Linking Individual Fish Movement to the Demographics and Genetics of Populations

Ecological theory predicts that small dispersal rates can have large effects on population persistence and gene flow.  Mark-recapture studies of individual stream fish typically conclude that movement is restricted to the reach scale (10¹-10³ m).  In contrast, less direct studies of dispersal (e.g., occupancy models, genetic studies) often find watershed-scale (10⁴-10⁵ m) connectivity among populations.  This contradiction might be reconciled if mark-recapture data were analyzed using techniques that focus on the "tails", rather than the "centers", of movement-frequency distributions.  In this study, we assessed the fit of a suite of theoretical models to published movement data from North American stream fishes representing various taxa.  From best-fitting models, we extrapolated the spatial extents over which key dispersal thresholds relevant to population connectivity (e.g., 10% dispersal rate, one migrant per generation) might be reached.  Study design exerted a strong influence on estimates of mean and maximum movement distance, whereas taxonomic affiliation did not.  Models that allowed for intra-population heterogeneity in movement behavior generally provided a better fit to data than did those assuming homogeneity.  Using heterogeneity-based models, we projected non-trivial movement over spatial extents much greater than traditional movement analyses have suggested.  Such long-distance movements form the key, understudied link between individual movements and the population dynamics and evolution of stream fishes.