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

Abstract

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 should therefore be recognised as an over estimation. In addition, the extent of the suitable area varied under different climate change scenarios where it was projected to decrease under Representative Concentration Pathway (RCP) 4.5 scenarios and increase under the RCP 8.5 scenarios. This 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 second component of the study assessed how O. mykiss invasions in the upper Blyde River have affected the community structure and function of indigenous species by evaluating the composition, relative abundance, and community structure in invaded and uninvaded river sections. There were significant differences in composition and abundance between fish communities that could be attributed to O. mykiss invasion and variation in habitats. Populations of indigenous species that historically occurred throughout the upper catchment, such as Enteromius treurensis, were greatly reduced and fragmented in the presence of O. mykiss. However, instream migration barriers such as waterfalls have prevented upstream migration of O. mykiss, and these invasion-free areas have remnant populations of native fishes that appear to be largely intact. This finding is consistent with other studies that have shown that introduced alien predatory fish can have a significant effect on fish communities and highlighted the need to prevent human-facilitated introductions in biodiversity-sensitive areas, such as mountain headwater streams, that are inhabited by endemic and range-restricted minnows.

The third component assessed trophic interrelationships of fish communities in invaded and uninvaded river reaches within the upper Blyde River catchment. 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. 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. The study 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. These 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.

The last component of the thesis provided as assessment of the risks associated with the six trout species that are known to occur in South Africa using the recently developed Risk Analysis Framework for Alien Taxa (RAAT). The RAAT framework offered a simple, objective, and transparent process of identifying risk associated with the introduction of alien species in South Africa and the development of appropriate management strategies. Four species (O. mykiss, O. kisutch, O. tshawytscha, and S. trutta) were considered to have a high risk of invasion because they are already present in South Africa and have a high potential to cause moderate to major impacts. Salmo salar and Salvelinus fontinalis were classified as low risk because they are not present in the country and are known to cause minor to moderate impacts in their invasive range globally. The recommendations from the risk analyses largely support the current management interventions as prescribed by the NEM:BA A&IS Regulations. The two key recommendations from the risk analyses are that O. mykiss and S. trutta should be included on the NEM:BA A&IS list of regulated species, and that National Management Plans should be developed for each trout species that is known to be present in South Africa. The tenet of these management plans should be to preserve the benefits while limiting the negative impacts. This can be achieved by managing trout by area, where utilisation is allowed in areas where trout have naturalised and/or were historically stocked while preventing introductions into new areas. There is also a need for a broader consultative process when these management plans are been developed to minimise contestations around the proposed management plans.