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<br />920
<br />
<br />COPEIA, 1998, NO.4
<br />
<br />are particularly encouraging for a species of
<br />concern. However, these results may be mislead-
<br />ing; the adult population may appear numeri-
<br />cally steady because adults are long-lived, rather
<br />than because of extensive recruitment. This was
<br />the situation with X. texanus in Lake Mohave,
<br />Arizona, where adult numbers were steady for
<br />more than a decade (Minckley, 1983; Minckley
<br />et aI., 1991; Dowling et a!., 1996b). Despite
<br />abundant seasonal reproduction, a chronic lack
<br />of recruitment (Horn, 1996) will inexorably re-
<br />duce the Lake Mohave X. texanus population to
<br />extinction unless effective intervention occurs
<br />(as per Dowling et aI., 1996b:125-126).
<br />Longevity is a recognized and effective adap-
<br />tive strategy for catostomids in western North
<br />American rivers (Smith, 1981; Douglas, 1993).
<br />It provides a mechanism by which vagaries of
<br />the environment are counterbalanced over evo-
<br />lutionary time. Adults may miss numerous se-
<br />quential spawning seasons due to a variety of
<br />physiological or environmental reasons (e.g.,
<br />lack of accrued fat resources; shortage of suit-
<br />able breeding habitat due to flow characteris-
<br />tics; restricted access to tributaries; ete.). If re-
<br />production does occur, prolonged drought and
<br />concomitant low-water may restrict develop-
<br />ment of backwater nursery habitat. Other fac-
<br />tors known to decimate a year class (Horn,
<br />1996) are extreme predation, exogenous nutri-
<br />ent depletion, and entrainment of larvae within
<br />current. The species maintains itself because
<br />probability of achieving an eventual abundant
<br />year class is enhanced due to adult longevity.
<br />This adaptation can also foster benign ne-
<br />glect by managers because potential for swift re-
<br />covery in these long-lived fishes is deemed
<br />great. But adult senescence can be rapid and
<br />drastic (as noted in Lake Mohave X. texanus).
<br />Thus, it is important not only to establish a base-
<br />line for adult longevity but also to define sur-
<br />vival rates according to age. Studies that attempt
<br />to determine age of individual catostomids of-
<br />ten have conflicting results, and suitability of bi-
<br />ological materials used in these analyses is ques-
<br />tioned. McAda (1977) and McAda and Wydoski
<br />(1985) used scales to ascertain an age of eight
<br />or nine years for upper basin G. latiPinnis. Other
<br />researchers (Usher et aI., 1980; Minckley, 1983;
<br />McCarthy and Minckley, 1987) argued that the
<br />regenerative capability of scales make them in-
<br />appropriate media from which to determine
<br />maximum age. In addition, scale annuli are of-
<br />ten unreadable after the first few years of life, a
<br />condition which adds to unreliability of (and el-
<br />evated variance in) this medium. Usher et aI.
<br />(1980) and Carothers and C. O. Minckley
<br />(USBR, 1981, unpubI.) used opercular bones to
<br />
<br />estimate a maximum age of 10 years for G. la-
<br />tipinnis in Marble/Grand Canyons. Minckley
<br />(1991) suggested these were underestimates.
<br />His otolith data (unpub!., 1991) estimated
<br />Green River G. latipinnis (TL = 530 mm) at 30
<br />years, whereas Scoppettone (1988) judged five
<br />individuals (TL = 530-590 mm) from the same
<br />area at > 17 years of age
<br />Given inherent limitations of scales and oper-
<br />cular bones to age big-river endemic fishes, we
<br />grouped individuals for analysis by overall body
<br />length. This offers a simple and repeatable
<br />mechanism, even though cut points are recog-
<br />nizably arbitary. With this approach, we noted
<br />reductions in survivability among the largest
<br />size classes (i.e., 301-600+ mm TL). Yet, during
<br />this period, population estimates showed an
<br />overall increase. Predation is an improbable
<br />cause for a reduction in survivability because
<br />the decline occurred in larger fish. Alternative
<br />hypotheses that appear more viable include nat-
<br />ural variability in long-term movement patterns
<br />of large adults and senescence. Neither was test-
<br />able within the temporal framework of our
<br />study.
<br />
<br />Xyrauchen texanus and hybrids with Catostomus la-
<br />tipinnis.- The first record of X. texanus in Grand
<br />Canyon was by an angler in 1944 at Bright An-
<br />gel Creek (RM 87.8; National Park Service files;
<br />cited in S. W. Carothers and .C. O. Minckley,
<br />USBR, 1981, unpub!.). A single specimen was
<br />caught in 1963 somewhere in the region from
<br />Paria River confluence (RM 1) to Lee's Ferry
<br />(RM 0) [AZ Game and Fish records, cited in
<br />Carothers and Minckley, USBR, 1981, (un-
<br />pubI.); Minckley et a!., 1991; SWCA Ine., Grand
<br />Canyon Data Integration Project, Final Rpt.,
<br />U.S. Bureau of Reclamation, 1997]. In 1978, a
<br />gravid female was captured (and two others ob-
<br />served) in the PariaRiver 100 m above its con-
<br />fluence with the mainstem (Minckley and Ca-
<br />rothers, 1980). The last reported capture (spec-
<br />imen photographed and released) was above
<br />Bass Rapids (RM 107.7) in 1986 (cited in Ca-
<br />rothers and Brown, 1991:330). Extensive elec-
<br />trofishing by Carothers and Minckley (USBR,
<br />1981, unpub!.) in 1977-1979 never resulted in
<br />a mainstem capture of X. texanus. Similarly, Val-
<br />dez and Ryel [Life History and Ecology of the
<br />Humpback Chub (Gila CYPha) in the Colorado
<br />River, Grand Canyon, AZ, Final Rpt., U.S. Bu-
<br />reau of Reclamation, 1995, unpub!. (hereafter
<br />USBR, 1995, unpub!.)] failed to capture X. tex-
<br />anus during extensive fieldwork from 1990-
<br />1995.
<br />A putative hybrid X. texanus/G.latiPinnis was
<br />collected at the mouth of the Paria River by G.
<br />
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