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upstream spawning migrations of its close relative, the shovelnose sturgeon (e.g., <br />Goode 1884, Jordan and Evermann 1923) and it is likely that construction of mainstream <br />dams blocked spawning migrations of both species. Loss, reduction, or blockage of <br />spawning habitat would have reduced the number of sturgeon larvae and exacerbated <br />predation impacts, especially in less turbid environments. As a result of all these <br />factors, recovery of pallid sturgeon and a reduction in the rate of decline of the shovel <br />nose sturgeon population will require increased recruitment. In the present system few <br />easy options exist. Adult sturgeon are large fishes and their size provides safety from <br />predation by other native fishes. However, larvae and juvenile sturgeon are at risk. <br />The level of recruitment of sturgeons in the historic Missouri River is unknown. <br />However, present recruitment of shovelnose and pallid sturgeons is apparently very low <br />because sturgeon larvae are rarely captured (e.g, 2 sturgeon larvae in 147,000 fish <br />larvae captured from 1983 -1991; USFWS 1993). Among the factors that could be <br />associated with the low numbers of sturgeon larvae is a higher predation rate due to <br />increased predator density, or an increase in predator efficiency in the changed Missouri <br />River environment. Although the sturgeons evolved with predaceous fishes such as the <br />sauger, the present environment (e.g., less turbid, slower flows, less habitat complexity) <br />could enhance predation rate. Also, addition of so may new predators places prey at a <br />greater risk due to different foraging tactics of nonnative predators. In this case, <br />predator- induced defenses gained through evolution would be lacking. <br />— Other Species of Concern <br />This category includes paddlefish and seven species of minnows (sturgeon, <br />sicklefin, flathead, silver, and speckled chubs; plains and silvery minnows) that have <br />declined in distribution and abundance in the Missouri River. Little is known about the <br />life history of most of these fishes, and specific information about their reproductive <br />ecology is lacking. <br />Two species previously considered for federal listing as threatened or <br />endangered ( USFWS 1995, 2001) are the sturgeon and sicklefin chubs. The USFWS <br />(2001) recently removed the two chubs as candidates for listing for two reasons: (1) <br />collections with benthic trawls in some locations indicated that the fish was more widely <br />distributed than previously thought, and (2) conservation measures proposed for the <br />listed pallid sturgeon also would benefit the two species. But this action warrants review. <br />Sturgeon and sicklefin chubs have had extensive range reductions and threats to <br />their survival continue. A major concern in some fisheries is the destruction of benthic <br />life, which may include the younger life stages of the fishes and the invertebrates they <br />prey upon (DeAlteris and Morse 1997). Uniquely adapted to life under very turbid <br />conditions of the mainstream river, both of these small species are vulnerable to <br />predation from a variety of sight- feeding predators that have been introduced into altered <br />riverine habitats. <br />24 <br />