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<br />EXECUTIVE SUMMARY
<br />
<br />Between Shiprock, New Mexico and Chinle Creek, Utah, secondary channels are
<br />a common feature of the San Juan River. Prior to initiation of the San Juan River Seven-
<br />Year Research Program no studies specific to secondary channels had been conducted.
<br />As part of the Seven- Year Research Program, research was initiated in 1991 to
<br />characterize the fish fauna of San Juan River secondary channels, to discern seasonal-use
<br />patterns of secondary channels by common fishes, to evaluate the effects of different flow
<br />regimes on common fish species, and to characterize use of secondary channels by rare
<br />fishes (roundtail chub, Colorado pikeminnow, and razorback sucker). Summer
<br />inventories of secondary channels were initiated in 1991 and spring and autumn
<br />inventories began in 1993. Research on secondary channels occurred in Geomorphic
<br />Reaches 5, 4, and 3. Flows of 1500 cfs wetted most secondary channels and flows of
<br />2500 cfs inundated almost aiL
<br />
<br />During spring, when flows were at least 2500 cfs, native flannelmouth sucker and
<br />bluehead sucker and nonnative common carp and channel catfish numerically dominated
<br />secondary channel fish collections. Small-bodied fishes such as native speckled dace and
<br />nonnative red shiner and fathead minnow, were uncommon in spring collections. The
<br />sampling technique, raft-mounted electrofishing, was not an effective means of collecting
<br />small-bodied fishes. Abundance of flannelmouth sucker was less in 1997 than 1993 in all
<br />reaches, but the decline was most apparent in Reach 4. Although abundance evidently
<br />declined, mean total length, mean biomass, size-structure, and age-structure were not
<br />markedly different in 1997 from that found in 1993. Bluehead sucker abundance was
<br />least in 1995 in all Reaches and second greatest in 1997. Neither mean total length nor
<br />biomass was appreciably different in 1997 than that found in 1993. In Reach 5, where
<br />bluehead was typically most common, there was little change in size- or age-structure,
<br />but in Reach 3 (where least common), there was considerable variation in these
<br />population attributes among years. In all years in all Reaches, most common carp
<br />captured were::: 400 mm total length and thus adults. Common carp were slightly more
<br />common in all reaches in 1997 than 1993, but mean total length and biomass did not
<br />change. Abundance of channel catfish in Reaches 5 and 4 was slightly greater in 1997
<br />than 1993, but was considerably greater in Reach 3 in 1997 than 1993. Total biomass,
<br />however, was greater in 1993 than 1997 in Reaches 5 and 4. In Reach 3, total biomass
<br />was least in 1993, but from I 995 through 1997 did not change. Mean total length and
<br />biomass declined from 1993 through 1997 in all reaches, but most dramatically in
<br />Reaches 5 and 4. Changes in size- and age-structure of channel catfish were also noted;
<br />smaller and younger fish were more common in collections in 1997 than 1993. In most
<br />years (1994 through 1997) and Reaches flannel mouth sucker was slightly more abundant
<br />in secondary channels than the primary channel. Bluehead sucker, however, tended to be
<br />more common in the primary than secondary channels. In Reach 5, common carp were
<br />more common in secondary channels than primary channel in all years and usually so in
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