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<br /> <br />CROSS-MOAPA RIVER FISHES <br /> <br />TABLE 5.-Spatial overlap of Moapa River fishes <br />(excluding headwater springs data) for 1964-68 <br />(above) and 1974-75 (below). Numbers in column <br />headings refer to the respective species listed at <br />lelt. Native species denoted with an asterisk (.). <br />1964-68 data from Deacon and Bradley (J972). <br /> <br />2 3 4 5 6 7 8 9 10 <br /> <br />1 Gila rob una. 28 37 28 34 6 5.5 57 1 1 <br /> 21 52 21 47 o 42 58 0 0 <br />2 Rhinichthys osculus. 46 39 22 2 22 .51 0 0 <br /> 37 49 22 o 58 37 0 0 <br />3 Moapa coriacea. 93 14 o 23 68 0 0 <br /> 87 5 o 55 64 0 0 <br />4 Crenichthys baileyi. 14 o 23 65 0 0 <br /> 5 o 68 64 0 0 <br />5 Cyprinus carpio 72 34 25 67 26 <br /> 45 22 29 40 43 <br />6 N otropis lutrensis 8 6 97 40 <br /> 8 o 93 96 <br />7 Gambusia affinis 52 2 2 <br /> 63 1 4 <br />8 Poecilia mexicana 0 0 <br /> 0 0 <br />9 MicropteTUS salmoilles 38 <br /> 97 <br /> <br />10 Chaenobryttus cyaneUus <br /> <br />and Bradley 1972; Deacon et a1. 1964; Hubbs <br />and Deacon 1964; Wilson et a1. 1966), and <br />the physical and chemical deterioration of the <br />habitat. <br /> <br />Species Overlap <br /> <br />Establishment of nonnative species in the <br />Moapa River has led to changes in niche <br />utilization by the fishes (Deacon and Bradley <br />1972). An estimate of spatial overlap was <br />calculated based upon the extent of common <br />occurrence of two species at each station in <br />the river (Table 5) as outlined in Deacon and <br />Bradley. For example, 49%, 31%, and 4% of <br />the R. osculus sample came from Stations 3, <br />4, and 6 respectively, while 5%, 1.3%, and 37% <br />of the Cyprinus carpio sample was collected <br />at those stations. The spatial overlap was 5% <br />at Station 3, 13% at Station 4" and 4,% at <br />Station 6 for a total of 22%. High values for <br />percent spatial overlap indicate common use <br />of stream space suggesting potential competi. <br />tion (Deacon and Bradley 1972). <br />High overlap values existed between native <br />species and the shortfin molly, mosquitofish, <br />and carp [overlap values between native (C. <br />baileyi and M. coriacea) and introduced (P. <br />mexicana and G. affinis) fishes would have <br />heen higher if headwater springs data had <br />been included]. Overlap between M. coriacea <br /> <br />507 <br /> <br />and G. affinis increased since 1964-68 while <br />overlap decreased between the Moapa dace <br />and the other native species indicating the <br />decline in abundance of this endemic. Over- <br />lap between G. affinis and R. osculus and C. <br />baileyi also increased. Deacon and Bradley <br />(1972) concluded that P. mexicana and G. <br />affinis had partially taken over the niches of <br />the four native species. <br />Physical alterations, water quality deterio. <br />ration, and introduced fishes have led to a <br />disruption of the natural habitat of the native <br />Moapa River fish fauna. Deacon and Bradley <br />(1972) noted a shift upstream in the popu. <br />lation centers of R. osculus and G. robusta <br />from 1964 to 1968. Data from 1974-75 indi. <br />cate reductions in the range and abundance <br />of Moapa coriacea [an endangered species <br />(United States Bureau Sport Fisheries and <br />Wildlife 1973)] and Gila robusta. Native <br />fishes occupied only the upper half of the <br />Moapa River. The shrinking of the habitable <br />range of the native species seems to have <br />coincided with expansion of the range of <br />introduced species and deterioration of the <br />habitat. <br />The population of the relict endemic Moapa <br />coriacea appears to he distinctly threatened. <br />Survival of the native species in the Moapa <br />River may depend upon cessation of further <br />biotic, physical, and chemical alterations. <br /> <br />ACKNOWLEDGMENTS <br /> <br />The following people assisted in the field; <br />their help is gratefully acknowledged: H. A. <br />Berry, 1. E. Deacon, J. Holland, G. Kobetich, <br />P. J. Leary, J. Lockhart, R. Meinke, D. O. <br />Schreiweis, and D. Thomas. P. 1. Leary and <br />R. L. Summers kindly provided data on water <br />quality for the Moapa River in 1974. <br /> <br />LITERA TURE CITED <br /> <br />AMERICAN PUBLIC HEALTH ASSOCIATION. 1%5. SIan. <br />dard methods for the examination of water and <br />wastewater, 12th cd. American Public Health <br />Association, American Watcr Works Association, <br />and Watcr Pollution Control Fcdcration. Ncw <br />York. <br />ANONYMOUS, 1962. Moapa Valley pumping project. <br />Nev. Bur. Rec. Recon. Rcp. Bouldcr City, Nevada. <br />CROSS, J. N. 1975, Ecological distribution of the <br />fishes of the Virgin Rivcr (Utah, Arizona, Ne- <br />vada). M.S. Thesis. Univ. Nevada, Las Vegas. <br /> <br />"~" <br />