vertical lines delineate first and last capture of ripe roundtail chub; 1986 not included because sampling was initiated late in spring runoff. humpback chub and exhibited more robust tuberculation and more brilliant orange col- oration. Patterns of tubercles and breeding coloration were similar between the two chubs. Ripe male roundtail chub averaged 344 mm TL (n = 117, SD = 24, range 292-419 mm TL) and 329 g (n = 100, SD = 84, range 190-652 g), and ripe females averaged 363 mm TL (n 6, SD 15, range 343-380 mm TL) and 363 g (n 3, SD = 104, range 276-478 g). Nonripe tuberculate fish aver- aged 351 mm TL (n = 119, SD 29, range 264-447 mm TL) and weighed about 364 g (n 77, SD = 123, range 140-844 g). Ripe roundtail chub were captured in pools and shoreline runs and eddies during the period of declining spring runoff (Fig. 2). Humpback and roundtail chubs in breeding condition were collected syntopically on 13 occasions. Although this indicated overlap in use of shoreline eddies during spring runoff, ripe females of both species were not syntopic. DISCUSSION Humpback chub and roundtail chub were sympatric in DNM in the reach from upper Yampa Canyon to upper Whirlpool Canyon, although humpback chub were rare (<1% of total catch and only 8% of the two Gila species combined). Humpback chub were [Volume 50 most prevalent in, and presumably selected, eddy habitats in moderate- to steep-gradient reaches, whereas roundtail chub were ubiqui- tous in parks and most canyons in eddies, riffles, and runs. Both fishes were most abun- dant in Yampa Canyon; neither was captured in Split Mountain Canyon, and the humpback chub was absent and the roundtail chub rare in Lodore Canyon. The paucity of Colorado River chubs in Split Mountain and Lodore Canyon reaches indicates a general decline of Gila species rel- ative to earlier decades (e. g. , Banks 1964, Vanicek et al. 1970, Holden and Stalnaker 1975a). This may be related to the loss of historic temperature and flow regimes due to regulated flow releases from Flaming Gorge Dam, and to the proliferation of nonnative fishes, particularly channel catfish and com- mon carp. The current rarity of Colorado River chubs in Split Mountain Canyon was also noted by the authors in 10 hours of oppor- tunistic sampling and by the State of Utah during their 1988-89 studies (T. Chart, Utah Division of Wildlife Resources, personal com- munication). Capture of 133 humpback chub, including 39 breeding adults and 29 juveniles, indicates that a reproducing population exists in Yampa Canyon. However, only one ripe fish, a male, was collected in the Green River (i. e. , Whirl- pool Canyon), and it is unknown whether it spawned there or was a stray from the Yampa River. Collection of ripe roundtail chub in canyon reaches yielding ripe humpback chub indicates some temporal and spatial overlap in habitat use during the spawning period, as observed by others in the upper Colorado River (Kaeding et al. 1990). Ripe humpback and roundtail chubs were collected during declining spring flows and increasing river temperatures after highest spring runoff. This occurred in May and June in low- (e. g. , 1987, 1989) and average- (e. g. , 1988) flow years but extended into July in the 1986 high-flow year. No humpback chub in breeding condition were captured during pre- runoff and late postrunoff periods, and we presume the fish spawned only during the 5-6 week period following highest spring flows. Capture of only a few ripe female chubs (five humpback and six roundtail chubs, 4% of all breeding captures) suggested that females may be ripe for a limited time. Ripe