Evaluating the effects of three Alabama River dams on fish movements and population connectivity using otolith microchemistry

Abstract

Dams impede fish movement and can isolate riverine populations into defined areas. The Alabama River is divided into four major sections by three lock-and-dam structures. I used otolith microchemistry to quantify movements and population connectivity among these river sections by three fish species (Freshwater Drum, White Crappie and Blue Catfish) that differ in life expectancies, spawning strategies and swimming abilities. Water sample trace-element ratios (Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca) from throughout the study area varied spatially but were temporally consistent. Broad patterns in water chemistry were reflected in element:calcium ratios in otolith whole-transects (i.e., across entire life), edges (reflecting time of capture), and cores (reflecting early life). Correlations between otolith-edge and season-specific water Sr:Ca ratios from the areas where fish were collected were significant for all three species, while the associations between otolith-edges and water were mostly nonsignificant for Mg:Ca, Mn:Ca, and Ba:Ca ratios. Linear discriminant analyses (LDAs) were used to determine how accurately the multivariate element signatures in otolith-edges could classify fish back to the river sections from which they were collected, resulting in mixed accuracies across species. Otolith whole-transect LDA classifications were generally similar in accuracy to otolith-edge LDAs for each species, while core region LDA classification accuracies were typically lowest, likely due to enriched element concentrations within otolith cores. I developed criteria for identifying potential dam passage events within fish lifetimes based on otolith Sr:Ca profiles, and evidence that these species move past dams appears very limited. Otolith microchemistry results generally appear to suggest that these fish tend to remain in areas near where they hatched and while movements among habitats may occur, both upstream and downstream dam passages appear rare.