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~7 cwt ~--~ ~ o ECOLOGICAL INVESTIGATION OF A SUSPECTED SPAWNING SITE OF COLORADO SQUAWFISH ON THE YAMPA RIVER, UTAH Vincent A. Lamarra', Marianne C. Lamarra', and John G. Carter' t~ ARSTnncr.L On 5 July 1981, 13 adult Colorado squawfish were found in spawnvtg condition at river mile 16.5 in the Yampa River, a major tributary to the Green River. An investigation was undertaken to quantitatively describe this section of the river to gain insights on the spawning requirements of this endangered species. The substrate at the suspected spawning site was cobble with large interstitial spaces devoid of organics, silts, or clays. It appeared that larvae of several fish species utilized these cobble areas and the associated voids. Diurnal sampling indicated that larval drift occurred between 0100 and 0125 hours. Substrate size also appeared to be a dominant factor in fish distribution. Feeding intensities of these fish corresponded to macroinvertebrate drift. Spawning by the endangered Colorado squawfish (Ptychocheilus lucius) has not been documented in nature. On 1 July 1981, the Colorado River Fisheries Project [CRFP] of the U.S. Fish and Wildlife Service captured 13 adult Colorado squawfish in spawning condition in one localized area of the Yampa River, known as Cleopatra's Couch, 16.5 miles from the Green River confluence. This gave us the opportunity to intensively. sample the immediate area in an effort to locate the site and to document conditions in a natural spawning habitat. This survey was under- taken 24-26 July 1981. The objectives of this study were: 1. To characterize this river ecosystem at the sus- pected squawfish spawning site. 2. To document the interactions of the abiotie envi- ronment with the biotic community. 3. To document the interactions among and be- tween trophic levels. 4. To compare the structure and fimction of the eco- system at this site with that of other similar sites in adjacent tributaries where squawfish were not located. The above objectives were accomplished by intensively surveying the area where spawning squawfish were observed (mile 16.5) and comparing these data to another similar Yampa River site (mile 18.0) appar- ently not used for spawning during 1981, Furthermore, these data ate compared to a structurally similar area in the White River, a major tributary of the Green River, where successful squawfish spawning has been sus- pected but not documented. The results of 'Ecrosystem Research Institute, 975 South State Highway, Logan, Utah 84321. this study are important for recognizing po- tential squawfish spawning sites and for the future maintenance of the physical and bio- logical integrity of this important Yampa River site in the face of future development within the upper Green River Basin. .MATERIALS AND METHODS Field Two field seining sites were established (mile 16.5 and 18.0), and at each of these samples were taken from several different habitat types, e.g. riffles, eddies, and runs. Five seining sample locations were chosen for river mile 16.5 and seven for river mile 18.0: All fish samples were collected with seines (4 x 30 ft, with 1/4-in mesh). Upon capture, all fish were identified, counted, and measured for total lengths. Fish less than 100 mm were slit abdominally and placed in a 10% formalin solution. The diges- tive tracts of fish greater than 100 mm were removed and preserved in 10% formalin. A benthic sample (modified Surber sampler) was collected at each seine haul location. Insects floating in the water column were sampled with drift nets (27.5'cm diameter, 1.0 mm mesh size) at each seine sample loca- tion. These nets were anchored to the river bottom and set in the water column approx- imately 1 m upstream from the area to be seined. Each net was set for approximately 4 hours prior to fish collection and removed 127 ~y