Evaluating the Trade-Offs between Invasion and Isolation for Native Brook Trout and Nonnative Brown Trout in Pennsylvania Streams

A popular conservation strategy for native trout species in western North America is to prevent invasions by nonnative trout by installing barriers that isolate native trout populations into headwater streams. In eastern North America, native Brook Trout Salvelinus fontinalis are frequently replaced in coolwater habitats by nonnative Brown Trout Salmo trutta and relegated to small headwater streams. In this study, we compared the effects of isolation and invasion by nonnative Brown Trout on the distribution and demographic structure of Brook Trout populations from 78 trout streams in northwestern Pennsylvania. The Brook Trout and Brown Trout distributions varied in predictable ways along the stream size gradient, with Brown Trout becoming dominant in larger streams. However, there was a prominent barrier effect, with streams 12 times more likely to have Brook Trout than Brown Trout when a downstream barrier was present between the sample site and the nearest Brown Trout stocking location. In comparison, 91% of the streams with Brown Trout had no downstream barrier, suggesting that barriers are important in creating refugia for Brook Trout. Brown Trout also appeared to have a negative impact on Brook Trout population demographics, as Brook Trout populations in sympatry with Brown Trout had fewer age-classes and lower population densities than allopatric Brook Trout populations. Isolating Brook Trout to small headwater streams with downstream barriers that prevent Brown Trout invasion could be a viable conservation strategy in regions where barriers would serve to reduce the negative impacts from Brown Trout. Since barriers could further fragment local Brook Trout populations, however, they would need to be strategically placed to allow for seasonal movements to maintain metapopulation structure and ensure population persistence. A frequently documented pattern in North American streams is the gradual replacement of native trout Oncorhynchus and Salvelinus spp. with nonnative trout. However, it is often unclear whether this is due to replacement or displacement of the native species by a nonnative species. In some cases, nonnative trout replace native trout by filling open “ecological niches” in the wake of native extirpations resulting from land use changes and habitat degradation (Dunham et al. 2002; Rieman et al. 2006; Hudy et al. 2008; Al-Chokhachy et al. 2016). In other instances, nonnative trout have displaced native trout by directly reducing native population viability via hybridization (Allendorf and Leary 1988) or interference competition and intraguild predation (Fausch and White 1981; Grant et al. 2002; Peterson et al. 2004; Hoxmeier and Dieterman 2013; Hitt et al. 2017). Preventing the establishment of nonnative trout has been an important part of native trout conservation because native trout often recover following the removal of nonnatives (Kanno et al. 2016). One management strategy for conserving native trout in the face of nonnative introductions has been the “isolation management” strategy, whereby barriers are intentionally installed to prevent invasion by nonnative trout (Novinger and Rahel 2003; Rahel 2013). An unfortunate *Corresponding author: kirkma18@gmail.com Present address: Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming 82071, USA. Present address: College of Fisheries and Ocean Sciences, University of Alaska–Fairbanks, Fairbanks, Alaska 99709, USA. Received March 2, 2018; accepted May 17, 2018 Transactions of the American Fisheries Society 147:806–817, 2018 © 2018 American Fisheries Society ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1002/tafs.10078