Geochemistry and Genetics Reveal Scale and Ontogeny of Dispersal of Smallmouth Bass in a River-Tributary Network

Dispersal of stream fish between mainstem river and tributary habitats is central to connectivity of populations, expansion of invasive species, and spatial patterns in community composition across river-tributary networks.  We used an approach combining geochemical isoscape modeling, otolith isotopic composition, and population genetics to determine the timing and scale of dispersal events across life history for smallmouth bass Micropterus dolomieu in the James River basin of Virginia.  Geochemical modeling revealed heterogeneity in strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) that distinguished tributaries from mainstem habitats.  Analysis of otoliths demonstrated that exchange between rivers is common; half of all fish analyzed moved between rivers at some point in their life.  Most dispersed from their natal river in the first year of life within a few months post-spawn, revealing a much earlier and broader pattern of juvenile dispersal than previously described.  There was no evidence for asymmetry of recruitment exchange between tributaries and mainstem.  Population genetic structure indicated isolation-by-distance within the mainstem and some spatial partitioning of genetic clusters, suggesting that longitudinal dispersal within mainstem habitats may be comparatively limited. Early juvenile dispersal appears to be a key driver of demographic exchange among habitats and should be an important consideration in spatial management strategies.