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65
<br />the isolated backwater. Taxa and sizes most
<br />strongly selected as food by larvae in the backwater
<br />were quantitatively more abundant in the reser-
<br />voir, yet selection by reservoir larvae for these
<br />same sizes was negative or neutral. This phe-
<br />nomenon may have been partly a function of food
<br />density differences (e.g., Paloheimo 1979, Chesson
<br />1983). We are aware of no distinctions in behavior,
<br />pigmentation, habitat, etc., between zoopiankters
<br />in the two habitats that would help explain ob-
<br />served differences in selection.
<br />More importantly, our comparisons indicate
<br />availability of larval foods of appropriate type, 1
<br />number, or size does not account for apparent total
<br />mortality of larvae in the reservoir. Papoulias (un-
<br />publ. data) found that larval razorback --ur....-val
<br />(but not growth) was independent of productivity
<br />(level of fertilization) among experimental ponds
<br />at Dexter National Fish Hatchery (DNFH), New
<br />Mexico.
<br />Declines of razorback sucker and other native
<br />fishes of the Colorado River basin have generally
<br />been attributed to a suite of physical modifications,
<br />which render habitats inhospitable to life-cycle
<br />completion, or to introduced fishes that have direct
<br />or indirect impacts on native kinds (e.g., Miller
<br />1961, 1972, Minckley & Deacon 1968, Johnson &
<br />Rinne 1982, Minckley 1983). Physical and biolog-
<br />ical effects often cannot be separated, since major
<br />modifications to the river system and widespread
<br />introductions of exotic species were approximately
<br />concomitant in time and place.
<br />Explanations for apparent disappearance of lar-
<br />val razorback sucker from Lake Mohave include
<br />migration to inaccessible habitats within the reser-
<br />voir, down-lake transport out of the area, and mor-
<br />tality. The first is rejected since extensive collec-
<br />tions using gears that have effectively taken
<br />thousands of individuals of other species in the size
<br />range of juvenile suckers failed to take small razor-
<br />back. This sampling has been conducted since the
<br />early 1970s and covered all available habitats ex-
<br />cept the deepest profundal (Minckley 1983, un-
<br />publ. data). These same techniques have suc-
<br />cessfully captured juvenile suckers reintroduced
<br />into several central Arizona streams (unpubl.
<br />data).
<br />Down-lake transport may occur, but razorback
<br />populations in the Colorado River below Davis
<br />Dam, in downstream Lake Havasu, and below. are
<br />very small; only sporadic individuals have been
<br />recorded in recent years (Minckley 1983, Ulmer
<br />1987). If down-lake transport is an important mech-
<br />anism removing young razorback from Lake Mo-
<br />have, it must thus result in high mortality. Validity
<br />of the out-of-system transport hypothesis should be
<br />evaluated and tested.
<br />Last is mortality. Data continue to accumulate to
<br />strengthen the idea that predation by introduced
<br />fishes playes an important role in declines of native
<br />fishes, and contributes to the continued perilous
<br />status of several species. Unanticipated events
<br />.. .., .,..,,U la.C 1.7i: 5 inves"I'vation support
<br />this contention. High reservoir level (196.8 m)
<br />combined with storm-driven waves on 4 April 1985
<br />to breach the berm that separated the Arizona Bay
<br />backwater from Lake Mohave. Razorback sucker
<br />larvae were abundant in the backwater and rela-
<br />tively large (>20mm TL, middle postlarvae) at
<br />that time. Within the next few days, non-native
<br />fishes including threadfin shad, carp, channel cat-
<br />fish, largemouth bass, green sunfish, and bluegill
<br />had invaded, and nearly 40% of green sunfish cap-
<br />tured over a 24 h period contained an average of
<br />four razorback sucker larvae. Larvae could no
<br />longer be collected by 5 May, when the backwater
<br />again was isolated, and we attribute total mortality
<br />of the stock to predation. Predation on razorback
<br />sucker eggs and larvae has further been docu-
<br />mented in Senator Wash Reservoir, California,
<br />and Lake Mohave (Medel-Ulmer 1983, Louder-
<br />milk 1985, Langhorst 1987, Marsh 1987). Marsh &
<br />Brooks (unpubl. data) recorded potentially signifi-
<br />cant predation by introduced ictalurid catfishes on
<br />juvenile razorbacks stocked into the middle Gila
<br />River, Arizona, and Brooks (unpubl. data) deter-
<br />mined predation loss of young suckers to be in
<br />i direct relation to density of green sunfish in experi-
<br />mental ponds at DNFH. Additional evidence
<br />comes from study of other fishes in the Region
<br />(e.g., Meffe et al. 1983, Meffe 1985) and elsewhere
<br />(e. a., Selgeby et al. 19.78, Lemly 1985, Loftus &
<br />Hulsman 1986).
<br />Most studies of predation on wild razorback
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