|
I q ~3
<br />THE SOUTHWESTERN NATURALIST 28(2):165-187 MAY, 1983
<br />STATUS OF THE RAZORBACK SUCKER,
<br />XYRAUCHEN TEXANUS (ABBOTT), IN
<br />THE LOWER COLORADO RIVER BASIN
<br />W. L. HINCKLEY
<br />Aesraecr.-The razorback sucker fwpula[ion of the lower Colorado River basin is now reduced
<br />to scattered individuals in all but Lake Mohave, Arizona-Nevada. That population consists of
<br />large, slowly-growing fish, which are proposed to be nearly 30 years old, presumably having
<br />hatched when the reservoir was filling in the early 1950s. The species comprised about 12.5% of all
<br />fishes taken by trammel netting in Lake Mohave in the period 1979-82. No recruitment into the
<br />population has been detected in that period, despite repeated observations o[ spawning and
<br />records of fertilized eggs and hatched larvae. Sex ratios, sexual dimorphism, and fecundity all
<br />indicate the fish [o be reproductively capable despite a high incidence of disease. The species is
<br />now under anificial propagation, and is being reintroduced within its native range. Predation by
<br />introduced Eishes on early lice-history sages of razorback sucker is considered [he most importam
<br />factor in their decline.
<br />Of four endemic "big-river" fishes of the Colorado River basin, Colorado
<br />squawfish (Ptychocheilus Lucius Girard), humpback chub (Gila cypha
<br />Miller), bonytail chub (Gila elegans Baird and Girard), and razorback sucker
<br />(Xyrauchen texanus [Abbott]), only the last has yet to be listed as endan-
<br />gered by the U.S. Department of Interior (Johnson and Rinne, 1982). Habi-
<br />tats for all these fishes have been reduced by development of the river for
<br />domestic and industrial (including agricultural) water supplies and power,
<br />and further reductions may be expected as human demands increase
<br />(Wydoski et aL, 1980).
<br />Sampling of fish populations by myself, students, and colleagues since
<br />1963, throughout the lower Colorado River basin, has documented changes
<br />in disu~ibution and abundance of indigenous species (Minckley, 1965, 1973,
<br />1979; Minckley and Deacon, 1968). Although all native fishes have demon-
<br />strated reductions in range, local populations of many species yet remain
<br />abundant. Razorback suckers have become increasingly rare in all but Lake
<br />Mohave, Arizona-Nevada, and systematic sampling to determine life-history
<br />characteristics of that population was commenced in 1974. Lake Mohave is
<br />inhabited mostly by introduced fishes, with only two native species, razor-
<br />back sucker and bonytail chub, persisting as a small percentage of the over-
<br />all fauna. Proposed changes in powerplant specifications and operations of
<br />Hoover Dam, development o[ pump-storage facilities, and re-regulation o[
<br />water levels in Lake Mohave (Paulson el al., 1980a-b) soon may influence
<br />these fishes, and little is known of their requirements in the system. In tact,
<br />there has long been a critical lack of information of these and other big-river
<br />fishes of the Colorado River basin (Miller, 1946, 1961; Branson, 1966; Minck-
<br />ley and Deacon, 1968; Minckley, 1965, 1973, 1979; Wydoski et al., 1980).
<br />This report reviews information on razorback sucker in the lower Colo-
<br />rado River basin, presents results of studies o[ the species in Lake Mohave
<br />during the period 1974-82, and examines reasons for decline of the "big-
<br />~~~~7
<br />.~---
<br />~ 1-.~• i' ~• ~ ~-
<br />~~yt!~•~, .i. ~.v
<br />~ ~'~~
<br />
<br />
|