THE CONTROL OF STREAMBED GRAIN SIZE UPON SALMONID HABITAT IN THE SOUTHERN APPALACHIANS

Encroachment of exotic trout species are confining brook trout (Salvelinus fontinalis), the only salmonid native to the southern Appalachians, to headwater streams. Previous region-specific studies have investigated the impact of hydrologic, biologic, and land use controls as a function of stream reach elevation on salmonid distributions. Streambed grain size availability may be an additional control on species fecundity since brook trout use a significantly smaller grain size than the invasive salmonids in the construction of redds, the streambed mounds in which salmonids deposit their eggs.

We created a streambed grain size model in order to compare stream reach-averaged median grain size with surveys of native and nonnative salmonid locations obtained from previous studies. We used the Shields equation to model grain sizes in four streams in the Smoky Mountains of the southern Appalachians. Watershed and stream morphometrics obtained from a digital elevation model and field surveys provided inputs and validation for the model, respectively. The observed median grain size in field surveys of all study streams decreased upstream, a seemingly counterintuitive result that may be linked to the more prevalent trapping of finer-grained sediment upstream of log jams in headwater reaches. The minimum elevation of stream reaches containing only brook trout are positively correlated with stream gradient similar to findings of previous studies. Model results will be presented and their relationship with observed salmonid distribution will be discussed.

Further transgression of brook trout upstream will likely result in isolated populations of dramatically decreased numbers of inbreeding brook trout. Determining if grain size is a control upon habitat selection may motivate land managers to relax fishing regulations on invasive salmonids and focus habitat restoration efforts onto particular streams.