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172 The Southwestnn Naturalist vol. 2B, no. 2 May 1983 Minckley-Stains of Razorback Sucker 173
<br />TABLE 3.-(,at[hes OJ razorback suckers per 100 m= of trammel Hefting per 24-Nom period of aar-
<br />lous localities in !hc upper folarado Rroer basin. Data re-ralculaled Jrom McAda and N'ydoski
<br />11980); percen[ages are in parrn(heses,
<br />Tonal (a,ch pn I'ni~ Effort
<br />[-acalisies ~'o. Fish Razorhack A11 Fish
<br />Echo Park, UT
<br />May 1975 102 0.24 19.90
<br /> (L2)
<br />Walker N'ild~life Area, CO
<br />Apri11975 543 5.95 190.64
<br /> (9.2)
<br />May 1975 514 1.25 35.52
<br /> (3.5)
<br />June 1975 600 0.46 38.52
<br /> (3.5)
<br />Green River, UT
<br />December 1975 103 1.13 29.36
<br /> (0,5)
<br />Apri11976 61 0.29 6.79
<br /> (3.6)
<br />thus no indications o[ a population decline in razorback suckers in Lake
<br />Mohave based on trammel netting.
<br />Hybrids between razorback and [lannelmouth suckers comprised a small
<br />percentage o[ the total catch in 1975. They were not again taken by me, but
<br />Paulson (UNLV, pers. comm.) captured hybrids in 1982. All were large, >50
<br />cm TL, indicating ages comparable to those of the razorback sucker popula-
<br />tion. Flannel mouth suckers have not been collected from Lake Mohave
<br />since the 1950s. This hybrid combination has previously been recorded from
<br />the upper Colorado River basin (Hubbs and Miller, 1953), and from the
<br />Grand Canyon region (Suttkus and Clemmer, 1979).
<br />McAda and N'ydoski (1980) provided the only other quantitative data
<br />known to me on relative abundance of razorback suckers. They used similar
<br />trammel nets at various locations in the upper Colorado River basin in
<br />1974-76, finding the sucker as 3.3 to 9.0% of all fishes caught at Echo park,
<br />Colorado (near the confluence of the Green and Yampa rivers), 0.8 to 4.2°k
<br />in a gravel pit near Grand Junction, Colorado (Colorado River), and 0.0 to
<br />3.3X on the Green River, Utah, at various localities. Some of their data on
<br />catch per unit effort are reproduced in Table 3.
<br />Age and Growth.-I have no evidence for recruitment into the adult
<br />razorback sucker population o[ Lake Mohave since sometime prior to 1964.
<br />On the basis of size-frequency distributions of wild-taught fish, growth since
<br />1964 has proceeded at an average o[ about 0.5 cm per year (Fig. 2). Larger
<br />fish are females (97 to 74 cm TL) and smaller ones are males (37 to 64 cm).
<br />McAda and Wydoski (1980) reported similar size limits for adult fish from
<br />the upper Colorado River basin, with females from the Colorado River
<br />bett,'een 50 and 62 cm TL and those from the Yampa and Green rivers from
<br />46 to 54 cm. Males from those areas were from 48 to 52 cm and 44 to 52 cm
<br />TL, respectively.
<br />Bias toward large fishes by major sampling techniques used in Lake
<br />Mohave (trammel nets and electro[ishing) may be rejected since numerous
<br />83
<br />7O 223
<br />30 ]3 !69
<br />5]
<br />6
<br />60
<br />st
<br />~o 'z (a zo -
<br />iz /'~
<br />50 - - -
<br />40
<br />FS 7C] 75 BO
<br />YEARS
<br />Fse. 2.-Total lengths of razorback suckers collected in Lake Mohave, Arizona-Nevada, in 1969-
<br />69 and 1975-81. Horizontal lines are means, bars represem tl standard error, and vertical lines
<br />represent the range of lengths; numbers are total sample sizes; open bats are females, closed bars
<br />are males, when sexes were determined.
<br />fishes of other species <15 cm TL are readily caught by both methods. Vari-
<br />ation in TL [or razorback suckers taken by trammel nets in the period 1975-
<br />8] was, however, far greater than that indicated from electrofishing in earlier
<br />years. Perhaps the former technique samples more diversity of habitats than
<br />the latter, providing a broader picture of the population. Hubbs and Miller
<br />(1959) also noted a "paucity of young in collections," and suggested that
<br />young moved with adults "into larger and deeper waters, which have been
<br />neglected by collectors." It is possible that young razorback suckers live at
<br />great depth in Lake Mohave, and have thus avoided capture. They have not
<br />occurred inshore. No razorbacks <37 cm TL have been collected in extensive
<br />seining operations by myself and others (Gail Kobetich, Don Toney, and
<br />Jerry Burton, USFWS, pers. comm.), in annual electrofishing surveys by
<br />AGFD or Nevada Game and Fish Department (NGFD) (Kraig Burkstrand,
<br />pers. comm.), or in my sampling with devices other than trammel nets.
<br />Any occurrence o[ small razorbacks suckers in Lake Mohave after 1978
<br />may result, unfortunately, from the escape of a substantial number of indi-
<br />viduals (ca. 200) <30 cm TL Nom Vlrillow Beach (James E. Johnson,
<br />USFWS, pers. comm.). This perhaps explains the report of " 25-30 cm"
<br />razorback suckers by Paulson et a1. (]980bj and Liles (1981) in upper Lake
<br />Mohave near Hoover Dam, in the vicinity of the hatchery.
<br />Scales of razorback suckers are highly variable, small in size and irregular
<br />in shape on an individual fish. Width of posterior, anterior, and lateral radii
<br />was determined [or each of 2 to 5 scales of 20 fish sacrificed in 1975. Regres-
<br />sion for each o[ these variables against TL gave correlation coefficients of
<br />0.55, 0.57 and 0.61 for each radius, respectively. The regression for lateral
<br />radius (LR) against TL was LR = 2.899 4- 0.126 (TL). Agreement among
<br />
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