The Influence of Hydrological Patterns on Brook Trout (Salvelinus fontinalis) and Rainbow Trout (Oncorhynchus mykiss) Population Dynamics in the Great Smoky Mountains National Park.

Hydrologic processes impact the functioning of aquatic ecosystems and influence fish population dynamics. The flow regime of a stream affects the structure, composition, and productivity of fish communities by regulating abiotic habitat conditions and biotic community processes. In the Great Smoky Mountains National Park (GRSM), native brook trout (Salvelinus fontinalis) populations have declined in some watersheds over the past decade, believed to be primarily due to episodic acidification. The potential affects long-term hydrologic patterns, temporal hydrologic trends, and hydrologic extremes have on brook and rainbow trout (Oncorhynchus mykiss) populations were explored in this study. The current GRSM fish sampling program began nearly two decades ago; a total of 69 streams, including 369 sites, are routinely sampled by GRSM fisheries biologists with standard methods. Detailed data is collected on the trout populations. The Nature Conservancy¡¯s Indicators of Hydrologic Alteration (IHA) was used to quantify the flow regime of each stream into 67 ecological relevant parameters. Because the trout sites were located in remote ungaged streams, the Hydrologic Simulation Program FORTRAN (HSPF) was used to simulate flows at each trout site for a study period lasting 18 years (1990-2007). Using local climate data the model was calibrated by adjusting parameters including storage, infiltration, runoff, and ground water for three elevation classes (low < 800 m ¡Ü middle ¡Ü 1,200 m < high) to observed stream flows from two USGS gaging stations and one NPS gaging station. The parameters defined by IHA included the a) magnitude, b) frequency, c) duration, d) time, and e) rate of change of hydrological events. IHAs were statistically compared to the surveyed trout populations. Results indicated the abundance of young-of-the-year (YOY) brook and rainbow trout significantly declined after extreme floods and droughts. In particular, low-flows during droughts significantly reduced recruitment for both brook and rainbow trout, which is likely due to decreased spawning habitat. Brook trout populations in larger low-elevation streams showed more stability compared to smaller headwater streams. The study provided GRSM resource managers a calibrated hydrology model with unique flow parameters characterizing GRSM watersheds, and possible influences hydrologic conditions may have on trout populations.