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Part I INTRODUCTION History Pallid sturgeon (Scaphirhynchus albus) evolved from an ancient group of bony fishes, the subclass Paleopterygii, which was dominant during the Paleozoic Era. This group continued to flourish through the late Paleozoic Era and early Mesozoic Era. Most species in this subclass became extinct sometime in the Mesozoic Era. The living descendants of this group in North America include paddlefish (Polyodontidae), and eight species of sturgeon (Acipenseridae). The North American species of sturgeon, in addition to pallid sturgeon, are the shovelnose sturgeon (Scaphirhynchus platorynchus Rafinesque); white sturgeon (Acipenser transmontanus Richardson); green sturgeon (Acipenser medirostris Ayres); Atlantic sturgeon (Acipenser oxyrhynchus Mitchill); shortnose sturgeon (Acipenser brevirostrum LeSueur); and lake sturgeon (Acipenser fulvescens Rafinesque). An eighth species, the Alabama sturgeon (Scaphirhynchus suttkus Williams), was recently described from the Mobile Basin in Alabama and Mississippi (Williams and Clemmer 1991). This location is outside the range of pallid sturgeon and is not included in further discussions of Scaphirhynchus species in this plan. The pallid sturgeon was first described by S.A. Forbes and R.E. Richardson in 1905 from nine specimens collected from the Mississippi River near Grafton, Illinois, in June 1904 (Forbes and Richardson 1905). Known locally as the white sturgeon, they named it Parascaphirhynchus albus and suggested it be considered as its own genus. Later classifications, however, placed it in the genus Scaphirhynchus where it has remained (Bailey and Cross 1954). Electrophoretic analysis of the two species of Scaphirhynchus have suggested a very close genetic relationship. Phelps and Allendorf (1983), using electrophoretic techniques, were unable to distinguish the species by looking at 37 enzyme systems. Pallid sturgeon from the Missouri River in Missouri and shovelnose sturgeon from the Missouri, Yellowstone, and Mississippi Rivers were analyzed. The authors concluded that the close genetic similarity of pallid sturgeon and shovelnose sturgeon was due to recent or incomplete reproductive isolation. General Description: Pallid sturgeon have a flattened, shovel- shaped snout; long, slender, and completely armored caudal.peduncle; and lack a spiracle (Smith 1979). As with other sturgeon, the mouth is toothless, protrusible, and ventrally positioned under the snout. The skeletal structure is primarily cartilaginous (Galbraith et al. 1988). Pallid sturgeon are similar in appearance to the more common, darker shovelnose sturgeon. Pflieger (1975) reports the principal features distinguishing pallid sturgeon from shovelnose sturgeon are the paucity of dermal ossifications on the belly, 24 or more anal fin rays, and 37 or more dorsal fin rays. 1