Biology and Life History of Paddlefish in North America: An Update
Cecil A. Jennings and Steven J. Zigler
Abstract.—Paddlefish Polyodon spathula are among the largest and longest lived of the freshwater fishes (e.g., more than 2.2 m long; 72 kg; 30 years old) and can be distinguished by the presence of a large mouth and a long, paddle-shaped snout. Smooth skin, small eyes, a large, tapering operculum flap, bluish- gray to black coloration dorsally, and a deeply forked heterocercal caudal fin all serve to distinguish paddlefish from other species. Paddlefish become sexually mature and spawn at a later age than many other freshwater fishes; males mature at an earlier age than females, but maturity varies by latitude. Male paddlefish typically spawn each year, but spawning periodicity may be variable for females. Paddlefish spawn over gravel or other hard surfaces and require specific photoperiod, water temperature, and water flow for successful spawning. Paddlefish are relatively fecund (9,000–26,000 eggs per kilogram of body weight); mature eggs range from about 2.0–4.0 mm in diameter, and time from egg fertilization to hatching is directly related to water temperature. Optimum temperature for hatching is about 18_C. Newly hatched larvae average about 8.5 mm total length (TL) and are passive drifters until they are about 17 mm long when the yolk sac has been absorbed and the larvae begin active feeding on zooplankton and insects. Paddlefish complete fin ray development at 145–160 mm TL; at this size, they are considered juveniles and are similar in appearance to adults. Few paddlefish reach the maximum known age; instead, the median age for most populations is 5–8 years and maximum age is 14–18 years. Paddlefish growth seems to be directly related to the length of the growing season and food abundance. Generally, paddlefish length increases rapidly for about the first 5 years. After 5 years, paddlefish weight increases rapidly and may double during this time. Paddlefish feed primarily on zooplankton but occasionally consume small insects, insect larvae, and small fish. Traditionally, paddlefish inhabited slow-moving waters of side channels and river-lakes. In regulated rivers, paddlefish congregate where current velocities are reduced. In large rivers, paddlefish tend to congregate in the deep waters, usually selecting areas with depths greater than 3 m and current velocities less than 0.5 m/s. Further, paddlefish are highly mobile and make extensive movements within a system. Most of this movement is random, but paddlefish also make extensive nonrandom movements in spring during upstream migration to spawning areas. Some aspects of paddlefish life history and biology make them highly vulnerable to human activities. High prices for paddlefish roe or flesh periodically have stimulated fishing pressure and overexploitation followed by rapid declines in some populations. Dredging, flow manipulation, and the construction of dams have altered much of the traditional paddlefish habitat. Increasing levels of recreational and commercial boat traffic may also contribute to the mortality of paddlefish. Understanding and considering paddlefish biology and ecology can contribute to scientifically sound stewardship of all paddlefish populations, whether management is for conserving healthy populations or restoring decimated stocks.