Dispersal between Tributary and Main-Stem Rivers by Juvenile Smallmouth Bass Evaluated Using Otolith Microchemistry

Juvenile dispersal and the exchange of offspring among fish populations in river-tributary networks are difficult to characterize, but they may play a substantial role in the dynamics of fisheries in these systems. We used geochemical signatures in otoliths to identify the natal origins of young-of-year smallmouth bass Micropterus dolomieu in the James River and a tributary, the Maury River, in Virginia. The trace element signatures (Sr:Ca, Rb:Ca, Mg:Ca, and Ba:Ca) in otoliths differed significantly between these two rivers, resulting in approximately 87% accuracy in classifying known-origin fry to their natal rivers. The variation in otolith composition reflected the variation in water chemistry within the system. We subsequently used classification functions and stock mixture analyses based on these signatures to identify the natal origins of age-1 juveniles collected from the same rivers 1 year later. The results indicate that approximately 50% of the age-1 smallmouth bass collected in the James River were spawned in the Maury River, while no evidence for reciprocal main stem-to-tributary exchange was found. These results suggest that the downstream movement and dispersal of smallmouth bass is common in their first year of life. Such inputs from tributaries may play an important role in the dynamics of smallmouth bass river fisheries, which are often characterized by substantial annual variation in recruitment. Characterizing such exchange at broader scales will probably be an important step toward linking the spatial ecology of this species with effective management and conservation strategies in river-tributary networks.