Spawning Migrations and Reproductive Dynamics of Paddlefish in the Upper Missouri River Basin, Montana and North Dakota

A reproducing population of paddlefish Polyodon spathula in the upper Missouri River basin of Montana and North Dakota has spawning access to the free-flowing Yellowstone River, the thermally and hydro-logically impacted Missouri River below Fort Peck Dam, and the Milk River (a tributary to the Missouri River below Fort Peck Dam). In this study, we quantified paddlefish spawning migrations into these potential spawning areas based on 4 years (2002-2005) of telemetry information and quantified spawning success based on collections of larvae over a 5-year period (2001 2005). The initiation of spawning migrations into the Yellowstone River and Missouri River was consistent among years (early to mid-April) and typically occurred following slight increases (3.8-18.1%) in discharge. Each year, about one-third and two-thirds of the radio-implanted population of paddlefish migrated into the Missouri River and Yellowstone River, respectively. Spawning migrations into the Milk River primarily occurred only following substantial increases (>200%) in discharge. Larval paddlefish were collected annually in all rivers except the Milk River in 2003 when discharges were low and paddlefish did not migrate into the river. Spawning was initiated earliest in the Yellowstone River, and there was strong evidence that spawning in the Yellowstone River and Milk River was initiated by a discharge pulse. Results from this study provide an understanding of the reproductive dynamics of paddlefish and the environmental factors associated with paddlefish reproduction in the upper Missouri River basin.