Picture by Chad Thomas, Texas State University-San Marcos

Polyodon spathula

paddlefish

Type Locality

None given (Walbaum1792).

Etymology/Derivation of Scientific Name

Polyodon: many toothed, perhaps in reference to the numerous gill rakers because adult paddlefish lack teeth, although young fish possess numerous small teeth; spathula: spatula, in reference to the elongate, paddle shaped snout (Ross 2001).

Synonymy

Squalus spathula Walbaum 1792:522.

Polyodon folium Wailes 1854:332.

Polyodon spathula Hildebrand and Towers 1928:110; Cook 1959.

Characters

Maximum size: 2200 mm, 86.6 in (Allardyce 1992).

Coloration: Dull colored and often mottled; color ranges from bluish-gray to black dorsally, light gray-black on the sides and white ventrally (Jennings and Zigler 2000).

Counts: Gill rakers number 550-600 on the outer row of the leading arch (Weisel 1973).

Body shape:  Body shark-like with prominent paddle shaped snout and heterocercal tail (Boschung and Mayden 2004).

Mouth position: Subterminal (Goldstein and Simon 1999).

Morphology: A distinctive paddle shaped snout accounts for approximately one-fourth to one-third of the length of the body; smaller fish have proportionally longer paddles (Ross 2001). Smith (1979) noted that the spatulate snout is believed to serve as a stabilizer to prevent the nose diving that would otherwise occur as a result of the drag created by water entering the gaping mouth during filter feeding. Two small barbels present on underside of head in front of mouth (Ross 2001). In adult paddlefish teeth are diminutive or are absent (Ross 2001). In a 630 mm (25 in) paddlefish, teeth where found to be less than 1 mm (0.04 in) in length (Becker 1983).Teeth are small, fanglike, irregularly positioned and deciduous in single rows along upper and lower jaws and on the floor of the mouth on the basal portions of the gill arches. Gill rakers long, slender, numerous; gill cover extending posteriorly as a long, pointed fleshy flap (Boschung and Mayden 2004). Smooth-bodied, generally lacking scales; very small scales may occur on the upper lobe of the caudal fin, the base of the pectoral fin, or above the anterior portion of the lateral line (Russell 1986, Ross 2001). Skeleton primarily cartilaginous, with bone limited to the jaws (Ross 2001). Male paddlefish can be distinguished from females by the papillae that surround the urogenital opening (Carlander 1969), which is somewhat raised in males, and is more flattened and softer in females (Meyer and Stevenson 1962).

Distribution (Native and Introduced)

U.S. distribution: Originally ranged throughout much of the Mississippi River drainage and eastward of the Appalachian Mountain range and the Great Lakes (Hubbs et al. 1991).

Texas distribution: Species occurred in every major river drainage from the Trinity Basin eastward, but its numbers and range had been substantially reduced by the 1950’s (Hubbs et al. 1991). Warren et al. (2000) listed the following drainage unit for distribution of paddlefish in the state: Red River (from the mouth upstream to and including the Kiamichi River).

Abundance/Conservation status (Federal, State, Non-governmental organizations)

Species is vulnerable in the southern United States (Warren et al. 2000). Graham (1997) reported that in the state of Texas no sport or commercial fisheries exist; also that the species is classified as endangered in the state, but that its status is increasing.  Hubbs et al. (1991) listed the species as endangered. Beginning in 1990 the state of Texas had been utilizing a stocking program to recover paddlefish populations in east Texas, with the last recorded stocking taking place in 2000 (Texas Parks and Wildlife Department 2010).

Habitat Associations

Macrohabitat: Large river systems and their tributaries (Graham 1997; Ross 2001); paddlefish thrive in backwaters, oxbows, and deepwater channel habitats (Paukert and Fisher 2000; Boschung and Mayden 2004). These fish typically inhabit low-gradient areas of moderate to large-sized rivers, sluggish pools, backwaters, bayous, and oxbows with abundant zooplankton (Wallus et al. 1990). Large reservoirs make good feeding areas, with paddlefish moving from reservoirs into flowing streams in the spring for spawning (Russell 1986).

Mesohabitat: Paddlefish prefer large, free-flowing rivers rich in zooplankton, but will occupy impoundments with access to spawning sites (Burr 1980). This species has also been found to reside in saline waters (Burr 1980; Wallus et al. 1990; Boschung and Mayden 2004). In altered reaches of large rivers, paddlefish occur in areas where they may find protection from strong currents, such as near dikes, revetments, or bridges (Southall and Hubert 1984; Russell 1986). In the winter, paddlefish usually move into deep water, as in the Nueces River system, Texas, where spring to fall capture depths averaged 3.9-5.0 m (12.8-16.4 ft), increasing to 7.6 m (25 ft) in the winter (Pitman and Parks 1994). Large river populations make extensive spawning migrations in the spring (Russell 1986; Paukert and Fisher 2000); their movement on these occasions associated with pools during high water, and with tailwaters (where dams exist) and turbulent main-channel habitats (Southall and Hubert 1984; Moen et al. 1992; Paukert and Fisher 2000). Optimum temperatures for this species have been shown to range from about 12-24°C, 53.6-75.2°F (Crance 1987); Paukert and Fisher (2000) reported that selected water temperatures ranged from 24-29°C (75.2-84.2°F) in the Keystone Reservoir, Oklahoma.

Biology

Spawning season: Spawning occurs between late February and late June when water temperatures are 10-17°C,

50.0-62.6°F (Purkett 1961; Pasch et al. 1980; Alexander and McDonough 1983; Pitman 1992; Lein and DeVries 1998). Even at optimum temperatures, research has found that only a period (10-14 days) of increased and prolonged river flow will attract fish to the preferred spawning habitat (Russell 1986; Pitman 1992).

Spawning habitat: Nonguarders; open substratum spawners; lithopelagophils – rock and gravel spawners with pelagic free embryos (Simon 1999). Gravel and gravel plus cobble are the dominant substrates in spawning; velocity, depth, or substrate may be used as cover, either singly or in combination (Crance 1987; Wallus et al. 1990). Usually in swiftly flowing water over large gravel bars (Purkett 1961; 1963). In the Cumberland River, Tennessee, Pasch et al. (1980) observed spawning over gravel-rubble substrate in waters 2-12 m (6.5-39.3 ft) deep.

Reproductive strategy: Mature padddlefish do not reproduce every year, but spawn every four to seven years (Vasetskiy 1971; Ross 2001). Pitman (1991) noted that mature males are typically capable of annual spawning; however, it is thought that  females require two or more years to develop mature ova. In the upper Alabama River system, paddlefish may spawn annually; the capture of tagged gravid females participating in spawning migrations in successive years indicated the potential for spawning each year (Lein and DeVries 1998). Female paddlefish release eggs in the upper water column, often breaking the surface with their caudal fins during spawning (Purkett 1961; Pitman 1992). External fertilization, occurs after males release sperm in the water (Pitman 1991; Pitman 1992). Unfertilized eggs are non-adhesive, but become adhesive, sticking singularly at first contact after fertilization (Purkett 1961; Yeager and Wallus 1982; Russell 1986; Pitman 1992; Jennings and Zigler 2000).

Fecundity: About 15,000 to 35,000 ova are produced per kg of body weight. Gravid females have ovaries which comprise 15-25% of their body weight (Purkett 1961; Pitman 1992). In Lake Ponchartrain, Louisiana, fecundity of fish from 876-1,137 mm (34.5-44.8 in) EFL ranged from 65,000-136,800 eggs/female (mean, 100,319; SE, 9,349), and from 6,600-13,300 eggs/kilogram body weight (mean, 9, 484; SE, 696); fecundity was highly variable among individuals of approximately the same size; egg diameters ranged from 2.1-3.1 mm (0.08-0.12 in) and averaged 2.67 mm, 0.11 in (SD, 0.2; Reed et al. 1992). Eggs are demersal and adhesive (Yeager and Wallus 1982); oval; dark brown, blackish, or grayish in color; bipolar; late embryos with little pigment (Wallus et al. 1990); eggs range from 2.0 to 3.9 mm (0.08-.15 in) in diameter when mature (Larimore 1950; Purkett 1961; Rosen et al. 1982; Yeager and Wallus 1982). Incubation time reported to range from 155-166 hours at 14.4-18.8°C, 58-66°F (Yeager and Wallus 1982); 9 days at 14°C (Ballard and Needham 1964); and 7 days or less at 18.5-21°C, 65.3-70.0°F  (Purkett 1961).

Age at maturation: Varies by population; males reach maturity between 4-9 years and females, between 6-12 years (Pitman 1992).

Migration: Upstream migration in large free-flowing rivers with silt-free gravel bars (Pasch et al. 1980; Wallus et al. 1990). Fish move extensively at times, particularly during spring spawning migrations, when they may travel 160-322 km (99-200  mi) upriver, followed by downstream movements after spawning (Russell 1986; Pitman and Parks 1994; Paukert and Fisher 2001; Ross 2001; Stancill et al. 2002). Where upstream dams impede their upstream travel, fish often become concentrated in the tailwaters (Pasch et al. 1980; Southall and Hubert 1984).

Longevity: 30+ years (Purkett 1963; Pitman 1992; Jennings and Zigler 2000).

Food habits: First and second trophic classifications are invertivore/planktivore, and filter feeder, respectively; this species is likely the best known example of a planktivore ram filtration feeder, straining food with large mouth (Goldstein and Simon 1999). Wagner (1908) reported diet of plankton material including small crustaceans, algae, and ephemerid larvae, and Forbes and Richardson (1920) listed food items of entomostracans, larval mayflies, dragonflies, chironomids, aquatic insects, amphipod crustaceans, and leeches. Ruelle and Hudson (1977) reported that fish generally feed at night. Optimal temperature for feeding ranged from 7-20°C, 45-68°F (though fish occurred in temperatures up to 28°C, 82.4°F; Rosen and Hales 1981). Blackwell et al. (1995) reported fish in the lower Trinity River, Texas, feeding at temperatures greater than 20°C (68°F); stomach analysis of specimens indicated rotifers were being consumed. Active feeding begins at yolk-sac absorption (Yeager and Wallus 1982), with larvae consuming zooplankton and insects (Ruelle and Hudson 1977). Young paddlefish (<120 mm, 4.7 in TL) are selective, capture feeders (Rosen and Hales 1981; Wallus et al. 1990). Young paddlesfish may be cannibalistic under intensive culture conditions (Yeager and Wallus 1982). Active feeding on individual prey organisms continues until fish reach 120-250 mm (4.7-10.0 in) TL, and the gill rakers are sufficiently developed to be used as a filter (Rosen and Hales 1981; Michaeletz et al. 1982; Jennings and Zigler 2000). Larval and juvenile fish feed on zooplankton and all stages of aquatic insects (Ruelle and Hudson 1977; Rosen and Hales 1981; Jennings and Zigler 2000).

Growth: Newly hatched larvae average about 8.5 mm (3.3 in) TL (Purkett 1961; Pasch et al. 1980; Yeager and Wallus 1982; Jennings and Zigler 2000). Early growth is rapid (Wallus et al. 1990). Food abundance seems to be the prime factor limiting fish growth, and in instances where food was not limited, some young-of-year reached more than 508 mm (20 in) TL (Russell 1986; Pitman 1992). In Fort Gibson Reservoir, Oklahoma, fish grew at a rate of 4.3 mm/day (0.17 in/day) and averaged 721 mm (28.3 in) TL by December of their 1^st year (Houser and Bross 1959; Pitman 1992). According to Pitman (1991), growth usually slows after the 1^st year; by age 5, growth is about 51 mm/year (2.0 in/year); however, growth is highly variable.

Phylogeny and morphologically similar fishes

No other freshwater fish in North America has an elongate rostrum that is broader at its distal end compared to the base. The paddle, along with the smooth skin and heterocercal caudal fin makes this a very distinctive species (Ross 2001).

Host Records

Trematoda (6), Cestoda (3), Nematoda (2), Leech (1), Crustacea (1; Hoffman 1967). Becker (1983) noted that this species is heavily parasitized and is used as a food source by lampreys of the genus Ichthyomyzon.

Commercial or Environmental Importance

       In Texas, habitat destruction and water quality continue to be major concerns (Graham 1997). Species is vulnerable to commercial (both legal and illegal) harvest (Graham 1997).

References

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