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<br />C) <br />c' <br />....~ <br />CD <br />l',) <br />o <br /> <br />8 <br /> <br /> <br />.Side Channel 1994 <br />IBr!IMain Channel 1994 <br />- Side channel 1995 <br />- Main Channel 1995 <br />-Side channel 1996 <br />. Main Channel 1996 <br /> <br />6 <br /> <br />4 <br /> <br />2 <br /> <br />o <br /> <br />April <br /> <br />October <br /> <br />July <br /> <br />August <br /> <br />Figure 3.19. Macroinvertebrate density in main and side channel habitats for sampling <br />during 1994 through 1996 (Source: Brooks et al. 2000). <br /> <br />The studies in the San Juan River found that invertebrate densities are variable. Brooks et al. (2000) <br />concluded that sediments from late sununer and fall thunderstorm activity in the San Juan River likely <br />caused the drop in invertebrate numbers during that time period. They also noted that this reduction in <br />density may limit food for young fish during that time period. Archer (2000) compared food availability <br />and growth of stocked YOY Colorado pikeminnow from two <br />reaches of the San Juan River, a portion of Reach I (RM 8,0 to RM 13.0) and a portion of Reach 4 near <br />RM 120.0. Food availability (benthic invertebrates) was lower in Reach 1 than in Reach 4, but growth, <br />condition factor, and lipid content of Colorado pikeminnow were not different between the two reaches. <br />This lack of difference between the two study areas suggests that food was not limiting in Reach I. Both <br />study areas have extensive populations of red shiner, fathead minnow, and other nonnative fishes, <br />suggesting that competition for food, even in a reach (Reach 1) with relatively low benthic invertebrate <br />densities and during the fall when densities are at their lowest, is not occurring in the San Juan River. <br /> <br />Trammell and Archer (2000) showed that growth of Colorado pikeminnow stocked in the San Juan <br />River in 1996, 1997, and 1998 was excellent (Figure 3.20) and similar to growth of wild <br />Colorado pikeminnow in the Colorado River. Once Colorado pikeminnow switch to piscivory, their <br />diets only have high overlap with other obligate piscivores, which occur in lower numbers than the <br />abundant, omnivorous red shiner and fathead minnow, This suggests that food for YOY <br /> <br />September 2000 <br /> <br />3-48 <br /> <br />Program Evaluation Report <br />