Budy, P., T. Walsworth, G.P. Thiede, P.D. Thompson, M.D. McKell, P.B. Holden, P.D., Chase, W.c. Saunders. 2021. Remarkably rapid recovery of native trout following removal of a dominant non-native trout sub-population: evidence of resilience and conservation potential. Conservation Biology: Conservation Practice and Policy 2021;3:e325. doi.org/10.1111/csp2.325.
Abstract
While the importance of reducing the impacts of non-native species is increasingly recognized in conservation, the feasibility of such actions is highly dependent upon the stage of invasion and the size of the ecosystem being restored. As such, invasive species management actions face three questions regarding the ultimate success of the action: (1) will the removal efforts be successful (i.e., will the invasive species remain absent), (2) will the native species respond positively to non-native species removal, and (3) how long after restoration activities can the native species be expected to reach a recovered state without further intervention. Here, we present the results of a multi-year, non-native brown trout (Salmo trutta) removal and native Bonneville cutthroat trout (Oncorhynchus clarkii utah) recovery in a small tributary in the Intermountain West, USA. We monitored trout populations for nine years prior to the onset of eradication efforts, which included two years of mechanical removal followed by two years of chemical treatment. Cutthroat trout populations were then seeded with low numbers of both eggs and juvenile trout. We monitored population demographics and estimated population growth rates and carrying capacities for both populations from long-term depletion estimate data, assuming logistic growth. Prior to eradication, brown trout populations fluctuated around their carrying capacity. Following brown trout eradication and initial seeding efforts, cutthroat trout populations have demonstrated exponential growth. Within five years, cutthroat trout have approached their estimated carrying capacity. Population projections suggest that cutthroat trout are between 70 and 80% of their carrying capacity and are likely to be at 90% of their carrying capacity within 10 years. Additionally, at least four age-classes are present including adults large enough to satisfy angling demand. These results demonstrate native trout species have substantial capacity to rapidly recover following the removal of invasive species in otherwise minimally altered habitats. While these locations are likely limited in extent individually, collectively they may serve as source populations for larger connected systems. In such cases, these source populations may provide additional conservation potential through biotic resistance.