|
f t
<br />AGE ESTIMATION FOR RAZORBACK SUCKER
<br />(PISCES: CATOSTOMIDAE) FROM LAKE MOHAVE, ARIZONA AND' NEVADA
<br />MICHAEL S. McCARTHY and W. L. MINCKLEY
<br />Arizona State University
<br />Department of Zoology
<br />Tempe, AZ 85287
<br />ABSTRACT
<br />Seven bony structures (opercular, cleithral, and branchiostegal bones, vertebrae, scales, pectoral fin rays, and otoliths) were
<br />examined for usefulness in age estimation for razorback sucker, Xyrauchen texanus (Abbott), from Lake Mohave, Arizona
<br />and Nevada. Of these, only sectioned otoliths appeared reliable in larger (older) fish. Seventy specimens were estimated
<br />to vary from 24 to 44 years old at capture in 1981-83. Recruitment is low or non-existant, with all individuals taken in
<br />recent years being large adults. If a pattern of extirpation ca. 40 years after completion and closure of other mainstream
<br />dams remains consistent, the Lake Mohave stock should disappear before the year 2000.
<br />INTRODUCTION
<br />The razorback sucker, Xyrauchen texanus (Abbott), is
<br />a large catostomid fish endemic to the Colorado River basin
<br />of western North America (Miller 1959, Minckley et al.
<br />1986). It was once widespread, ranging from Green River,
<br />Wyoming, south to the Colorado River delta of north-
<br />western Mexico (Minckley 1983). The species was histori-
<br />cally abundant, comprising a major food fish for Indians and
<br />early settlers (Rostlund 1952, Miller 1955, 1961, Hubbs
<br />1960, Minckley 1983). Although still present in large
<br />streams of the upper Colorado River basin (McAda and
<br />Wydoski 1980, Tyus et al. 1982, 19861 recent occurrences
<br />in the lower Colorado basin have been infrequent and
<br />typically of large adults (Minckley 1983, Loudermilk 1985,
<br />Ulmer in press). Large adults remain widespread, but
<br />uncommon, in the upper Colorado and Green rivers, and
<br />records of small (young) fish are sporadic and geographi-
<br />cally disjunct (Tyus et al. 1982, Wick et al. 1982).
<br />Mainstream impoundments of the lower Colorado
<br />River basin were initially colonized by substantial numbers
<br />of razorback sucker (Minckley 1983). However, the fish
<br />disappeared from Gila River system reservoirs in the 1950s
<br />(Minckley 1973), about 40 years after construction and
<br />closure of dams. It is becoming increasingly rare in reser-
<br />voirs on the mainstream Colorado River that are now of
<br />similar or greater ages. Although some specimens are still
<br />taken from lakes Powell (Tyus et al. 19861 and Havasu (Paul
<br />C. Marsh, Arizona State University [ASU], unpubl. data),
<br />a large population persists only in Lake Mohave, Arizona
<br />and Nevada (Minckley 1983, Bozek et al. 1984).
<br />Razorback suckers collected from Lake Mohave are
<br />consistently large (females averaged 50 cm and males 45
<br />cm standard length in 1981) and presumed old (Minckley
<br />1983). Annual spawning and production of larvae occur, but
<br />since young disappear prior to reaching greater than 12 mm
<br />total length, there is no evidence for recruitment (Minckley
<br />1983, Langhorst et al. 1985, Langhorst in press). High
<br />incidences of blindness, tumors, and other maladies in Lake
<br />Mohave specimens also suggest considerable age for
<br />individual fish. Minckley (1983) was unable to determine
<br />age by examination of scales, but nonetheless proposed the
<br />stock to have originated before or coincident with con-
<br />struction and closure (1942-51) of Davis Dam and final
<br />filling (1953-54) of the reservoir (Allan and Roden 1978).
<br />This paper assesses values of various bony structures for
<br />age estimation in razorback sucker, tests the hypothesis of
<br />substantial age for Lake Mohave fish, and delineates
<br />apparent patterns of individual growth.
<br />MATERIALS AND METHODS
<br />Razorback suckers were collected from Lake Mohave
<br />in 1980-83 by trammel nets and beach seines, sexed,
<br />measured, weighed, and prepared as dried skeletons or
<br />frozen for study. A total of 88 wild-caught fish were
<br />examined: 59 specimens (reported in part by Mpoame 19831
<br />collected May 1980 and January 1981; 9 broodfish that died
<br />February 1982 and January 1984 at Dexter National Fish
<br />Hatchery (DNFH), Dexter, New Mexico; 4 individuals
<br />provided by Arizona Game and Fish Department (AZGFD),
<br />March 1982; and 16 fish (reported in part by Buth and
<br />Murphy 1984) provided by California Department of Fish
<br />and Game (CADFG), January 1983. Specimens exhibited
<br />variations in size and condition typical of the Lake Mohave
<br />population in the period 1980-84. Sex of 4 fish was
<br />McCarthy, M. S., and Minckley, W. L. 1987. Age estimation for Razorback Sucker (Pisces: Catostomidae) from Lake Mohave,
<br />Arizona and Nevada. Journal of the Arizona-Nevada Academy of Science 21:87-97.
|