Paddlefish (Polyodon spathula) COSEWIC assessment and status report: chapter 6
Biology
General
Nothing is known of the biology of paddlefish in Canada and all available information comes from populations in the United States.
Reproduction
Little is known about the life history of this fish, but spawning is reported to occur during April and May when water temperatures range from 10 to 17ºC (Purkett 1961; Wallus 1986; Lein and DeVries 1998). Mature fish move upstream to the vicinity of spawning areas where they congregate in deeper areas of the river (Purkett 1961; Pasch et al. 1980). Movement onto the spawning areas is initiated by increases in water velocity, with optimal velocities being 60 to 140 cm/sec (Crance 1987). The act of spawning is apparently triggered by increased water flow (Purkett 1961; Pasch et al. 1980; Jennings and Zigler 2000).
Paddlefish spawn over gravel bars, rock or rip/rap at water depths of 2 – 12 m (optimal depths 3 – 6 m), in areas with enough current to keep the eggs aerated and silt free (Pasch et al. 1980; Wallus 1986; Crance 1987). The eggs are adhesive and stick to substrate (Purkett 1961), hatching in 7 to 9 days. Females weighing 9 to 36 kg may release 70 – 300,000 eggs (Dillard et al. 1986), although larger females may be capable of producing over 500,000 eggs.
Growth
Larvae (newly hatched young) look very different from adults and swim actively from top to bottom in the water column, apparently to take advantage of currents to carry them from high velocity spawning areas to nursery habitats. Current velocities of 30 – 122 cm/s are thought to be required to achieve this transport (Wallus 1986). Currents in nursery areas are much lower, permitting the young fish to feed without expending large amounts of energy (Crance 1987). Growth rates between populations are variable (Bonislawsky 1977; Pasch et al. 1980; Combs 1982), but growth is rapid, and at one month the young begin to resemble adults, and by 18 months individuals may attain average weights of 2 – 3 kg. Under ideal conditions, juveniles may add up to 4.5 kg per year to their body weight (Dillard et al. 1986).Lentic (reservoir) populations had faster growth at age 0 than lotic populations, but growth rates were similar for ages 2 – 10, suggesting that initial growth differences produced the divergence between lentic and lotic populations. Maximum age was not different between lentic and lotic populations.
Mean total lengths of adults from various populations in the midwestern states range from 70 to 150 cm (Carlander 1969). Males apparently grow more slowly than females, and are usually one-half to two-thirds the weight of females (Dillard et al. 1986). Sexual maturity is estimated to be at 7 – 10 years for males and possibly 14 - 17 for females (Parker 1988; Scarnecchia and Schmitz 2003; NatureServe 2007). In Missouri, maturity is attained at lengths of about 140 cm (TL) for females, and 127 cm (TL) for males (Carlson and Bonislowsky 1981). Males apparently spawn every year, but females may require 2 to 7 years to develop mature ova before spawning again (Parker 1988; Jennings and Zigler 2000).
Survival
There is little, or no, information on rates of natural mortality but the species is highly susceptible to exploitation as are most Acipenseriformes. Piscivorous species, such as walleye (Sander vitreus), and sauger (Sander canadensis), may be significant predators on young paddlefish (Mero et al. 1994). Average life expectancy is thought to be about 15 years (Burkhead and Jenkins 1991), but some individuals may live up to 50 years or longer (Scarnecchia and Schmitz 2003). Most large fish (18 – 40 kg) caught in the mid–west are females ranging in age from 15 to 50 years (average about 26 years). Most of the smaller fish (4.5 – 18 kg) are males ranging from 9 to 50 years of age and averaging about 16 years (Scarnecchiea and Schmitz 2003).
Diet
All life stages of paddlefish are primarily invertivores, feeding mainly on zooplankton, small invertebrates and insect larvae, using their long gill rakers to filter food from the water (Dillard et al. 1986). Small fishes may also be eaten (Jennings and Zigler 2000), as well as algae and aquatic plants (Becker 1983).
Dispersal/migrations
Paddlefish move from overwintering areas in deeper waters of large rivers or reservoirs to spawning areas in the spring (see Habitat). Paddlefish appear to be very highly mobile especially during the spring migration (see Parker 1988), sometimes moving over long distances. For example, tagged fish in Missouri moved over 1900 km down the Missouri River into the Mississippi (Unkenholz 1983), although spawning runs of 80 to 270 km are more common (NatureServe 2007). In Missouri, the maximum activity range over the course of a year was 92 km (Moen et al. 1992).
Interspecific interactions
The young and smaller fish may be the prey of piscivorous fishes, but predation on larger fish has not been reported (Mero et al. 1994).
Behaviour/adaptability
Successful migration to spawning areas and initiation of spawning is dependent on barrier–free migration routes, specific water temperatures and flow rates. Thus, populations may be negatively impacted by any anthropogenic change to their habitat. However, paddlefish do display some degree of adaptation. They have been known to move through reservoir floodgates and spillways into areas below spillways (Pitman and Parks 1994), and may pass open dam gates (Southall and Hubert 1984). They may also actively select artificial features such as rock piles, revetments, locks, and tailwaters of dams, etc., as holding areas (Parker 1988). Paddlefish may be successfully cultured and hatchery stocks now support several paddlefish populations in the United States (Parker 1988; NatureServe 2007).
Improvements to water quality in non–wadeable Ohio rivers have also been associated with greater abundance and distribution of paddlefish (Yoder et al. 2005).
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