Freshwater Fishes of North America: Volume 1: Petromyzontidae to Catostomidae

Chapter 6 Polyodontidae: Paddlefishes

weak electric fields emitted by its prey. Zooplankton is captured by filter feeding in adults; Paddlefish pass large volumes of water into their huge mouths and filter it across extremely numerous long gill rakers, capturing the zooplankton . The name of the family and genus is derived from “poly-” (many) and “-don” (tooth), referring to the many gill rakers. The spawning habitat for the Paddlefish was unknown for >100 years until the spring of 1960 when spawning was observed on a flooded gravel bar after a rapid river rise of 2.7m (9 feet). Upstream spawning migrations can cover >322km (200 miles). Paddlefish populations are far below historical levels due to commercial harvest and large-river alterations (dams and channelization) that block migratory routes and destroy habitat. Paddlefish eggs are processed into caviar and are used as a substitute for Sturgeon (Acipenseridae) caviar. DIVERSITY AND DISTRIBUTION The Polyodontidae are restricted to the Northern Hemisphere and consist of only two extant species. The Chinese The family Polyodontidae, the Paddlefishes, has only two living species, the Chinese Paddlefish, Psephurus gladius, and the North American Paddlefish, Polyodon spathula (Fig. 6.1), although numerous fossil Paddlefishes date back to >100mya. The Paddlefish was originally described as a Shark (Chondrichthyes, Cartilaginous Fishes) in the late 1700s due to its cartilaginous skeleton, jaw structure, and shark-like tail, but Paddlefishes are actually ancient bony fishes. Both species get large, with the North American Paddlefish reaching 2.15m (7.1 feet) and 74kg (163 pounds). Paddlefishes get their common names and the specific epithet spathula (spatula) of the North American species from the long spatula- or paddle-shaped snout that overhangs extremely small eyes and a large mouth. The Paddlefish also goes by the name spoonbill catfish in reference to the paddle and the lack of obvious scales on the body, but Paddle- fishes are not closely related to Catfishes (Siluriformes). The paddle is absent in small larvae but is one-half of the body length in juveniles and one-fourth to one-third of body length in adults. The North American Paddlefish is a riverine species that feeds on zooplankton using tens of thousands of electrosensory organs covering its paddle to detect Chapter 6 Polyodontidae: Paddlefishes Bernard R. Kuhajda Plate 6.1. Paddlefish, Polyodon spathula 208 FRESHWATER FISHES OF NORTH AMERICA Inter- and Intraspecific Variation Other than a footnote alluding to an undescribed species of Polyodontidae in the Mississippi River basin (Myers 1949), no other species of Paddlefish or any subspecies of Polyodon spathula are known from North America. Lower Mississippi River Paddlefish reportedly attain a greater size than those from the Ohio and upper Mississippi Rivers (Stockard 1907), but the largest and heaviest specimens recorded are from the upper Mississippi and Ohio Rivers (Nichols 1916; Forbes & Richardson 1920). Variation occurs in riverine versus oxbow lake adults from the lower Mississippi River with riverine Paddlefish possessing more slender bodies and shorter and broader paddles (Fig. 6.3) (Stockard 1907). Morphological variation in juvenile Paddlefish (61.9–403.7mm TL, 2.4–15.9 inches) exists between populations in the southeastern United States. Hatchery-reared Paddlefish from the Mermentau River, Louisiana, possess shorter, narrower leaf-shaped paddles and asymmetrical caudal-fin lobes; hatchery-reared specimens from the Tombigbee River, Alabama, have longer, broader spoon-shaped paddles and more symmetrical caudal lobes; and field-collected juveniles from the Mississippi River, Mississippi, have the longest and broadest (paddleshaped ) paddles and symmetrical caudal lobes. Overall larger basins with higher gradients and discharges have juvenile Paddlefish with longer and broader paddles and Paddlefish (Psephurus gladius) occurs in the Yangtze River drainage, China, with adults occasionally migrating into the East China and Yellow Seas (Liu & Zeng 1988). The Paddlefish (Polyodon spathula) is a freshwater species that historically occurred in the Mississippi River basin of North America from New York to Montana and south to Louisiana and in adjacent Gulf Coast drainages from the Mobile Basin, Alabama, to Galveston Bay, Texas, as well as the Great Lakes in the United States and Canada (Table 6.1; Fig. 6.2). Although still present in most of its historical distribution in 22 states, the Paddlefish has disappeared from part of its peripheral range within the Mississippi River basin, from several western Gulf Coast drainages, and throughout the Great Lakes (Hubbs & Lagler 1964; Burr 1980; Gengerke 1986; Parker 1988...