Hydrologic and hydraulic factors affecting passage of paddlefish through dams in the Upper Mississippi River

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 (≥1 m). In these cases, we hypothesize that paddlefish moved upstream through the lock chamber.