Exploring metapopulation‐scale suppression alternatives for a global invader in a river network experiencing climate change

Abstract

Article impact statement: An invasive brown trout metapopulation may persist in Grand Canyon under climate change, requiring adaption of current suppression strategies.

Impeding access to tributary spawning habitat and releasing experimental fall-timed floods increase brown trout immigration into a dam’s tailwater

“…Regardless, we posit that our findings are biologically significant, because even a small number of colonizers could overcome an Allee threshold by offsetting depensation, and lead to rapid population growth toward carrying capacity (Drake and Lodge 2006). Salmonid populations are known to quickly rebound from low abundances following mortality events in both native (Vincenzi et al 2016;Budy et al 2021) and introduced ranges (Meyer et al 2006;Saunders et al 2015;Healy et al 2022b). Brown trout expansion also coincided with the rapid population decline of a potential competitor, rainbow trout, due to intraspecific competition (Korman et al 2021).…”

“…Our results suggest that the combination of impeding access to spawning grounds and release of Fall HFEs contributed to increased straying to the tailwater ecosystem, setting the stage for increased recruitment reported elsewhere (Runge et al 2018;. Expansion of brown trout into the Lees Ferry reach, which contains high-quality habitat capable of supporting a large population, has important implications for invasive species management and maintenance of native species in the Grand Canyon ecosystem (Runge et al 2018;Healy et al 2022b).…”

“…The annual suppression program was expanded spatially and temporally in 2012 to include depletion electrofishing throughout 16 km of Bright Angel Creek and weir installation and operation between October and February, which was effective in suppressing the brown trout population by >90% (Healy et al 2020), and coincided with a decline in catch rates in the adjacent mainstem (Rogowski and Boyer 2019). Regardless, brown trout populations may rebound rapidly, even when few adults remain, and when ideal conditions exist during incubation and emergence periods (Saunders et al 2015;Healy et al 2022b).…”

“…2), and desynchronized from spring native fish reproductive periods when floods could be of benefit to recruitment (Healy et al 2020;Humphries et al 2020). Late winter or spring-timed high flows also limit brown trout (Lobón-Cerviá 2004;Healy et al 2022b) and rainbow trout recruitment in some tailwaters beyond our study system (Dibble et al 2015), and may potentially benefit aquatic macroinvertebrate communities (Carlisle et al 2017). Thus, shifting HFEs or other types of designer flows to the spring, or even early summer, may provide greater benefits to both native fish and rainbow trout and disadvantage brown trout.…”

“…Understanding responses of fishes to flow manipulation is particularly important in flow-regulated systems inhabited by both native and introduced species as their response may differ dramatically. For instance, native fishes of the southwestern US may experience population-level benefits from spring or early summer floods (Gido and Propst 2012;Van Haverbeke et al 2013;Healy et al 2022a;Yackulic et al 2022a), while recruitment of introduced trout can be limited by high magnitude spring flooding (Fausch et al 2001;Kawai et al 2013;Healy et al 2022b; but see Avery et al 2015). The impacts of fall-timed floods on native and introduced species in the southwestern US are less well understood; however, introduced species like brown trout (Salmo trutta) that evolved in systems with a wide variety of flow regimes and spawn during the late fall might benefit from fall flooding (reviewed in Unfer et al 2011).…”