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. . . v. Study Mathods/Approach: Passive drift-net sampling is an effective and proven method for capturing Colorado squawfish larvae. In properly conducted, results can provide & reascmal:lle estilllace of annual reproductive output frOlll spawning agrees without over-sampling larvae. Drift-net sampling in the Yampa and Green rivers, as part of FGFRI, was started in 199,0 and will be conducted again in 1991. In 1991, two drift- net sta.t:ionswere established, i. e., lower Yampa River at Rcho Park and Green River just belew the Split Mountain boat ramp. At each station, three flow-metered plankton nets were set near the shore, daily at dawn for 1-2 h, frOlll end of June through early August. The objectives of this sampling were to doCUlllent timing and duration of spawning by Colorado squawfish in the lower Yampa River, determine peaks in abundance of transported Colorado squawfish larvae transported frOlll the lower Yampa River spawning area into nursery habitats in the Jensen-Ouray area; nursery-habitat sampling conducted by Utah DWR (Core-Research study) in fall 1990 was, intended to Subsequently determine abundance of young Colorado squawfish in backwater habitats. Results of the 1990 drift-net sampling were provided in an annual progress report sul:lmi tted to the 0. S . Bureau of Reclamation. After discuseions with the cooperative FGFRI team in December 1990, the drift-net sampling scheme used in 1990 was changed in favor of the design proposed here (this new study design will be employed in 1991) . Reproductive output will be measured by daily captures of drifting Colorado squawfish larvae at two sites, one at mouth of the Yampa River and one below Grey and Desolation canyons in the lower Green River. Both of these sites are below known spawning grounda, and data frOlll sampling at these sites will provide estimates of relative abundance of Colorado squawfish larvae being transported frOlll spawning areas and into potential nursery habitats. Colorado squawfish in the Yampa River spawn on the descending limb of the hycirograph, often asso<:iated with abrupt increases (flow spikes) in base flew (Nesler et al. 1988; Tyus and Karp 1989). Sampling for Colorado squawfish larvae will be initiated based on those data ,and stream-flow conditions prior to sampling (probably start in mid-late June). Duration of the sampling period will depend on number of larvae collected in late- season samples, past data, and stream-flow conditions probably end early- mid August) . Colorado squawfish larvae are most abundant in drift-net samples at dawn and nearshore and midetream nets capture roughly equivalent numbers of fiSh/unit volume of water sampled (Haynes et al. 198.; Nesler 1986). This was supported by results of out nearshore sampling during SUlllller 1990; catch rates of three nearshore drift nets were similar (one-way ANOVA) . Results of the 1990 sampling showed that about 60t of drifting Colorado period on only four sampling days; .. determined that daily sampling over a 6-S-week period was necessary to detect and measure peaks in larval abundance and brack6t the period of larval transport. At each station, three .-m-long conical plankton nets (560 micron mesh) will be set deily at dawn. Nets will be attached to rectangular steel frames (0.15 m') and staked into the stream substrate adjacent to the shore in water 0.5-1.0 m deep. A rttDlClVahle collection bucket for trapping filtered lIIaterial and fishes will be attached to the cod end of each net. Flow meters for measuring velocity will be suspended inside the mouth of each net, and net sets will be timed to determine volume of water sampled. Duration of each set will be 1-2 h depending on debris load. Samples will be fixed and preserved in 95-100t ethanol (for subsequent otolith-ageing Flaming Gorge Studies A.