For over a century, rainbow trout (Oncorhynchus mykiss) has been widely introduced into lakes and rivers in South Africa to create and enhance sport‐fishing opportunities. Despite its long history of introduction, naturalized populations of rainbow trout are still localized to a few areas with suitable habitats and climate. This study assessed the spread debt (i.e., the increase in area invaded by invasive species over time) of rainbow trout in Mpumalanga Province, South Africa to highlight areas with known introductions, the extent of the invasion, and to identify areas that are suitable for establishment but are still invasion‐free. The total river length that was predicted as suitable for rainbow trout under current climate was about ca. 3,500 km in an extension of about ca. 15,000 km. Current occupancy (river length predicted as suitable with known rainbow trout occurrence records) was ca. 1,220 km (35%) and the invasion debt was therefore estimated as 65%. While these data infer a large invasion debt, they are confounded by a lack of knowledge on sampling effort and verified true absence and should therefore be recognized as an estimate. In addition, the extent of the suitable area varied under different climate change scenarios where it was projected to decrease under RCP 4.5 scenarios and increase under the RCP 8.5 scenarios. This study demonstrates some of the difficulties of quantifying the potential future extent and impacts of biological invasions and how the invasion debt concept can be applied to provide an important link between invasion biology, management, and policy.

The rainbow trout (Oncorhynchus mykiss) is a popular angling fish native to North America that has been introduced across the world, mainly for aquaculture and angling purposes (Crawford and Muir 2007). In some cases, introduced O. mykiss have become invasive and have caused adverse impacts to native fish and ecosystems (McDowall 2006;Young et al. 2010;Kadye et al. 2013;Stanković et al. 2015). For more than a century, O. mykiss has been widely introduced into lakes and rivers in South Africa to create and enhance sport-fishing opportunities (Ellender and Weyl 2014). It is a cold-water species, often introduced into cool, well-oxygenated mountain headwaters. However, these habitats are also inhabited by some threatened and endemic species (Weyl et al. 2020). These introductions of O. mykiss have, in some cases, caused declines in native invertebrates, frogs, and fish, extirpating some populations (Karssing et al. 2012;Rivers-Moore et al. 2013;Shelton et al. 2015a). In addition, O. mykiss introductions have altered food web structures (Shelton et al. 2015b), changed habitat coupling, as well as causing other overall ecosystem-level impacts (Jackson et al. 2016). These studies represent a small, but growing number of studies within South Africa, that provide evidence that trout invasions in areas of conservation concern can lead to adverse impacts on biodiversity, as demonstrated in other parts of the world (McDowall 2006;McIntosh et al. 2010).This study investigated the impact of the O. mykiss on community structure of native fish species in the upper catchments of the Blyde River in Mpumalanga province, South Africa. The upper catchment of the Blyde River is located on the northern part of the Great Drakensburg Escarpment where O. mykiss has been introduced into several headwater streams and has established naturalised populations (Engelbrecht and Roux 1998). These upland areas form part of trout angling areas where the stocking and utilisation of trout is allowed (such as the Crocodile River from its source up to its confluence with the Santa River, the Sabie River from its source up to the waterfall in the town of Sabie, and the Elands River from its source up to its confluence with the Swartkops River) by the Mpumalanga Nature Conservation Act No. 10 of 1998.These headwater streams are also inhabited by range-restricted and endemic river minnows, such as the Treur River barb (Enteromius treurensis), that are listed as critically endangered, because of alien predatory fish, such as O. mykiss, brown trout (Salmo trutta) and smallmouth bass (Micropterus dolomieu

The introduction of alien predatory fish such as rainbow trout (Oncorhynchus mykiss) can have a significant effect on indigenous fish communities such as altering the structure and dynamics of food webs. Quantifying the trophic niche utilised by the alien fish species is therefore important to aid our understanding of how their feeding strategies might aid establishment, define their functional role and inform on potential impacts. This study assessed food web interactions between fish communities in river reaches that are invaded by O. mykiss and sections that are free of invasions in the upper Blyde River catchment, South Africa. It specifically evaluated the hypothesis that O. mykiss invasion is likely to lead to a decrease in the trophic functional diversity through predation and that indigenous fish will shift their trophic niche to either minimise competition with, or avoid predation, by O. mykiss. Gut content and stable isotope analyses were used to determine trophic interactions. Fish communities in uninvaded areas utilised fewer and similar food sources and occupied lower trophic levels, than fish communities in invaded areas. Oncorhynchus mykiss fed mainly on invertebrates and at trophic levels similar to and or lower than indigenous fishes. This suggests that their current impact on indigenous fish communities is mainly through competition for resources. We posit that O. mykiss invasions reduced the abundance of indigenous fishes and thereby reduced predation pressure on aquatic invertebrates, with a knock-on effect on the trophic interrelationships among fish assemblages. Our findings are consistent with other studies in South Africa that have shown that the impacts of O. mykiss invasion can lead to a decline and fragmentation of indigenous species populations and niche shifts.