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<br />C:J <br />C:I <br />~ <br /><.0 <br />..... <br />en <br /> <br />limiting factor among a myriad of other adverse impacts to native fish populations (Gido and Propst 1999) <br />can be even more difficult. Unrelated species, such as the endemic fish fauna of the San Juan River and <br />nonnatives introduced largely from the Mississippi River and other eastern river systems, will often compete <br />if resources and habitats are limited, A number of factors and complicated interactions can affect <br />competitive interactions, including temporal shifts in resource use and ontogenetic size-related changes in <br />preferred habitats and diet (Wooten 1990, Gido and Propst 1999). <br /> <br />Nonnative fishes numerically dominate low-velocity habitats in the San Juan River (Archer and Crowl <br />2000a, Propst and Hobbes 2000). Yet whether nonnative fishes force native fish species into less-suitable <br />habitat where resources are limited, or whether they out-compete the native species for food sources in <br />short supply, thereby reducing their ability to grow, reproduce, and recruit into the population, still needs <br />to be determined. Studies completed on the San Juan River provided some preliminary answers. <br /> <br />There was considerable overlap observed in habitat use among native and nonnative fish species in the San <br />Juan River (Gido and Propst 1999). Speckled dace did not appear to suffer from competitive interactions <br />with nonnative fishes because they occupied riffle habitats as juveniles and adults, No nonnative fishes <br />currently inhabiting the San Juan River preferred riffle habitat, although red shiner and juvenile channel <br />catfish may have used riffles at certain times (K, Lawrence, Ecosystems Research, Personal <br />Communication). Gido and Propst (1999) reported a temporal separation between native and nonnative <br />larval fishes (excluding common carp) because of the earlier spawning of native species. This may reduce <br />the potential for competitive interactions among larval fishes, Although bluehead sucker and flannelmouth <br />sucker decline was observed since 199 I below Reach 6, juveniles are stilI common and condition factors <br />of both species increased over the same time period. Stocked Colorado pikeminnow and razorback <br />sucker reintroduced into the San Juan River in 1996 to 1997 and 1994 to 1997, respectively, had relatively <br />high swvival rates and appeared to find adequate habitat. Therefore, there is little evidence to suggest that <br />competition from nonnative species negatively affected native species in the numbers present during the 7- <br />year research period, <br /> <br />Brooks et al, (2000) investigated food limitations by looking at dietary overlap and food availability for <br />smalI native fishes and nonnative fishes. They found that most small fish consume benthic invertebrates, <br />especially dipterans and ephemeropterans, and that channel catfish and red shiner have the greatest dietary <br />diversity, The food-availability studies showed that invertebrates in the San Juan River were generally more <br />abundant in spring samples than in autumn samples (Figure 3. I 9) and that density of invertebrates was <br />lower than in other upper Colorado River Basin streams, but that there was considerable variation in the <br />data, Dipterans and ephemeropterans were generally the most-abundant forms of benthic invertebrates <br />in both main and secondary channels. <br /> <br />September 2000 <br /> <br />3-47 <br /> <br />Program Evaluation Report <br />