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<br />In riverine environments razorback suckers spawning has been reported on clean cobble rifiles at
<br />water temperatures of 12-180C (Model-Ulmer 1983, Langhorst and Marsh 1986), Spawning
<br />occurs in a 4 to 5-week period, extending from mid-April to mid-June (Mcada and Wydoski
<br />1980, Valdez et ai, 1982), Reproduction in the wild has not been observed in river environments,
<br />but the fish apparently stage in deep pool!, large quiet backwaters, or riverside impoundments
<br />that are flooded by spring runoff (Tyus and Karp 1990), Males and females make spawning
<br />forays onto cobble rifiles that are 1.3 feet deep with a velocity of 1.3 feet per second (McAda and
<br />Wydoski 1980, Tyus and Karp 1990), In the lower Colorado River basin, spawning has been
<br />observed in sheltered riverside impoundments and washes where feIIlales build redds ( nests)
<br />scraped in cobble and gravel (Medel-Ulmer 1980, Bozek et aI. 1984),
<br />
<br />The number of eggs per female is quite variable. Ten females from the Green River carried an
<br />average of46,740 eggs per fish with a range of27,614-76,576 (McAda and Wydoski 1980),
<br />Eggs incubated in a hatchery at 14,4-17,20C had a 95% hatching rate, while those incubated at
<br />lU.e had a high mortality (Toney 1974), Hatching success may be related to acclimation
<br />temperature (Marsh 1985). The eggs are small, generally less than 2 mm in diameter, and hatch in
<br />5-6 days, The larvae are also small at 7-9 mm long.
<br />
<br />Not much is known about the behavior of newly-hatched larvae, Like the other native Colorado
<br />River fishes, they are transported downstream by river currents into shallow, sheltered, warm
<br />areas where food is abundant (Muth pees, comm,), Since spawning occurs shortly after peak
<br />runoff: while flows are high, the mainstem, low flow nursery backwaters used by Colorado
<br />squawfish are not available, and young razorback suckers use flooded bottomlands, inundated
<br />tributary mouths and other low velocity riverine habitats (Tyus and Karp 1990, Muth 1995), The
<br />young may eventually move into backWaters as they develop during receding flows in late
<br />summer,
<br />
<br />Recruitment of razorback sucker in the upper basin has been limited since the Colorado, Green,
<br />San Juan, and Gunnison rivers were impounded in the early 1960's. Very few young have been
<br />found in the upper basin in spite of intensive fishery surveys starting in the late 1960's (Holden and
<br />Stalnaker 1975a) and the 1970's and 1980's (Miller et ai, 1982), The fish that currently occupy
<br />the upper basin are believed to be a renullUlt population composed primarily of old and senile
<br />individuals (Lanigan and Tyus 1989, Modde et ai, 1996), These fish spawn successful1y but the
<br />young appuently fiil to survive beyond the first year in significant numbers (Modele 1996) and
<br />the ability of the upper basin, Green River population to persist without intervention is in
<br />question.
<br />
<br />Aie and nrowtb.
<br />
<br />Razorback suckers exhibit quick growth to adult size under optimum conditions and are large and .
<br />long-lived as adult fish, Razorback suckers taken from Lake Mohave in 1980 and 1981 showed
<br />ages of 24-44 years (McCarthy and Minckley 1987), indicating that the fish were probably
<br />spawned from about 1935-55, Fish aged from the Green River in 1987 were 12, 16, 19, and 26
<br />
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