The legacy of over a century of introductions: Spread debt of rainbow trout (Oncorhynchus mykiss) in Mpumalanga Province, South Africa

Abstract

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.

Biological invasions in rivers and associated ecosystems: New insights, challenges, and methodological advancements

“…As all these papers demonstrate, predicting the dispersal and establishment of INNS is difficult and often a result of a plethora of environmental factors which may differ based on species identity. Maimela, Chimimba, and Zengeya (2022) focus on predicting the invasion debt (the time delayed effects of species already present in a region; Rouget et al, 2016) of non‐native rainbow trout, Oncorhynchus mykiss , in South African rivers. Their study suggests that under current conditions this species has the potential to expand its range further given the suitability of many sites, but that future expansion is dependent on different climate change scenarios.…”

“…In this regard “Before‐After‐Control‐Impact” studies are particularly valuable as they enable a more realized extent of the factors associated with invasion to be evaluated (e.g., Guareschi et al, 2021; Mathers et al, 2020b). Second, the range of methodological techniques showcased by the papers highlights that tackling the challenges of biological invasions requires a combination of methods ranging from controlled experiments which may provide explicit evidence for small scale processes (e.g., Coughlan et al, 2022; Sanders & Mills, 2022), through to modelling approaches to gain a broadscale view of invasion dynamics (Larson et al, 2022; Maimela et al, 2022). Field studies which aim to investigate site specific / regional patterns provide in‐situ validation of theories and enable incorporation of potentially interacting and confounding factors to be considered (e.g., Blackman et al, 2022; Bolpagni & Dalla Vecchia, 2022; Mathers et al, 2022; Salgado et al, 2022).…”