Quantifying the movements of Paddlefish Polyodon spathula in the lower Alabama River and Mobile-Tensaw River Delta

Abstract

Paddlefish Polyodon spathula are mobile potamodromous fish that move among riverine, deltaic, and estuarine habitats in coastal drainages. Despite a reasonable amount of research having been conducted on Paddlefish life history and migrations, relatively little is known about their use of downstream deltaic or estuarine (low salinity) areas. To address this knowledge gap, I tagged and tracked Paddlefish in order to quantify their movements in the lower Alabama River and Mobile-Tensaw River Delta (MTRD), Alabama; identify distinct movement behaviors, assess interannual consistency in those behaviors, and evaluate associations between MTRD use and several environmental factors (i.e., temperature, salinity, gage height). From December 2024 through January 2026, 148 Paddlefish were tagged with acoustic-radio transmitters and monitored using both passive (N = 17 receivers in the Alabama River and 18 receivers throughout the MTRD) and active telemetry during 8 seasonal surveys of the lower Alabama River and MTRD. Movement metrics were analyzed with k-means clustering, and weekly MTRD use was modeled using generalized linear mixed models. Of the 148 tagged fish, 131 provided sufficient detections for inclusion in the data set (N=4,057,456 total detections), and 110 (83%) used the MTRD. Three distinct movement behaviors were identified in Year 1, two of those unique movements were also identified in Year 2. Among the 43 fish that could be classified in both years, 40% showed similar behaviors. MTRD use increased with upstream salinity and temperature but decreased with river stage. These results state extensive use of deltaic habitat during non-spawning periods and suggested that some individuals may repeat similar movement behaviors between years. Paddlefish conservation and management must consider the importance of these downstream estuarine habitats due to their importance as a summer and fall refuge for this species.