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. Table l.--Larval Colorado squawfish'collections, Yampa Canyon, 1980-83. 1980 values include collections made with dip nets; 1981-83 collections were made exclusively with seines aNo. of bDates cLocalities Year samples (mO N 1980 215(9) 23-25 Aug 0.1-8.6 46 1981 519(11) ?4 July-15 Aug 0.1-17.9 23 1982 310(10) 7-25 Aug 0.3-12.2 20 1983 245(17) 22 July-21 Aug 0.7-12.3 228 TL range ( in. ) Est. spawning period (dates) dC/E 0.79 0.18 5.59 0.55-1.14 0.35-0.87 0.39-0.83 0.34-0.66 2 July-8 Aug 6-30 July 24 July-8 Aug 24 July-11 Aug a bNumbers in parentheses represent numbers of collections which contained one or more larvae. Dates of earliest and latest larvae collection. ~Distance from mouth O2 Yampa River (mi 0). Number larvae/1000 ft specific habitat. In 1983, the use of fine mesh (560 micron) plankton nets was included in the sampling design to evaluate the possiblity of downstream drift following hatching. Three nets were deployed along the shoreline at each of two sites. One site, at approximately RM 20.5, was selected to determine if spawning occured above the previously documented RM 16-20 reach. The downstream site was approximately 2 mi above the river's mouth and was selected to follow any drifting larvae into the Green River. Sampling was initiated at water temperatures of approxi- mately 18 C in early June and continued through late August when flows were too low to permit travel. Collections were timed to evaluate the possibility of diel activity patterns. Seine sampling was continued and restricted to the lowermost 22 mi of Yampa Canyon. An additional unanticipated factor in 1983 was an unusually high flow3regime with flows of approximately 22,000 ft /sec in late May. A total of 331 larval Colorado squawfish were collected in 1983, 228 of which were collected by seine from shallow shoreline embayments. Of the drift specimens, only one was collected at the upstream site, strongly suggesting that spawning was indeed limited largely to the lowermost 20 mi of the Canyon. Diel periodicity of squawfish could not be demonstrated statistically. Inter- estingly, native fishes (predominantly larvae and juveniles) were far more abundant in the drift and compromised 83% of the total catch. Only channel catfish (Ictalurus punctatus), an intro- duced riverine species, were collected in numbers comparable with the most abundant native species. In 1980-82, squawfish numbers were too small to make realistic evaluations relative to spawning "intensity;" however, back-calculated spawning dates for 1983 revealed that while limited spawn- ing occurred as early as 11 July (mean water temperatures=18-20 C), a peak in spawning activ- ity occurred between 25 July and 10 August (mean water temperatures=22-25 C). This peak in spawning activity coincided closely with dates when radio-tagged fish were active over the spawning area (H. Tyus, CRFP, pers. comm.). Spawning after 10 August was sparse and appeared to cease by 20 August (mean water temperature = 29 C). Capture/effort (C/E) calculations made exclu- sively on seine collections for the years 1981-83 (1980 data were excluded since a number of dif- ferent methods had been used to collect la2vae) suggested that a 1983 value of 5.59/100 ft , 7-20 times greater than the two previous years, was indicative of greater larval production during the 1983 high-flow year than during the two previous years when flows had been substantially lower. Interestingly, although the mainstem Colorado River similarly experienced a very high flow regime in 1983, C/E in that year was lower than in 1982. When thermal regimes (in terms of summed 22 C+ degree-days) were compared, heat accumulation in the Yampa River in 1983 was comparable with 1982 when flows were considerably lower. On the Colorado River, 22 C+ degree-day accumulation in 1983 was substantially less than in the previous 2 years and only 65% that of the Yampa in 1983 (Haynes and Muth 1984). The combined seine-drift sampling design was repeated in 1984. Laboratory processing and data analysis are in progress and therefore not included in this synthesis. Tyus (pers. comm.) has reported the collection of larval and juvenile squawfish from Green River backwaters (Utah) and it is presumed that these individuals originated in the lower Yampa since no other upstream spawning aggregations have been documented. Only a single squawfish juvenile has been collected in Yampa Canyon since initiation of studies. Thus, larvae originating in the Yampa appear to be transported downstream via drift to nursery sites in the Green River. 192