Detecting local and regional controls of brook trout and mottled sculpin population dynamics through density manipulation experiments

The relative importance of local controls (e.g., survival and recruitment) versus regional controls (e.g., dispersal) of stream fish population dynamics remains largely unknown. We used a 2 x 2 factorial design (density treatment x spatial location treatment) along with capture-mark-recapture techniques to quantify the effects of density perturbations on the population dynamics of two headwater stream fishes — brook trout and mottled sculpin. A key consideration in this study was whether population recovery differed depending on whether the focal stream was highly isolated in the headwaters or drained directly into a larger river mainstem. Although population dynamics varied widely across all study sites, we found evidence of density dependence in some population vital rates, including immigration and individual growth rates. Nevertheless, the dominant pattern in the data was the importance of dispersal processes and the influence of spatial position on overall population dynamics. Our results indicate that the local dynamics of both mottled sculpin and brook trout were controlled simultaneously by local density dependent processes and regional dispersal processes. These results emphasize the importance of immigration and drainage network connectivity for maintenance of headwater stream fish populations.