Geochemistry and genetics reveal scale and ontogeny of fish dispersal in a river-tributary network

Background/Question/Methods

Dispersal of stream fish between mainstem river and adjacent tributary habitats is a process central to maintaining connectivity of populations, expansion of invasive species, and establishing spatial patterns in community composition across river-tributary networks.  Traditional marking- or telemetry-based methods provide incomplete information on this process due to limitations on the size of fish that can be employed, sample size, or the size of the area that can be studied effectively.  Dispersal in the early juvenile period is particularly hard to characterize, especially for nest-spawning species where young-of-year are less susceptible to advective transport.  We used an approach combining geochemical isoscape modeling, hard-part chemistry, and population genetics to determine the timing and scale of dispersal events across life history for smallmouth bass Micropterus dolomieu, a common centrarchid fish species with both native and introduced populations in North America.  Student collaborators were responsible for directing components of the research and refining methods from published examples to meet our study objectives.  Our study area was the James River basin of Virginia, which supports a large population of smallmouth bass introduced in the 1800s. 

Results/Conclusions

Modeling landscape geochemistry revealed areas of geologic heterogeneity where variation in strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) distinguished tributaries from mainstem reaches.  Subsequent analysis of water and otolith samples confirmed this heterogeneity and established a basis for determining residency and movement of individuals between the mainstem James River and its major tributaries via strontium isotope ratios in otoliths.  Analysis of fish collected from mainstem and tributary reaches demonstrated that exchange between rivers is common; half of all fish analyzed showed movement between rivers at some point in their life.  The vast majority (80%) of these fish dispersed from their natal river in the first year of life and mostly within the first few months post-spawn, revealing a much earlier and broader pattern of juvenile dispersal than previously described for the species.  There was no evidence for asymmetry of recruitment exchange between tributaries and mainstem.  Population genetic structure indicated a pattern of isolation-by-distance within the main stem and some spatial partitioning of genetic clusters, suggesting that longitudinal dispersal within the mainstem habitat may be limited in comparison. Early juvenile dispersal appears to be a key driver of exchange among habitats and therefore should be an important consideration in spatial management strategies.