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ISMP backwater sampling evaluation <br />STUDY AREA <br />Backwaters were sampled in a 55 km-long reach of the Colorado River, CO in the Grand <br />Valley from the Grand Valley Diversion to just downstream of the Loma boat ramp (Fig. 1). The <br />study reach was divided into four sub-reaches, two upstream of the Gunnison River and two <br />downstream. The most upstream sub-reach (sub-reach 1) was from river kilometer (RK) 298.2 <br />downstream to RK 285.8, sub-reach 2 was from RK 285.7 downstream to the confluence with <br />the Gunnison River at RK 274.2, sub-reach 3 was from RK 274.1 to RK 258.6, and reach sub-4 <br />was from RK 258.5 to RK 243.1. River gradient was relatively low and substrate consisted <br />mostly of cobble and gravel, which was overlain with silt and sand in low velocity backwaters <br />(VanSteeter and Pitlick 1998). River discharge was relatively high and variable during autumn <br />1997, but was lower and more stable in autumn 1998 (Fig. 2). <br />METHODS <br />Field sampling <br />We implemented a double sampling approach (Thompson 1992) in backwaters where fish <br />species richness and fish density was estimated by each of two techniques in autumn of 1997 and <br />1998. The first was the quantitative but relatively low effort two seine haul ISMP approach <br />(ISMP sampling), and the second was a relatively high effort 3-pass seine depletion or multiple <br />pass capture-recapture approach (DMR sampling). The goal of such sampling was to quantify <br />the bias of presence/absence and abundance data derived from ISMP sampling compared to that <br />gathered with the relatively higher effort, and presumably, more precise and accurate DMR <br />sampling. Backwaters chosen for sampling during this study were a minimum of 30 m2 in <br />-3-