Resilient and rapid recovery of native trout after removal of a non‐native trout

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

Invasive species can dramatically alter ecosystems, but eradication is difficult, and suppression is expensive once they are established. Uncertainties in the potential for expansion and impacts by an invader can lead to delayed and inadequate suppression, allowing for establishment. Metapopulation viability models can aid in planning strategies to improve responses to invaders and lessen invasive species' impacts, which may be particularly important under climate change. We used a spatially explicit metapopulation viability model to explore suppression strategies for ecologically damaging invasive brown trout (Salmo trutta), established in the Colorado River and a tributary in Grand Canyon National Park. Our goals were to estimate the effectiveness of strategies targeting different life stages and subpopulations within a metapopulation; quantify the effectiveness of a rapid response to a new invasion relative to delaying action until establishment; and estimate whether future hydrology and temperature regimes related to climate change and reservoir management affect metapopulation viability and alter the optimal management response. Our models included scenarios targeting different life stages with spatially varying intensities of electrofishing, redd destruction, incentivized angler harvest, piscicides, and a weir. Quasi-extinction (QE) was obtainable only with metapopulation-wide suppression targeting multiple life stages. Brown trout population growth rates were most sensitive to changes in age 0 and large adult mortality. The duration of suppression needed to reach QE for a large established subpopulation was 12 years compared with 4 with a rapid response to a new invasion. Isolated subpopulations were vulnerable to suppression; however, connected tributary subpopulations enhanced metapopulation persistence by serving as climate refuges. Water shortages driving changes in reservoir storage and subsequent warming would cause brown trout declines, but metapopulation QE was achieved only through refocusing and increasing suppression. Our modeling approach improves understanding of invasive brown trout metapopulation dynamics, which could lead to more focused and effective invasive species suppression strategies and, ultimately, maintenance of populations of endemic fishes.

Assessing the Predatory Effects of Invasive Brown Trout on Native Rio Grande Sucker and Rio Grande Chub in Mountain Streams of New Mexico, USA

Invasive predators pose a critical threat to native taxa. Body size plays an important role in mediating the interactions of predator and prey. For piscivorous fishes, increased predator body size can be accompanied by the selection of increasingly larger prey or may reflect a mix of small and large prey. Knowledge of such interactions helps determine how predation affects population vital rates. Here, we assessed the predatory effects of invasive Brown Trout (Salmo trutta) on populations of native Rio Grande Sucker (Catostomus plebeius) and Rio Grande Chub (Gila pandora) in streams of the Jemez River watershed (New Mexico, USA). Trout diets were sampled every two weeks during the 2020 growing season. Predator and prey body lengths were measured to examine relationships to better understand patterns of piscivory and quantify the threat Brown Trout pose to populations of Rio Grande Chub and Rio Grande Sucker. Across all streams and sampling dates, 7% of Brown Trout diets contained fish. Predator–prey length relationships reflected a ‘wedge’ pattern, indicating that Brown Trout consumed an increasing range of prey body sizes as they grew larger. Rio Grande Sucker and Rio Grande Chub comprised 46% of consumed fishes. The findings demonstrated that Rio Grande Sucker and Rio Grande Chub experience constant predation over the growing season by Brown Trout. Moreover, our study provides evidence that these invasive predators pose a threat to the viability of Rio Grande Chub and Rio Grande Sucker populations. Conservation efforts to protect these chub and sucker populations must account for and directly address predation by invasive Brown Trout.