Population Statistics for Paddlefish in the Wisconsin River

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.