Laserfiche WebLink
Nebraska. Detectable concentrations of chlordane, DDE, DDT, and dieldrin also were found (Ruelle <br />and Keenlyne 1994). Prolonged egg maturation cycle of pallid sturgeon, combined with a <br />bioaccumulation of certain contaminants in eggs, could make contaminants a likely agent adversely <br />affecting eggs and embryo, development or survival of fry, thereby reducing reproductive success. <br />Further investigations are needed to identify sources of contaminants in the Missouri and Mississippi <br />Rivers and to assess the role of contaminants in the decline of pallid sturgeon populations. <br />Hybridization - The previous lack of genetic information on the pallid and shovelnose sturgeon, has led <br />to a hybridization debate. In recent years, however, several studies have increased our knowledge of <br />the genetic, morphologic, and habitat differences of those two species. Campton et al. (1995) collected <br />data that support the hypothesis that pallid and shovelnose sturgeon are reproductively isolated in less - <br />altered habitats, such as the upper Missouri River. Campton (2000, in press) suggested that natural <br />hybridization, backcrossing, and genetic introgression between pallid and shovelnose sturgeon may be <br />reducing the genetic divergence between those species. Sheehan (2000, pers comm) has identified 86 <br />separate loci for microsatellite analysis that are being used to differentiate between pallid, shovelnose <br />and suspected hybrid sturgeon. <br />Bramblett (1996) found substantial differences in habitat use and movements between adult pallid and t <br />shovelnose sturgeon in less altered habitats of the Yellowstone River. Presumably, the loss of habitat <br />diversity caused by human- induced environmental changes inhibits naturally occurring reproductive <br />isolating mechanisms. Campton et al. (1995), and Sheehan et al. (1997b) note that hybridization <br />suggests that similar areas are currently being used by both species for spawning. <br />Carlson et al. (1985) studied morphological characteristics of 4,332 sturgeon from the Missouri and <br />middle Mississippi Rivers. Of that group, he identified 11 pallid sturgeon and 12 pallid/shovelnose <br />sturgeon hybrids. Suspected hybrids recently have been observed in commercial fish catches on the <br />lower Missouri and the middle and lower Mississippi Rivers (K. Graham, Missouri Department of <br />Conservation, pers. comm. 1992; B. Reed, Louisiana Dept. of Wildlife and Fisheries, pers. comm. <br />1992). Bailey and Cross (1954) did not report hybrids, which may indicate that hybridization is a <br />recent phenomenon resulting from environmental changes caused by human- induced reductions in <br />habitat diversity and measurable changes in environmental variables such as turbidity, flow regimes, and <br />substrate types (Carlson et al. 1985). A study by Keenlyne et al. (1994) concluded that hybridization <br />may be occurring in half the river reaches within the range of pallid sturgeon and that hybrids may <br />represent a high proportion of remaining sturgeon stocks. Hybridization could present a threat to the <br />survival of pallid sturgeon through genetic swamping if the hybrids are fertile, and through competition <br />for limited habitat (Carlson et al. 1985). Keenlyne et al. (1994) noted few hybrids showing <br />intermediacy in all characteristics as would be expected in a first generation cross, indicating the hybrids <br />are fertile and reproducing. <br />Hubbs (1955) indicated that the frequency of natural hybridization in fish was a function of the <br />Status Range Wide -PS 115 <br />