Movement of Paddlefish once past a lock-and-dam structure on the Alabama River

Objective: Movement of fish past dams can be facilitated by dedicated fish passage structures, navigational locks, and crested spillways, with the efficacy of a passage structure depending strongly on the nature of the system (height of the dam, flow rate, etc.) and the fish's behavior and swimming capability. However, once the fish have passed a dam, whether by using a mitigation structure or due to active translocation, they encounter a different habitat upstream versus in the tailrace, potentially affecting their ability to continue their upstream migration. Methods: Here, we had two objectives. First, we determined whether Paddlefish Polyodon spathula that successfully passed a structure continued their upstream migration in the Alabama River. Because assuring passage by tagged fish required that we move the fish past the dam, our second objective was to determine whether translocated fish exhibited fallback behavior (downstream drift or movement postrelease that would compromise their continued migration). We used both active and passive telemetry methods to quantify the postpassage movements of tagged and translocated Paddlefish. Result: Fish that were translocated above Claiborne Lock and Dam (CLD) exhibited upriver movements once translocated, and they exhibited no fallback (i.e., down-stream movement with delayed or no continuation upstream). Timing of movement relative to spawning periods did not influence initial fish movement or the likelihood of reaching the next upstream dam (Millers Ferry Lock and Dam [MFLD]; similar to 100 river kilometers upstream) within the first 30 days of observation, but more fish that were tagged and released during the early prespawn period made it to within 4.83 km of MFLD (the location of our closest receiver below MFLD) relative to fish from the prespawn or spawning period. Fish that were released above CLD had a lower probability of being subsequently detected downstream of their release sites compared to fish that were released below CLD. Conclusion: Our findings support that Paddlefish will continue their upstream migration once they have passed a structure, despite changes in habitat.