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. . - 4 - arrangement. The duration of each sample will depend on the volume of debris in the water, but will probably range from 15 to 40 minutes. The volume of water filtered through each net will be determined in order to compute the number of larval fish per 1000 cubic feet of water. Drift sampleB will be taken daily between the hours of 6:00 pm and midnight to maximize the likelihood of high drift rates of native BpecieB, aB found by Valdez et a1. (1985) in the Upper Colorado River above Grand Junction, CO. Drift sampling will begin &bout 4 days after the eBtimated spawning time, based on incubation time observed by Hamman (1981) in hatchery investigations, and continue for 5 daYB. The drift nets to be used are each 3 m long with a 30 x 45 cm opening. The nets are made of 560 micron mesh, and each is equipped with a removable sample cup. The drift samples will be preserved in individually labeled plaBtic bagB with 5' buffered formalin, and sorted at the BIO/WEST laboratories. All drift specimenB will be forwarded to the Larval FiBh Laboratory in Fort COllinB, CO. for Bpecific identification and length measurements. A Bimilar program of drift sampling will be conducted at two additional Btations, one at the upper end of Westwater Canyon (above Wild HorBe Cabin), and one at the lower end of the canyon (below Big Hole). Two crews will be deployed to monitor the three drift sample stations identified. Once larval fish become numerous immediately below a spawning site, based on on- site inspection of drift samples, additional drift nets will be deployed further downstream to determine the distance that the young chubs are being transported by currents. This additional drift netting will be complemented by sampling shorelines and eddieB with 1/32-inch mesh seines and larval dip nets to identify nursery areas. We hypotheBize that the young chubs are transported relatively short diBtances before being caught by eddy currentB that wash them toward backwaters, small concavities and pocket waters along the shoreline. We further hypothesize that these nursery habitats are critical to the Burvival of the year class and that their occurrence and abundance iB directly related to river flows. This investigation proposes to identify and map spawning as well as nursery habitats in order to quantify flow-habitat relationships. TASK 3: IDENTIFY AND DESCRIBE REPRODUCTIVE ISOLATING MECHANISMS Subtask 3A: Synthesize Past Data: Data from 1979-81 (Yaldez et al. 1982) and 1986-91 (Moretti and Chart 1991) will be assimilated together with hydrologic flow and temperature records from the Stateline USGS Gage. The catch rates and species composition of these data will be compared with data collected by this study. Catch rates of non-native Bpecies will be related to flows and temperature regimes to identify flow and temperature scenarios conducive to their invasion and proliferation. Past catch rateB of roundtail chub will also be compared with flows and temperature to asseSB the posBibility of hybrization with humpback chub. Subtask 3B: COordinate With ISMP. This investigation will incorporate the information collected by the ISHP Humpback Chub Monitoring Program. Data collected from that program will be analyzed to asseSB the occurrence of sympatric species in Westwater Canyon and Black RockB. These data will be compared to river flow regimes such as spring peaks, duration, timing, and temperature to determine if certain flows reduce the invasion of non-natives into these regions.