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Populations of paddlefish Polyodon spathula have been adversely affected by dams that can block their movements. Unlike high-head dams that preclude fish passage (unless they are equipped with fishways), the dams on the upper Mississippi River are typically low-head dams with bottom release gates that may allow fish passage under certain conditions. We evaluated the relation of dam head and river discharge to the passage of radio-tagged paddlefish through dams in the upper Mississippi River. Radio transmitters were surgically implanted into 71 paddlefish from Navigation Pools 5A and 8 of the upper Mississippi River and from two tributary rivers during fall 1994 through fall 1996. We tracked paddlefish through September 1997 and documented 53 passages through dams, 20 upstream and 33 downstream. Passages occurred mostly during spring (71%) but also occurred sporadically during summer and fall (29%). Spring passages varied among years in response to hydrologic conditions. We evaluated patterns in upstream and downstream passages with Cox proportional hazard regression models. Model results indicated that dam head height strongly affected the upstream passage of paddlefish but not the downstream passage. Several paddlefish, however, passed upstream through a dam during periods when the minimum head at the dam was substantial (Ն1 m). In these cases, we hypothesize that paddlefish moved upstream through the lock chamber.

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Although paddlefish Polyodon spathula are protected in Wisconsin and Minnesota, limited information exists on the status of populations in the upper Mississippi River. Paddlefish populations in the upper Mississippi River probably have never fully recovered from declines that occurred in the late 1800s and early 1900s following habitat loss and extensive fishing. In our study, we estimated population size, size and age structure, and mortality for a paddlefish population in the Wisconsin River, a tributary of the upper Mississippi River, during 1993 and 1994. We tagged 337 paddlefish and recaptured 33. Unequal probability of capture and small sample sizes violated the assumptions of published population models. To improve interpretation of results, data were adjusted for assumption violations, and estimates were generated using three widely used models (modified Jolly–Seber, adjusted Petersen, and Chapman's modified Schnabel). Based on Chapman's modified Schnabel adjusted for mortality and estimated ages of 5–23 years for all paddlefish sampled, we estimated the population to be 1,353 paddlefish. Eye‐to‐fork length for paddlefish ranged from 64 to 125 cm. Using catch‐curve analysis, we estimated annual mortality to be 26.7%. Size structure and age structure data indicated weak year‐classes for the 4–6 years preceding our sampling. Monitoring of the Wisconsin River paddlefish populations should continue because of the relatively high concentration of fish with the potential as a source population; investigations should also be implemented on other large tributaries and main stem of the upper Mississippi River.

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