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<br />
<br />OBSERVATIONS ON RECRUITMENT AND ECOLOGY OF
<br />RAZORBACK SUCKER: LOWER COLORADO RIVER,
<br />ARIZONA-CALIFORNIA-NEVADA
<br />Paul C. Marsh' and W. L. Minckley2
<br />WC d
<br />ABSTRACT.-The Colorado River system downstream from Lake Mohave yielded 42 adult, 19 juvenile, and 39 larval
<br />wild razorback suckers (Xyrauchen texanus) between 1962 and 1988. Forty-six additional young captured between
<br />1984 and 1987 may have been wild or stocked, hatchery-propagated fish. Wild juveniles of this endemic, imperiled
<br />species, with one exception, have not been otherwise known from the Colorado River basin downstream from the
<br />Grand Canyon since the 1950s. A majority of adults and larvae were from the river or its mainstream impoundments,
<br />while all but one wild juvenile and all presumably stocked fish were captured from irrigation canals. The ecology of
<br />artificial canals in which young razorback suckers survive and grow is poorly understood.
<br />The razorback sucker, Xyrauchen texanus
<br />(Abbott), a once abundant endemic fish of the
<br />Colorado River basin of western North Amer-
<br />ica, now occurs naturally in only a few places.
<br />Populations upstream from the Grand Can-
<br />yon are small in size and apparently declining
<br />(McAda and Wydoski 1980, Tyus 1987, Lani-
<br />gan and Tyus, in press). Downstream, a sub-
<br />stantial remnant population persists only
<br />above Davis Dam in Lake Mohave, Arizona-
<br />Nevada (Fig. 1, Minckley 1983). That stock is
<br />comprised of old individuals (McCarthy and
<br />Minckley 1987), which despite annual repro-
<br />duction have apparently failed to recruit for
<br />nearly four decades.
<br />In the 1950s, and before, razorback suckers
<br />commonly occurred as a reproductive popula-
<br />tion in the Colorado River downstream from
<br />Davis Dam, in Lake Havasu, and below
<br />(Jonez et al. 1951, Douglas 1952, Jonez and
<br />Sumner 1954, Minckley 1983). They are
<br />presently rare or sporadic in those river
<br />reaches (Minckley 1979, Loudermilk and Ul-
<br />mer 1985, Marsh and Minckley 1987); only a
<br />small number of adults and a few young fish
<br />have been taken in recent years (in part,
<br />Minckley 1983, Ulmer and Anderson 1985).
<br />Programs to reintroduce the razorback suck-
<br />ers into historic habitats have been initiated
<br />by the states of Arizona and California (John-
<br />son 1985, Ulmer and Anderson 1985). Sub-
<br />stantial stockings of hatchery-produced fish
<br />into the lower Colorado River mainstream
<br />were commenced in spring 1986, although a
<br />few experimental reintroductions were made
<br />earlier.
<br />Occurrences of larvae or juveniles collected
<br />before the times of reintroductions, or in
<br />places inaccessible to stocked fish, represent
<br />natural production in the system. The intent
<br />of this paper is to separate records of natural
<br />occurrences from those attributable to hatch-
<br />ery fish. We document historic collections of
<br />small-sized razorback suckers downstream
<br />from Davis Dam, which, in light of recent
<br />age-and-growth and larval studies (McCarthy
<br />and Minckley 1987, Marsh and Papoulias, in
<br />press), provide evidence of recruitment to the
<br />population. We then report probable or
<br />known occurrences of reintroduced fish in
<br />the system; Langhorst (1988) further details
<br />short-term recaptures of hatchery-produced
<br />fish recently stocked by California. Com-
<br />parisons of collection localities for juveniles
<br />before and following reintroduction efforts
<br />indicate that both wild and hatchery-propa-
<br />gated young of the species distribute them-
<br />selves in similar ways, providing information
<br />pertinent to both the recovery program and
<br />the ecology of razorback suckers.
<br />Acronyms used in the text for various agen-
<br />cies are as follows: AZGFD, Arizona Game
<br />and Fish Department; ASU, Arizona State
<br />University; CADFG, California Department
<br />of Fish and Game; NVDOW, Nevada Depart-
<br />ment of Wildlife; USBR, U.S. Bureau of
<br />Reclamation; and USFWS, U. S. Fish and
<br />Wildlife Service. Funding for this work was
<br />'Center for Environmental Studies, Arizona State University, Tempe, Arizona 85287-1201.
<br />2Departmen t of Zoology, Arizona State University, Tempe, Arizona 85287-1501.
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