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<br />DOUGLAS AND MARSH-GRAND CANYON CATOSTOMIDS
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<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
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<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%
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