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<br />January 1989 MARSH, MINCKLEY: RAZORBACK SUCKER
<br />The first major reintroduction of razorback
<br />suckers in the mainstream was in March 1986,
<br />when nearly 1.4 million larvae (10-18 mm TL)
<br />were released by CADFG and USFWS at
<br />various localities along the Colorado River
<br />from Devil's Elbow and Blankenship Bend (in
<br />the Topock Gorge area upstream from Lake
<br />Havasu, Fig. 1), downstream to Imperial Na-
<br />tional Wildlife Refuge near Yuma, Arizona.
<br />AZGFD and USFWS placed an additional
<br />70,000 juveniles (- 5.1 cm TL) into the Colo-
<br />rado River near Parker, Arizona, in May 1986,
<br />and CADFG stocked 4,163 juveniles (- 20 cm
<br />TL) in the same area in October-November
<br />1986. Since then, more than a million addi-
<br />tional larvae and juveniles have been stocked
<br />downstream from Parker Dam (Langhorst
<br />1988). These last stockings were all conducted
<br />later than the collections of all but one
<br />(Niland, December 1985) of the juveniles
<br />tabulated above, and of larvae reported by
<br />Marsh and Papoulias (in press).
<br />Captures, 1987-88
<br />Excluding the 1987 collection (Langhorst
<br />1988) of a two-year-old individual in the Colo-
<br />rado River mainstream upstream from Palo
<br />Verde Diversion Dam (thus wild-hatched), a
<br />total of 41 juvenile razorback suckers was cap-
<br />tured from canals downstream from Parker,
<br />Arizona, on the east (Arizona) side of the Colo-
<br />rado River in 1987 and 1988 (S. Yess, US-
<br />FWS, personal communication). Thirty-eight
<br />fish caught in 1987 averaged 28.8 cm, and
<br />three taken in 1988 averaged 45.1 cm TL.
<br />Unfortunately, none from the first group was
<br />aged, but based on mean size they could have
<br />been one-year-old fish and therefore origi-
<br />nated, at least in part, from the 1986 stock-
<br />ings. None could have been derived from ear-
<br />lier reintroductions, all of which were placed
<br />downstream from Headgate Rock Dam (Fig.
<br />1). Fish of the second (1988) group had otolith
<br />ages of three, four, and seven years, having
<br />hatched, respectively, in spring 1981, 1984,
<br />and 1985. These were naturally produced
<br />wild fish, since dates of hatching do not corre-
<br />spond with those of any reintroductions in
<br />areas from which they could have moved to
<br />the collection sites.
<br />DISCUSSION AND SUMMARY
<br />Captures between 1974 and 1988 of at least
<br />19 young, wild-hatched razorback suckers in
<br />75
<br />the lowermost Colorado River system down-
<br />stream from Lake Mohave provide convincing
<br />evidence of potential recruitment to that
<br />population. Numbers recruited nonetheless
<br />appear insufficient to maintain a population of
<br />adults, since fish of reproductive size are ex-
<br />ceedingly rare and scattered in distribution
<br />(42 adult individuals recorded in the period
<br />1962-1988). Further, artificial canals where
<br />most young fish were recorded may act not
<br />only as a refuge for early development but as
<br />death traps later, during annual dewatering
<br />for maintenance of the irrigation system. Be-
<br />cause of this, potential recruits may ultimately
<br />be lost to the population.
<br />Waterways of Colorado River irrigation sys-
<br />tems consist of two major components, canals
<br />and drains (or wasteways ). Canals vary down-
<br />ward from maximum flows of 400 m3/sec.
<br />Water is withdrawn by gravity at diversion
<br />structures (e.g., Headgate Rock, Palo Verde,
<br />Imperial, and Laguna diversions) or through
<br />pumps (CAP and Colorado River Aqueduct
<br />intake facilities; USBR 1980; Fig. 1). Small
<br />laterals, which deliver water to agricultural
<br />fields or other points of use, are the least
<br />permanent, carrying water for only a few days
<br />or hours per month. Most canal habitats from
<br />which razorback suckers have been taken are
<br />of intermediate sizes that are dewatered at
<br />least annually for cleaning and repairs. Some
<br />of the largest canals may not be dewatered for
<br />periods of years.
<br />Periodic cleaning and repair of canals is
<br />typically in the irrigation off-season, usually
<br />December or January. Fishes are decimated
<br />by dewatering and mechanical cleaning, and
<br />few survive (Marsh and Minckley 1982). How-
<br />ever, razorback suckers spawn early, in late
<br />January through March, and larvae are thus
<br />available (generally from February through
<br />April; Marsh and Langhorst 1988) to colonize
<br />canals as they are placed back in service. De-
<br />pleted populations of potential predators en-
<br />hance larval survival, and razorback sucker
<br />growth rates (to 25+ cm in six months; unpub-
<br />lished data) are such that they rapidly grow
<br />out of predation range of small, abundant,
<br />nonnative predators (e.g., green sunfish, Lep-
<br />omis cyanellus) and attain capabilities suffi-
<br />cient to avoid larger species (largemouth
<br />bass, Micropterus salmoides, and ictalurid
<br />catfishes, especially flathead catfish, Pylo-
<br />dictis olivaris). Further, annual drainage of
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