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interacting species, even including predator and prey. In this case, adaptations of the <br />sauger that were successful in obtaining prey, as well as those that reduce the <br />vulnerability of the prey to the sauger may have been important attributes of native fish <br />communities, but would not be effective with introduction of nonnative predator and prey <br />species. <br />The great difficulty for fishery managers is that these adverse interactions occur <br />underwater and are hard to detect. Not only is it difficult to detect predation by <br />examining stomach contents of suspected culprits, but seemingly innocuous forage <br />fishes like fathead minnows, rainbow smelt, and other species can prey on eggs and <br />larvae of native fishes, or compete for food and space (reviewed by Tyus and Saunders <br />1996, 2000). <br />— Endangered Species <br />The only fish in the Missouri River basin that is presently listed pursuant to <br />provisions of the Endangered Species Act is the pallid sturgeon Reasons for the decline <br />and endangerment of pallid sturgeon include blockage of spawning migrations, habitat <br />fragmentation due to dams, habitat alteration (both physical and biological), over <br />harvest; pollution and contaminants, and hybridization (USFWS 1993). Of special <br />relevance to this paper is the change in the biological components of habitat and how <br />the pallid sturgeon might be affected. This issue is addressed in the Pallid Sturgeon <br />(Scaphirhynchus albus) Recovery Plan (USFWS 1993; page 12): <br />The turbidity caused by suspended sediment also provided the pallid strugeon <br />and other native fish, adapted to living in a nearly sightless world, with cover while <br />moving from one snag or undercut bank to another. Today, water clarity has <br />increased dramatically, and this essential cover is gone. Under such conditions, <br />predation by sight- feeding predators, such as northern pike (Esox lucius), walleye <br />(Stizostedion virteum) and smallmouth bass (micropterus [sic] dolomieui), can be <br />expected to significantly impact native species not equipped by evolution with <br />good eyesight. <br />A living representative of ancient ray -fin fishes, it is presently listed as an <br />endangered species by the U.S. Fish and Wildlife Service, the pallid sturgeon is the <br />rarest fish in the Missouri - Mississippi River basin (USFWS 1993). Pallid sturgeon was <br />formerly common in portions of the Missouri River where they constituted about one -fifth <br />of the sturgeons captured (Forbes and Richardson 1905), but only a few individuals <br />have been recently reported (USFWS 1993). Only 11 of 4,366 sturgeons captured were <br />pallid sturgeons, and an additional 12 fish were pallid X shovelnose sturgeon hybrids <br />(Carlson et al. 1985), making hybrids (0.3 %) about as common as pure- strain fish <br />(0.2 %). <br />Pallid sturgeon is adapted to a benthic life in large turbid floodplain rivers. As a <br />group, sturgeons are migratory (Nikolskii 1961), but pallid sturgeons declined in <br />abundance before their specific migrations were charted. Early accounts documented <br />23 <br />