Dispersal and Within-Stream Spatial Population Structure of Brook Trout Revealed by Pedigree Reconstruction Analysis

Spatial patterns of spawning and early dispersal have important implications for the population dynamics of stream-dwelling salmonids, but the limitations of marking technology have made it difficult to measure these processes in wild populations. We used microsatellite DNA markers and sibship and parentage analyses to follow the dispersal, spatial distribution, and distribution of reproductive success in a small, isolated western Virginia population of brook trout Salvelinus fontinalis at 4, 16, and 28 months after fry emergence. For the 2004 year-class (high-recruitment cohort), we identified 180 full-sibling families representing individual spawning events. Offspring were unevenly distributed across families, with 16% of the families accounting for 50% of the offspring and 53% of the families being represented by fewer than three individuals. However, a large proportion of adults had some successful reproduction. Spatial and family size distributions at 4 months after emergence were similar between the 2004 and 2006 (low-recruitment) year-classes in spite of a threefold difference in abundance. The spatial locations of full sibs were closely associated, indicating limited dispersal in the first 4 months postemergence. The spatial locations of assigned parents were correlated with the locations of their offspring. For the 2004 cohort, sibling dispersal substantially increased after the 4-month sample, but neither fish length, family size (number of individuals), nor fish density was related to dispersal distance at any postemergence time interval. In this study, we demonstrate the ability of sibship and parentage analyses to reveal important aspects of brook trout population structure and movement. Our results suggest that limited dispersal by age-0 brook trout and their parents results in a high level of within-stream spatial population structure even in the absence of barriers to movement, and this must be accounted for in genetic surveys and management studies.