Spatial Patterns of Synchrony in Recruitment of Trout Among Streams

Synchronous recruitment has been documented among salmonid populations in streams draining mountainous regions, and to a lesser degree in low-gradient, groundwater-fed streams. Relatively little is known about the spatial extent of recruitment synchrony among trout populations in low-gradient streams. We mapped Brook Trout Salvelinus fontinalis, Brown Trout Salmo trutta, and Rainbow Trout Oncorhynchus mykiss populations in low-gradient Michigan, USA streams whose recruitment dynamics were synchronous based on correlations in annual densities of age-0, age-1, and age-2 fish, and used maps of correlated populations to estimate the spatial extent of synchrony. Significant correlations indicative of synchronous recruitment occurred for all three species. The maximum spatial extent of synchronous recruitment observed for each species was greater than in many studies to date. Most Rainbow Trout populations were adfluvial, resulting in our documenting synchrony in steelhead recruitment. The persistence of synchronous patterns in year-class strength among older age groups of trout highlights the importance of recruitment to trends in trout abundance among streams within a region. By controlling for spatial variation among sites through time, use of index sites enables a coherent picture of synchronous patterns in recruitment to emerge at the regional scale and better positions fishery managers to evaluate influences of local-scale factors and larger-scale processes on local stream trout populations.