Relating juvenile spatial distribution to breeding patterns in anadromous salmonid populations.

1. Spatial within-population heterogeneity in density probably affects competition intensity and may have a fundamental role in shaping population dynamics and carrying capacity. This may be particularly relevant for organisms where limitations on juvenile mobility cause maternal choice of breeding locations to influence the spatial distribution of younger life stages. 2. In this study, we mapped redd locations and the resulting densities of juveniles the following year along the entire reach (9.2 km) of a river holding natural populations of anadromous salmonids (Atlantic salmon and brown trout). These data were used to quantify the spatial scale over which breeding influences juvenile densities, and hence becomes important for density-dependent processes. 3. Although the observed cumulative distributions indicated a relatively uniform distribution of breeding along the river, autocorrelation analyses identified spatial patchiness of both breeding and resulting juveniles on a local scale. Furthermore, cross-correlations suggested a close spatial relationship between distribution of redds and juveniles. 4. Using spatially explicit hockey-stick stock-recruitment functions, we found juvenile salmonid density to be mostly influenced by the amount of breeding upstream of a given location. This influence decreased rapidly within the first 75-150 m. Thus, female choice with regard to breeding location gave rise to a heterogeneous distribution of offspring on a spatial scale of almost two orders of magnitude finer than that of the whole population (9.2 km). 5. The results are consistent with smaller scale experimental studies of salmonids, and emphasize the role for maternal choice of breeding locations in causing substantial spatial heterogeneity in juvenile densities within natural populations. Due to effects of density heterogeneity on overall levels of competition, this adds another layer of complexity to the dynamics of salmonid populations even in populations where breeding appears to be relatively uniformly distributed through space, and potentially also for a range of other organisms where juvenile dispersal is constrained.