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depths of 15.2 m and observed pallid sturgeon at depths of 7 and 21 m with greater frequency than <br />such areas were available. The range of depth used by pallid sturgeon is likely related to the available <br />habitat within the river segment ( Krentz, USFWS, pers. comm.). <br />Substrate: Pallid sturgeon are most frequently caught over a sand bottom, which is the predominant <br />bottom substrate within the species' range on the Missouri and Mississippi Rivers. Constant et al. <br />(1997) noted that pallid sturgeon spent considerable time associated with sand substrates. They noted <br />that preference for sand substrates in low slope areas suggests that pallid sturgeon use such areas as <br />current refugia (e.g., use sand -wave troughs created as bed - material moves along the river bottom <br />(Gordan et al. 1992)). The pallid sturgeon collected on the Yellowstone River in July 1991 by Watson <br />and Stewart (1991) was over a bottom of mainly gravel and rock, which is the predominant substrate <br />at that capture site. Reed and Ewing (1993) found sturgeon occurring in the man-made rip -rap lined <br />outfall channels of the Old River Control Complex in Louisiana. Bramblett (1996) found that pallid i �ri SM�4 <br />sturgeon preferred sandy substrates, particularly sand dunes and avoided substrates of gravel and ' <br />cobble. Pallid sturgeon have adhesive eggs. Thus, spawning is thought to occur over hard substrates <br />of gravel or cobble with moderate flow (Dr. Robert Sheehan, SIUC, pers. comm.). <br />Temperature: Pallid sturgeon inhabit areas where the water temperature ranges from 32• - 86• F (0• <br />C to 30• C), which is the range of water temperature on the Missouri and Mississippi Rivers. Sheehan <br />et al. (1998a) noted that sturgeon habitat use in the Middle Mississippi River did not change with <br />changes in temperature regimes and stated that temperature would not seem to have an affect on either <br />habitat use or habitat selection by Middle Mississippi River pallid sturgeon. Curtiss (1990) found no <br />relation between surface water temperatures and depth used by shovelnose sturgeon on the Mississippi <br />River and no indication that shovelnose sturgeon were moving into deeper, cooler water (if available) as <br />water temperature increased. Current research, however, indicates that pallid sturgeon spawning is <br />directly linked to water temperature. As water temperature increases to 62• -65• F (16.7• C - <br />18.3- C), pallid sturgeon initiate spawning activity (Steve Krentz, USFWS, pers. comm.). <br />Sheehan et al. (1990) found that swimming ability decreased and mortality increased for some river <br />species below 39• F (4• Q. Hurley (1996) evaluated the habitat associations and movement of pallid <br />sturgeon in the Middle Mississippi River at water temperatures below 39• F (4• C) and above 39• F <br />(4• C) yet below 50• F (10• Q. Below 39• F (4• C), study sturgeon were found in association with <br />current - disrupting habitat features such as downstream island tips, wing dams downstream, main <br />channel, and main channel border. Once winter temperatures rose above 39• F (4• C), habitat use <br />became more restricted with main channel border and main channel comprising 87 percent of all <br />relocations. When water temperatures rose to above 50• F (10• C) but below 68• F (20• C) during the <br />spring, relocations in habitats between wing dams increased to 40 percent of the contacts. <br />Food and Feeding Habits - Carlson et al. (1985) determined composition of food categories, by <br />volume and frequency of occurrence, in the diet of shovelnose sturgeon (n =234), pallid sturgeon (n =9), <br />and presumed hybrids (n =9). Although benthic macroinvertebrates characteristic of river habitats are <br />110 Status Range Wide -PS <br />