Using Large-Scale Population Manipulation to Understanding Source-Sink Dynamics of Invasive Brown Trout

Exotic species present one of the greatest threats to native fish conservation, and efforts to manage or remove exotics are often undertaken.  However, eradication of exotic species using chemical toxins is costly and impractical in many locations.  Alternatively, mechanical removal can be used to identify sources of invasive fishes, and thus increase the efficacy of management by targeting these hot spots.  Our goal was to evaluate whether, after mechanical removal of exotic brown trout from two reaches demonstrating high brown trout density, Biotic Resistance by native cutthroat trout would limit recolonization by brown trout.  We mechanically removed brown trout from 1.3 km of the Logan River, Utah between two small impoundments, and 5.6 km of Right Hand Fork, a tributary to the Logan River, using single-pass electrofishing during 2009 and 2010.  During 2009 we removed 4865 and 959 brown trout, from the tributary and mainstem reaches, respectively, whereas during 2010 we removed 5423 and 1995.  These results suggest that large-scale, single-pass removal had little negative effect on brown trout densities.  However, brown trout removal from Right Hand Fork resulted in a strong recruitment pulse, dramatically shifting the size distribution towards smaller, predominantly age 0, brown trout.  In contrast, although there was slightly greater brown trout recruitment observed in the mainstem after removal, there was little change in the population size distribution.  These results suggest that in the tributary, before removal, density-dependent mortality or emigration of YOY fish stabilizes adult population size and provides an annual source of brown trout out of the tributary.  In contrast, impoundments adjacent to the mainstem reach provide a source of adult brown trout that rapidly recolonized the Logan River.  Furthermore, we observed no change in habitat use after removals in 2010, suggesting that brown trout quickly resaturated all available habitat.  Single-pass removal of exotic brown trout also resulted in increased recruitment of native Bonneville cutthroat trout in both study reaches, where cutthroat trout recruitment was first documented in 2010.  These results suggest that in sites demonstrating strong density-dependent mortality or emigration, or in close proximity to sources of exotics, one-time mechanical removal of exotic brown trout is likely to have limited effects on brown trout populations.  While mechanical removal may not eliminate brown trout, multiple removals may allow cutthroat trout populations to reach densities at which Biotic Resistance can limit subsequent expansion and recolonization by brown trout, thus providing a valuable alternative to chemical eradication.