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invertebrates (principally the immature stages of insects) composed most of <br />the diet of shovelnose sturgeon, while pallid sturgeon and presumed hybrids <br />consumed a greater proportion of fish (mostly cyp )• Oter <br />also reported a higher incidence of fish in the diet of pallid sturgeon than <br />in the diet of shovelnose sturgeon (Cross 1967; Held 1969). <br />Most piscivorous Missouri River species eat large quantities ste owereuatic insect <br />larvae in early life and even as adults. and Psychomyiidae (Tricopteran to <br />consume large numbers of Hydropsyche spp. <br />larvae) (Modde and Schmulbach 1977). <br />Age and Growth: Little is known about age and growth of pallid thatrofon. The <br />total length of pallid sturgeon was significa y greater <br />shovelnose sturgeon in the lower Missouri and Mississippi Rivers for each age <br />group in which comparable data were available (Carlson et al. 1985). Fogle <br />(1963) estimated growth rates using cross sections of pectoral fin rays from <br />six pallid sturgeon from Lake Oahe in South Dakota. He estimated that growth <br />of these fish was relatively rapid during the first 4 years, <br />but that growth <br />decreased to approximately 70 mm (4 in) per year between ages 5 and 10. <br />Carlson and Pflieger (1981) presented data (n =8) from the Missouri and <br />Mississippi Rivers in Missouri, which showed slightly slower growths than from <br />pallid sturgeon in South Dakota. <br />By interpreting cross sections of pectoral fin rays, L. Jenkins (pers. comm. <br />1991) estimated that pallid sturgeon can live more than 40 years. <br />Reasons for Decline <br />Habitat Loss: Destruction and alteration of habitats by human modification of <br />the river system is believed to be the primary cause of Iec Ines unlikely that <br />reproduction, growth, and survival pallid <br />successfully reproducing populations of pallid sturgeon can be recovered <br />without restoring the habitat elements (morphology, hydrology, temperature <br />regime, cover, and sediment /organic matter transport) of the Missouri and <br />Mississippi Rivers necessary for the species continued survival. <br />On the main stem of the Missouri River, approximately 36 percent of riverine <br />habitat within the pallid sturgeon's range was eliminated by construction of <br />six massive earthen dams between 1926 and 1952 and another 40 percent has been <br />channelized. The remaining 24 percent has been altered due to changes in <br />water flows caused by dam operations. <br />These dams also are believed to have adversely affected pallid sturgeon by <br />blocking migration routes and by causing inundation of spawning and nursery <br />areas. <br />Channelization of the Missouri and Mississippi elizaters began the chann of ges watere <br />century and <br />continues to t he present. Chann <br />velocity, reduces the width of the river, and prevents water flow into <br />backwaters. In the channelized reach downstream of Sioux City, Iowa, the <br />Missouri River that was once a diverse assembledge of braided channels, <br />sandbars, and backwaters is now confined within a narrow channel of rather <br />10 <br />