Conservation implications of Rainbow trout (Oncorhynchus mykiss) invasions in the upper Blyde River system, South Africa

• Other Authors: Zengeya, Tsungai A., Chimimba, Christian Timothy, Maimela, Lerato Thabitha
• Format: Thesis
• Language: English
• Published: University of Pretoria 2023
• Subjects: UCTD; Climate change; Community structure and function; Enteromius treurensis; Invasion debt; Risk analysis; Trophic interrelationships; Treur River; Salmo salar
• Online Access: https://repository.up.ac.za/handle/2263/89553

Thesis (PhD (Zoology))--University of Pretoria, 2022. Salmonids (trout and salmon) have a long history worldwide of introduction into areas outside their native ranges for angling and aquaculture. In South Africa, for more than a century several salmonid species have been widely introduced into lakes and rivers in the country to create and enhance sport-fishing opportunities. These include species such as Oncorhynchus mykiss, Oncorhynchus kisutch, Oncorhynchus tshawytscha, Salmo salar, Salmo trutta and Salvelinus fontinalis. Management of these salmonid species is often difficult because they are conflict-generating invasive species that have major negative environmental impacts, but also have socio-economic benefits. This thesis investigated the nature, extent, and impact of rainbow trout, Oncorhynchus mykiss (Walbaum, 1792) on indigenous congenerics within the Upper Blyde River catchment in northern South Africa. The study invisaged to gain a better understanding of factors that have allowed rainbow trout to be a successful invader and also to gain insights into its invasion rate and conservation implications within South Africa. The first component of the study applied the spread debt concept (an aspect of invasion debt) to highlight the potential increase in the area invaded by O. mykiss over time in Mpumalanga Province, South Africa. It specifically, assessed the extent of the invasion, identified areas that are climatically suitable for establishment but are still invasion-free, and evaluated how its potential invasive range may be affected by climate change scenarios. The total river length that was predicted as suitable for O. mykiss under current climate was ca. 3500 km in an extension of ca. 15,000 km. Current occupancy (river length predicted as suitable with known rainbow trout occurrence records) was ca. 1220 km (35%) and the invasion debt was 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 ...