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environment, and the seriousness of consequences of hybridization depend on hybrid viability. <br />Hybridization can occur in fish if spawning habitat is limited; if many individuals of one potential parent <br />species lives in proximity to a limited number of the other parent species; if spawning habitat is modified <br />and rendered intermediate; if spawning seasons overlap; or where movement to reach suitable <br />spawning habitat is limited (Hubbs 1955). All those conditions exist to some extent within the range of <br />pallid and shovelnose sturgeon. Any of those conditions, or a combination of them, could be causing <br />the apparent breakdown of isolating mechanisms that prevented hybridization between these species in <br />the past (Keenlyne 1994). <br />Although Mayden and Kuhajda (1997) contend there is no empirical evidence indicating that <br />hybridization between shovelnose sturgeon and pallid sturgeon is common, they present no evidence to <br />support this contention. Based on meristic and morphological characters, Carlson et al. (1985) noted <br />hybrids prevalent in their samples, suggesting that hybridization between the species of Scaphirhynchus <br />may occur frequently. Field surveys of Scaphirhynchus stocks suggest a relatively high incidence of <br />hybridization between shovelnose sturgeon and pallid sturgeon in the middle Mississippi River (Sheehan <br />et al. 1997a, 1997b, 1998). Hybridization in the Lower Missouri and Middle Mississippi Rivers <br />increased from 1 in 361 river sturgeons (0.27 percent) in the late 1970s to 1 in 145 river sturgeons <br />(0.69 percent) in the late 1990s (Carlson et al. 1985, Grady et al. in prep). Sheehan et al. (1997b) and <br />Carlson and Pflieger (198 1) noted a 3:2 ratio of hybrid sturgeon to pallid sturgeon on both the Missouri <br />and Mississippi Rivers. Sheehan et al. (1997b) speculated that if that is representative of the sturgeon <br />population in the middle Mississippi River, hybridization may pose a significant threat to pallid sturgeon <br />as the species continues to introgress with shovelnose sturgeon. <br />Summary <br />Sturgeons exhibit unusual combinations of morphology, habits, and life history characteristics, which <br />make them highly vulnerable to impacts from human activities (Boreman 1997). Sturgeons generally <br />have low mortality rates, long life spans, and relatively low capacities for population increases <br />(Boreman 1997). As such, pallid sturgeon are well adapted to living in large rivers, where fluctuating <br />environmental conditions, such as discharge, can affect reproductive success. However, those <br />characteristics also make sturgeon species more sensitive to additional mortality factors, particularly <br />human activities. Many anthropogenic impacts, such as those that diminish spawning and nursery <br />habitat, primarily affect reproduction and survival of age -0 fish (Dr. Robert Sheehan, Southern Illinois <br />University at Carbondale (SIUC), pers. comm.). Sturgeon populations worldwide have declined <br />because of anthropogenic influences. The structure and magnitude of genetic diversity of natural <br />populations of sturgeon serves to buffer those fish against environmental variation and should be <br />maintained (Wirgin et al. 1997). <br />The loss of genetic variability can result in depressed fitness of the population (Spearman et al. 1994). <br />Pallid sturgeon distribution and abundance have drastically declined. In various studies, pallid sturgeon <br />have represented from 0.29 percent to 11 percent of total sturgeon collected. In commercial catch <br />116 Environmental Baseline <br />