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<br />.. <br /> <br />! <br /> <br />DOUGLAS AND MARSH-GRAND CANYON CATOSTOMIDS <br /> <br />H. Clemmer (unpub!. field notes, 1976, 1980; <br />Special Coli., Hayden Library, Arizona State <br />University). Similarly, R. D. Suttkus, G. H. Clem- <br />mer, C. Jones, and C. R. Shoop [GCNP survey <br />of fishes, mammals, and herpetofauna, 1976, <br />unpub!. (hereafter GCNP survey, 1976, un- <br />pub!.)] collected three hybrids (but no X. tex- <br />anus) in the Grand Canyon. Valdez and Ryel <br />(USBR, 1995, unpub!.) captured five hybrid X. <br />texanus/C. latipinnis (332-631 mm TL) during <br />intensive mainstem sampling (four near the <br />LCR confluence). These results suggest X. tex- <br />anus (and putative hybrids) are rare in the <br />Grand Canyon region. The low numbers of pu- <br />tative hybrids we report herein are in agree- <br />ment. <br />If X. texanus is a constituent member of the <br />indigenous big-river fish community, then why <br />is it so rare in the Grand Canyon region? Sutt- <br />kus et al. (GCNP survey, 1976, unpub!.) con- <br />cluded X. texanus had been displaced in Grand <br />Canyon. They believed hypolimnetic releases <br />from Glen Canyon Dam were too cold for main- <br />channel X. texanus spawning and that the dam <br />prevented upstream movement to other spawn- <br />ing areas. They also suggested X. texanus does <br />not utilize as breeding habitat the numerous <br />smaller tributaries in Grand Canyon. These re- <br />searchers argued that, during the period when <br />Lake Powell was filling (i.e., 1958-1963), main- <br />channel water temperatures remained suitable <br />for spawning, and X. texanus hybridized with C. <br />latipinnis. Today, spawning conditions are com- <br />pletely unfavorable for X. texanus, and it is being <br />genetically swamped by the latter. <br />We agree with Suttkus et al. that X. texanus <br />and hybrids now have low population ,numbers <br />in the Grand Canyon and that hybridization has <br />occurred historically between X. texanus and C. <br />latipinnis. However, it is unknown whether pu- <br />tative hybrids captured in Grand Canyon were <br />spawned there. These individuals may have <br />been produced either downstream in Lake <br />Mead or upstream in (now-inundated) Glen <br />Canyon and are now blocked from extensive up- <br />stream movement (as per Suttkus et a!.). Xy- <br />rauchen texanus is migratory and capable oflong <br />movements (Tyus and Karp, 1990), which is of- <br />ten a characteristic of large-river desert fishes <br />(Smith, 1981; Tyus, 1990). <br />We do not believe X. texanus was once more <br />abundant within Grand Canyon. Skeletal re- <br />mains of indigenous fish species (Gila CYPha, G. <br />elegans, Ptychocheilus lucius, C. latiPinnis, and C. <br />discobolus) found in 4000-year-old deposits of <br />Stanton's Cave (GCNP, RKM 50.7) did not in- <br />clude X. texanus (Miller and Smith, 1984). To- <br />day, these species (less the extirpated G. elegans <br /> <br />921 <br /> <br />and P. Lucius) persist and comprise the indige- <br />nous big-river fish community of the Grand <br />Canyon. This, as well as other anecdotal evi- <br />dence (see above), suggest it was not a historic <br />resident of Grand Canyon but instead a tran- <br />sient. Xyrauchen texanus may have used Grand <br />Canyon as a corridor to move up- or downriver <br />to more satisfactory habitat [i.e., wider, slower- <br />flowing, noncanyon reaches of river (as per <br />Tyus, 1987:112; Minckley, 1991)]. <br />Buth et al. (1987) used allozymes to examine <br />pOltential for hybridization in C. latipinnis and <br />X. texanus collected throughout the Colorado <br />River basin. Overall, the propensity was 0-3% <br />toward X. texanus and 0-5% toward C. latipinnis. <br />Of 41 putataive hybrids morphologically iden- <br />tifi.ed in this study, nine (22%) were examined <br />electrophoretically and with RFLP analysis of <br />mtDNA. Eight of these were backcrossed. This <br />suggests two points: our morphological identi- <br />fication of putative hybrids was reasonable [in <br />that 89% (8/9) of this small sample were cor- <br />rectly identified]; and a very small hybrid sub- <br />populaton exists within the Marble Canyon <br />reach of Grand Canyon. The hybrid subpopu- <br />lation averages 30 in number, is predominantly <br />male, and frequents the LCR in spring, on av- <br />erage 2.2 kIn above the confluence. <br />To avoid ambiguity, comments (above) per- <br />taining to efficacy of morphological evaluation <br />in identification of hybrid individuals should be <br />expanded. A different picture appears when <br />capture records for the seven individuals with <br />Xyrauchen mtDNA are examined. These seven <br />were recaptured a total of 13 times. When their <br />field determination at time of capture is con- <br />trasted against their genetic background, it be- <br />comes clear that field personnel had difficulty <br />in consistently recognizing hybrid individuals. <br />The seven were listed as C. latiPinnis 53% (i.e., <br />8/15) of the time, as C. latipinnis hybrid 40% <br />(6/15) of the time, and as X. texanus 7% (1/15) <br />of the time. There are several reasons for these <br />discrepancies. When field biologists weigh, mea- <br />sure, and tag large numbers of bulky fishes, de- <br />tail is often sacrificed for expediency. Individual <br />researchers also vary in their abilities to consis- <br />tently recognize key phenotypic characteristics <br />(i.e., "the art of seeing well"; Douglas et a!., <br />1989). These aspects are compounded when <br />phenotypic characteristics are muted by several <br />generations of backcrossing (as herein). Thus, <br />given the above, variance in field designation to <br />species is to be expected. Nevertheless, on oc- <br />casion, anecdotal phenotypic information is re- <br />corded which aids in diagnosis. For example, <br />individual "7F7DIB780C" was captured seven <br />times; it was designated as C. latipinnis in 43% <br />