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

Abstract

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.

Spatial assemblage and interference competition of introduced Brown Trout (Salmo trutta) in a Himalayan river network: Implications for native fish conservation

“…The results of our study are in congruence with the other documented observations of the impacts of the invasive trout on the distribution of the native species across the globe (Larson and Moore, 1985;Fausch 2008;Hoxmeier and Dieterman, 2013). Owing to its aggressive territoriality, the Brown Trout has been found to exclude the native fishes, distinctively relegating them to the headwater or the smaller order tributaries (Fausch and White, 1986;Kirk et al, 2018). Owing to higher numbers of the Brown Trout adults in the downstream segments, the Snow Trout seemingly maintains higher numbers in the upstream segments.…”

Factors Associated with Detection and Distribution of Native Brook Trout and Introduced Brown Trout in the Driftless Area of Iowa

“…Instead, one of our plausible models indicated that Brown Trout CPUE was positively related with Brook Trout occupancy, potentially because Brown Trout use similar thermal and physical habitats as Brook Trout for feeding and reproduction (i.e., pools and coarse substrates; Fausch and White 1981; Witzel and MacCrimmon 1983) and Brown Trout have likely colonized—either through past stockings or immigration—the last remaining stream segments where Brook Trout persist. Despite finding no evidence for adverse interactions between species in this study, proactive conservation actions, such as the isolation of Brook Trout populations above barriers or the removal of Brown Trout from sympatric sites, have resulted in improved survival, abundance, and size structure of Brook Trout (Hoxmeier and Dieterman 2016; Kirk et al 2018; Miller et al 2019) and may be pertinent options for states attempting to manage fisheries for both species.…”

Dendritic prioritization through spatial stream network modeling informs targeted management of Himalayan riverscapes under brown trout invasion

“…With surmounting evidences of native ecosystem collapses resulting from its invasions in most mountain systems (Boddy, 2018;Fausch & White, 1981;Kirk et al, 2018;Ohlund et al, 2008), the condition in Himalayan stream networks is assumingly detrimental owing to limited research available on its interactions with natives.…”