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<br />red shiner were either the first or second most-abundant species in main channel nursery areas in late
<br />summer and falI (Archer and Crowl 2000a). These data suggest that, numerically, red shiner are likely one
<br />of the most-abundant fishes in the San Juan River. Other predatory fish species of concern were walleye
<br />(Stizostedion vitreum) and striped bass (Morone saxatilis), which were collected from 1995 to 1997
<br />after they gained access to the San Juan River when a waterfall barrier was inundated by Lake Powell
<br />during 1995, Numerically, these two lacustrine predators were not collected in large numbers and
<br />comprised less than 0.1 % of total fishes collected. Overall, the San Juan River has a high density of
<br />potential predators on eggs, larvae, juveniles, and adult native fishes.
<br />
<br />Diets of potentially piscivolUs nonnative fishes collected during main channel adult monitoring efforts during
<br />199 I to 1993 were examined by Brooks et ai, (2000), as were diets of fishes collected from 1994 to 1996
<br />in low-velocity habitats. Empty stomachs were excluded from the diet analysis. Because of their high
<br />abundance in the river, channel catfish were a primary target of the analysis. Piscivory by channel catfish
<br />in the San Juan River was infrequent, with only 7% of the stomachs containing fish (Brooks et al, 2000),
<br />Fish consumed by channel catfish were primarily flanneImouth sucker, although speckled dace, bluehead
<br />sucker, and red shiner were also consumed, Channel catfish diets primarily included a variety of aquatic
<br />insects and Russian olive fruit, as well as other vegetation, and piscivory increased with size of the catfish.
<br />
<br />Eleven striped bass stomachs were examined, and five contained fish (the other six were empty). 0f38
<br />walleye stomachs examined, 17 contained fish and 21 were empty. Black bullhead (Amerius melas),
<br />green sunfish (f-epomis cyanellus), bluegill (Lepomis macrochirus), smalImouth bass (Micropterus
<br />dolomieu), largemouth bass (Micropterus salmoides), and brown trout (Salmo trutta) stomachs were
<br />also examined for food. Of the 24 stomachs examined I3 contained fishes, and identifiable remains
<br />included flanneImouth sucker, speckled dace, red shiner, fathead minnow, and mosquitofish (Brooks et al.
<br />2000), Although not broken out by species, it was reported that all species except bluegill had fish in their
<br />stomachs. None of the nonnative fish species colIected from low-velocity habitats from 1994 to 1996
<br />exhibited piscivory.
<br />
<br />Predation by large-bodied nonnative fishes on native fish species in the San Juan River was documented
<br />by the SJRIP studies, but the extent of predation appeared to be low, Channel catfish exhibited piscivory
<br />in only I year (1993) of six sampled, and at total levels similar to the 7% reported in the Green and Yampa
<br />rivers (Tyus and Nikirk 1990). Other nonnative fishes demonstrated piscivory, but none of those species
<br />made up more than 0.1 % offish collected in standardized electrofishing sampling from 1991 to 1997.
<br />
<br />Predation on larval native fish by the numerous small bodied nonnative cyprinids, such as red shiner, in the
<br />Upper Basin was recently highlighted as, perhaps, a greater concern than predation by larger piscivores
<br />(Ruppert et al. 1993, Bestgen et al. 1998), Brandenburg and Gido (1999) investigated larval predation
<br />by a number of potential predators from San Juan River backwaters in early sununer when many native fish
<br />larvae were present. Only eight of 529 predators contained larvae, and less than 1 % of 4 I 4 red shiner
<br />contained larvae. This study showed that nonnative fish, including red shiner, eat larval native fish, but
<br />suggested the incidence of predation was not large,
<br />
<br />September 2000
<br />
<br />3-45
<br />
<br />Program Evaluation Report
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