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necessary to monitor recovery efforts. Studies designed to develop the necessary data to <br />produce population estimates began in Yampa Canyon, Black Rocks and Westwater Canyon <br />in 1998. Similar studies for the other populations are ongoing. This report presents the <br />population estimate for Black Rocks for 1998-2000. <br />METHODS <br />Study Area <br />Black Rocks is a 1-mi section of deep-water habitat formed by erosion-resistant igneous <br />rocks in the river channel. This unique area is about 4 mi upstream from the Colorado-Utah <br />state line and extends from about river mile 135.5 to 136.5 (Figure 1}. The river channel is <br />narrow and turbulent eddies, pools and runs are located throughout the short reach. Deep <br />areas along the rock faces provide important habitat for chubs. Black Rocks is substantially <br />deeper than other parts of the Colorado River, with an average depth of about 17 ft and <br />maximum depth of about 60 ft (Valdez et al. 1982). <br />Field Sampling <br />Sampling was done in late August to early October, after water temperatures began to <br />cool for the year. To develop reliable population estimates, sampling trips were scheduled to <br />correspond to amark-recapture sampling design. Sampling was done for four days (one <br />sampling rotation) with one week separating sampling trips. Three sampling trips were made <br />in 1998 and 2000 and four trips were made in 1999. Most sampling was done with multi- <br />filament trammel nets (1-in inner mesh) set along shoreline eddies or in other quiet habitats. <br />However, fish collections were supplemented with electrofishing and trap nets in 1998 and <br />with angling in all years. Four to six trammel nets were set mornings and evenings and <br />checked at 1 to 2 hr intervals (median 1.5 hr). Nets were left in the same location for 4 hr <br />unless no fish were collected or excessive amounts of trash required the net to be moved <br />sooner. As much of the Black Rocks area as possible was sampled during each trip to ensure <br />that all humpback chubs had a possibility of being captured; however, a shallow riffle at about <br />RM 136.3 limited sampling above that point. <br />2 <br />