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<br />002835 <br /> <br />overemphasized based on the evidence of historic hybridization within the Gila complex that <br />predates human modification of the river system (Dowling and DeMarais 1993), Hybridization <br />in Gila is in part a natural phenomenon (McElroy and Douglas 1995), Given that some <br />hybridization between bonytail, humpback chub, and roundtail chub in the wild is possible and <br />natural, it may be a negligible risk considering the 1) niche separation among these three <br />species, 2) factors contributing to the bony tail's decline and disappearance, and 3) preliminary <br />evidence for hybridization, <br /> <br />Kaeding et al, (1990) and Vanicek and Kramer (1969) suggest ethological mechanisms <br />may separate bonytail spatially from humpback and roundtail chub during spawning based on <br />field collection results. In looking at the apparent niches occupied by the three Gila species in <br />the Upper Basin, roundtail chub appear to be largely a tributary stream species, being most <br />abundant in the Yampa, White, upper Colorado, Gunnison, and Dolores rivers; while the <br />humpback chub is largely a deepwater, canyon species, being found in Yampa, Westwater, <br />Cataract, Desolation-Gray and Black Rocks canyons, This leaves the main stem rivers and <br />floodplains as an "available" niche for another Gila species, such as the bonytail. As <br />speculation, it is possible the significant loss of floodplain habitat with increasing abundance of <br />nonnative fish predators and competitors in remaining floodplain habitat could account for the <br />disappearance of bonytail populations except for the population of large, senile adults inhabiting <br />Lake Mohave, If the dramatic disappearance of bonytail were the result of hybridization <br />catalyzed by habitat modification, there has been no evidence of such a significant event in the <br />genetic material of the remaining Gila populations sampled for such via moxpho-meristic, <br />allozyme, and DNA techniques (Dowling and DeMarais 1993, McElroy and Douglas 1995), <br />This hybridization event would have, in effect, subsumed the bony tail population genome into <br />the genetic material of the other Gila species virtually without a trace, This is difficult to <br />accept, and makes the supposition that hybridization between stocked bony tail and humpback <br />chub or roundtail chub to the detriment of the latter two species also less credible, The risk of <br />extinction and permanent loss of wild bony tail populations are considered to outweigh the risk <br />of potential hybridization from reintroduction stocking, <br /> <br />DRAFT - June 4, 1997 <br /> <br />11 <br />