AppearanceAmerican paddlefish are among the largest and longest-lived freshwater fishes in North America. They have a shark-like body, average 1.5 m in length, weigh 27 kg, and can live in excess of thirty years.
For most populations the median age is five to eight years and the maximum age is fourteen to eighteen years.
American paddlefish are smooth-skinned and almost entirely cartilaginous. Their eyes are small and directed laterally. They have a large, tapering operculum flap, a large mouth, and a flat, paddle-shaped rostrum that measures approximately one-third of their body length.
During the initial stages of development from embryo to hatchling, American paddlefish have no rostrum. It begins to form shortly after hatching. The rostrum is an extension of the cranium, not of the upper and lower jaws or olfactory system as with the long snouts of other fishes.
Other distinguishing characteristics include a deeply forked heterocercal caudal fin and dull coloration, often with mottling, ranging from bluish gray to black dorsally grading to a whitish underbelly.
NamingAmerican paddlefish are closely related to sturgeons in the order Acipenseriformes, an order of basal ray-finned fishes that includes sturgeon and paddlefish, several species of which are now extinct.
Paddlefish are among the oldest of fishes as evidenced in the fossil record which dates their first appearance approximately 300 to 400 million years ago, almost 50 million years before the dinosaurs. Fossils of a second extinct species, ''P. tuberculata'', which date back approximately 60 million years ago, were found in the Lower Paleocene Tullock Formation in Montana.
DistributionAmerican paddlefish are endemic to the Mississippi River Basin, historically occurring from the Missouri and Yellowstone rivers in the northwest to the Ohio and Allegheny rivers of the northeast; the headwaters of the Mississippi River south to its mouth, from the San Jacinto River in the southwest to the Tombigbee and Alabama rivers of the southeast.
BehaviorAmerican paddlefish are long-lived, sexually late maturing pelagic fish. Females do not begin spawning until they are seven to ten years old, some as late as sixteen to eighteen years old. Females do not spawn every year, rather they spawn every second or third year. Males spawn more frequently, usually every year or every other year beginning around age seven, some as late as nine or ten years of age.
American paddlefish begin their upstream spawning migration sometime during early spring; some begin in late fall. They spawn on silt-free gravel bars that would otherwise be exposed to air or covered by very shallow water were it not for the rises in the river from snow melt and annual spring rains that cause flooding.
Young American paddlefish are poor swimmers which makes them susceptible to predation. Therefore, rapid first-year growth is important to their survival. Fry can grow about 1 in per week, and by late July the fingerlings are around 5–6 in long.
Their rate of growth is variable and highly dependent on food abundance. Higher growth rates occur in areas where food is not limited. The feeding behavior of fingerlings is quite different from that of older juveniles and adults. They capture individual plankton one by one, which requires detection and location of individual ''Daphnia'' on approach, followed by an intercept maneuver to capture the selected prey.
By late September fingerlings have developed into juveniles, and are around 10–12 in long. After the 1st year their growth rate slows and is highly variable. Studies indicate that by age 5 their growth rate averages around 2 in per year depending on the abundance of food and other environmental influences.
HabitatAmerican paddlefish are highly mobile and well adapted to living in rivers. They inhabit many types of riverine habitats throughout much of the Mississippi Valley and adjacent Gulf slope drainages.
They occur most frequently in deeper, low current areas such as side channels, oxbows, backwater lakes, bayous, and tailwaters below dams. They have been observed to move more than 2,000 mi in a river system.
FoodThe American paddlefish's diet consists primarily of zooplankton. Their electroreceptors can detect weak electrical fields, which signal not only the presence of zooplankton, but also the individual feeding and swimming movements of zooplankton appendages.
When a swarm of zooplankton is detected, the paddlefish swims forward continuously with their mouth wide open, forcing water over the gill rakers to filter out prey. Such feeding behavior is considered ram suspension-feeding. Further research has indicated that their electroreceptors may also serve as a navigational aid for obstacle avoidance.
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