Evaluating the effects of three Alabama River dams on movements and population connectivity of Freshwater Drum and White Crappie using otolith microchemistry and genetics

ABSTRACT Objective 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. Here, we used otolith microchemistry and genetic techniques to quantify potential movements and population connectivity among these river sections for two fish species (Freshwater Drum Aplodinotus grunniens and White Crappie Pomoxis annularis) that differ in life expectancies, spawning strategies, and swimming abilities. Methods We collected water samples throughout the study area to quantify spatial variation in trace element ratios, and we collected fish from the four river sections defined by the three lock-and-dam structures to quantify spatial variation in their otolith trace element ratios and their population genetic structure. Results Trace element ratios (Mg:Ca, Mn:Ca, Sr:Ca, and Ba:Ca) in water samples collected throughout the study area varied spatially but were temporally consistent. Broad patterns in water chemistry were reflected in element : Ca 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 both species, while the associations between otolith edges and water were mostly nonsignificant for Mg:Ca, Mn:Ca, and Ba:Ca ratios. Linear discriminant analyses 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 between species. Genetic structure was present in both species but was weaker in Freshwater Drum than in White Crappie, indicating that barriers have had a greater influence on White Crappie genomics than on Freshwater Drum genomics. Overall, there was little support for either species showing substantive movement past the lock-and-dam structures. Conclusions Our results based on both otolith microchemistry and genetics suggest that movement of these species past dams is very limited. Individuals of both species generally appear to remain in areas near where they hatched; although movements among habitats may occur, both upstream and downstream dam passage events appear rare. Efforts at increasing connectivity, if successful in attracting and moving fish, would increase opportunities for movement, thus increasing gene flow beyond its highly restricted current state.