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<br />vol. 28, no. 2
<br />Fte. l.-Sketch map of the lower holorado River basin, United States and Mexico, showing fea-
<br />tures mentioned in text and locality records (or razorback sacker. Symbols: ~ =record localities,
<br />1972-81; O =record localities, 1850.1971; and O =records from archaeological sites. This map
<br />supersedes and corrects that published by Holden (1980).
<br />river" fishes in general. Current attempts to perpetuate and re-establish the
<br />species also are reviewed. It is concluded that predation by introduced fishes
<br />on early life-history stages of razorback sucker is the most important single
<br />factor in their extirpation.
<br />STCDY AREA.-The lower Colorado River is defined as that portion of the system downstream
<br />from 1,ra6,ECRY„Irir~, The mainstream lies within some of the most arid lands in North Amer-
<br />ica, and (lows through ahemating reaches of spectacular canyons and broader strucwtal valleys.
<br />Only tour major tributaries enter this reach, from up- to downstream the';;,
<br />~?+~+td. 1).
<br />hake Mohave is a mainsveam reservoir about midway on the lower Colorado River. h was
<br />filled in the early 1950s m form a long (ca. 109 km) and relatively deep (99 m maximum)
<br />impoundment. The lake is generally less than 1.5 km wide between canyon walls, but widens to
<br />just more than 6.0 km at a central basin. Surface area at lull pool (197 m elevation) is about 119
<br />kmr (Jonez and Sumner, 1959; LaRivers, 1962). The shoreline (length ca. 900 km) is complex and
<br />irregular as a result of numerous washes, and the surroundings are typical Mohave desertscrub
<br />vegetation (Brown and Lowe, 1978). The reservoir is subject to severe, seasonal wind action as a
<br />result of its long south-north (etch and prevailing winds from south southwest. Bottoms along
<br />shore are gravel and cobble, resulting from waves (to >1.5 m high) exhuming ancient riverine
<br />terraces. Deeper areas have sand-silt substrates. Near-shore turbidity is caused by wave action, but
<br />the lake otherwise remains clear since water ewers directly from the hypo]imnion of Lake Mead.
<br />The reservoir is stable by southwestern standards, with a seasonal draw-down of only about 5.0 m.
<br />MErrtous AND Materials.-Review of past and present records for razorback sucker in the lower
<br />Colorado Ricer basin depended heavily on [aRivers (1962), Minckley (1979), reports by the U.S.
<br />Fish and Wildlife Service (Anonymous, 1980, 1981), and on unpublished reports and personal
<br />communications from workers active in the region (noted in text). William Loudermilk, Califor-
<br />nia Fish and Game Depanmem (CFGD), provided data Gom California not otherwise acknowl-
<br />edged in text. Robes R. Driller, University of Michigan Museum of Zoology (UMh1Z), provided
<br />data on specimens housed at that institution.
<br />May 1989 Minckley-Stains of Razorback Sucker 167
<br />Fishes were sampled from 1975.1977 and 1979-82. Field work was concentrated within 5.0 km
<br />up- and downlake bum Carp Cove, Mohave County, Arizona (rest of CottonwuuJ Lending, Clark
<br />County, Neaada), in March 1975 immediately south o[ Hoover Dam, in March 1981 on the Nevada
<br />side weso-southwest of Carp Cove, and in January 1982 in Arizona Bay. Collections from through-
<br />out the central basin of the reservoir }•ielded similar data. Voucher specimens are housed in the
<br />Arizona Stare University Collection of Fishes (ASL').
<br />Trammel nets Gom 805 to 219.9 m long were used In collect fishes. They were 1.9 to 2.4 m deep
<br />and had meshes (bar measure) o[ 25.9 to 95.6 crrr over walls and 2.5 to 9.1 cm inner walls (most
<br />were 90.5 and 8.8 cm, respeaivefy). Nets were set horizoma]ly, 25 to 200 m offshore with float
<br />lines 0.5 to 5.0 m beneath the surface, bouyed at imervals along their lengths, and marked with
<br />appropriate identification as navigation hazards and scientific devices. A few were set near or on
<br />bottom on each sampling date. Attempts were made to equitably sample coves, open lake, and
<br />areas o[ turbulence downwind of submerged terraces. Nets were generally set at right angles to
<br />prevailing waves, which most often resulted in angles of about 60° relative to local shorelines.
<br />Nets were run and cleared oI fishes at 2- to 8-hour intervals, day and night, depending upon catch
<br />rates and weather, and generally were allowed to bsh m the same position for at least two 29-hour
<br />periods. Catch rates are expressed as numbers of individuals per 100 m' of netting per 29-hour
<br />period
<br />Other gear employed to compete a species Irst, anempt to catch smaller lishes, and to collect
<br />aduhs for hatchery stocks included experimental gill nets with meshes that varied from 1.8 to 5.1
<br />cm, tyke and hoop nets of various sizes and meshes, seines varying from 1.0 to 22.9 m long, and
<br />with meshes from 8.2 mm to L5 cm (all bar measure), and 110- and 220.vo1t, A.C.!D.C. electro-
<br />fishing gear. Data collected by these methods x~ere rarely quamified. Fishes taken by Arizona
<br />Game and Fish Department (AGFD) were by electrofishing barge at night, generally in shallow
<br />water along shore. All reported lengths of fishes are to the nearest cm from snout to eip of
<br />depressed caudal tin (TL; total length). Standard lengths (SL; snout to end of hypural platy) were
<br />used (or some morphological comparisons and for compmation o[ relative fecundity. SL = 0.789
<br />t 0.012 TL in 100 fish 92 to 69 cm TL. Mean values of length and other features are given t one
<br />standard error unless otherwise noted.
<br />Scales for attempts at aging razorback suckers were removed from midway between the dorsal
<br />tin and lateral line, cleaned of tissue in potassium hydroxide solution, and mourned between glass
<br />slides for examination at appropriate magnification on a Bausch and Lomb Scale Reader. Bark
<br />calculation of ratios of scale radius to TL was by direr proportion. Laboratory and hatchery
<br />information on growth of razorback suckers was obtained at ASU and at Willow Beach and Dexter
<br />National Fish hatcheries (in pan Toney, 1979).
<br />A total of 89 adult razorback suckers ryas analyzed for sexual dimorphism: all were collected
<br />from Lake Mohave between 1966 and 1975 and are housed at ASLI or UMMZ. Measurements of
<br />body pans followed methods of liubbs and Lagler (1979). Breeding coloration and mbereulation
<br />were recorded in the field. -
<br />Mature ovaries of five irmale razorback suckers were excised for investigation of fecundity.
<br />Number of eggs within ovaries was estimated volumevicallp (Kandler and Piriwitz, 1957). Eggs
<br />were separated from ovarian tissue, total volume of ova for each fish was recorded, and two ].0 ml
<br />subsamplrs were counted. Relative fecundity estimates are expressed in terms of ova cm SL
<br />(Bagenal and Braum, 1978).
<br />ResutTS.-Historical and Present Distrihutions.-Razorback suckers have
<br />been widely recorded in the lower Colorado River basin. Early distributional
<br />records include the original description (Abbott, 1861) from the "Colorado
<br />and New Rivers," and a re-description (as Catoslomus cypho) by Locking-
<br />ton (1881) from the Yuma area. The species penetrated far onto the Colo-
<br />rado Delta in Mexico (Follett, 1961), and upstream throughout the main-
<br />stream Colorado River (Jordan, 1891; Jordan and Evermann, 1896;
<br />Evermann and Rutter, 1895; Gilbert and Scofield, 1898; Grjnnell, 1914;
<br />Snyder, 1915; Douglas, 1952; Rostiund, 1952; Miller, 1955, 1961; Stewart,
<br />1957). The Fish occupied the SaNon Sea in aboriginal times (Wilke, 1980),
<br />supporting a Lakeshore fishery until presumably extirpated by evaporative
<br />
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