et al. 2017a, b). In South Africa, the two species have been linked to the decline, and
in some cases local extinction, of native invertebrates, frogs and fishes (Karssing
et al. 2012; Rivers-Moore et al. 2013; Shelton et al. 2015a; Jackson et al. 2016;
Avidon et al. 2018). Shelton et al. (2015a) for example, demonstrated that the mean
densities and biomass of the native Breede River Redfin Pseudobarbus burchelli,
Cape Kurper Sandelia capensis and Cape Galaxias Galaxias zebratus, were 5–40
times higher in streams where O. mykiss were absent. Based on comparisons of
insect communities, Shelton et al. (2015b) also demonstrated that, in the Breede
River, O. mykiss do not functionally compensate for the native fishes that it has
replaced, being weaker regulators of herbivorous invertebrates than native fishes. As
a result, algal biomass is significantly higher at sites containing trout than at sites
without (Shelton et al. 2015b). On a broader scale, Jackson et al. (2016) working on
S. trutta invaded streams in the Drakensberg and Amathole mountains, demonstrated
that emerging aquatic insects were less important in the diet of populations of
terrestrial spiders alongside streams that were invaded by S. trutta than in those
that were not. As emerging aquatic insects are an important source of energy and
nutrient transfer from aquatic to terrestrial environments, the loss of this trophic
subsidy is likely to have further reaching consequences than the reduced spider
abundance reported by Jackson et al. (2016).
Siluridae
There is evidence for the introduction of two catfish species, the Highfin Pangasius
(Pangasius sanitwongsei) and the Vermiculated Sailfin Pterygoplichthys
disjunctivus. The occurrence of the P. sanitwongsei in the Breede River is considered incidental, based on the lack of evidence for reproduction (Mäkinen et al. 2013).
Pterygoplichthys disjunctivus, an armoured catfish native to the Amazon River in
Bolivia and Brazil is an important species in the pet trade that has colonised the
Mthlatuze and Nseleni Rivers in KwaZulu-Natal after escaping from captivity (Jones
et al. 2013). Its impacts are yet to be evaluated in South Africa, but elsewhere include
siltation and shoreline instability resulting from the burrows constructed by breeding
males into which females lay eggs; and potential displacement of native fishes (Jones
et al. 2013; Hill et al. 2015).
6.3 Conclusion
South Africa’s geographic position and diverse landscape provides opportunities for
establishment of both temperate and tropical freshwater species. This is illustrated in
Fig. 6.7, which shows the extent of suitable habitat for S. trutta, a typical cold-water
fish, a warm-water tilapia O. niloticus, and M. salmoides, which has a wide temperature tolerance (Fig. 6.7). Consequently, few introductions have failed and most of
the freshwater biota that have naturalised have also become invasive (Table 6.1).
While many taxa are considered to have the potential for high impacts (e.g. Nunes
et al. 2017; Marr et al. 2017), evidence for actual impacts in South Africa is scant
6 Alien Freshwater Fauna in South Africa
175
in some cases local extinction, of native invertebrates, frogs and fishes (Karssing
et al. 2012; Rivers-Moore et al. 2013; Shelton et al. 2015a; Jackson et al. 2016;
Avidon et al. 2018). Shelton et al. (2015a) for example, demonstrated that the mean
densities and biomass of the native Breede River Redfin Pseudobarbus burchelli,
Cape Kurper Sandelia capensis and Cape Galaxias Galaxias zebratus, were 5–40
times higher in streams where O. mykiss were absent. Based on comparisons of
insect communities, Shelton et al. (2015b) also demonstrated that, in the Breede
River, O. mykiss do not functionally compensate for the native fishes that it has
replaced, being weaker regulators of herbivorous invertebrates than native fishes. As
a result, algal biomass is significantly higher at sites containing trout than at sites
without (Shelton et al. 2015b). On a broader scale, Jackson et al. (2016) working on
S. trutta invaded streams in the Drakensberg and Amathole mountains, demonstrated
that emerging aquatic insects were less important in the diet of populations of
terrestrial spiders alongside streams that were invaded by S. trutta than in those
that were not. As emerging aquatic insects are an important source of energy and
nutrient transfer from aquatic to terrestrial environments, the loss of this trophic
subsidy is likely to have further reaching consequences than the reduced spider
abundance reported by Jackson et al. (2016).
Siluridae
There is evidence for the introduction of two catfish species, the Highfin Pangasius
(Pangasius sanitwongsei) and the Vermiculated Sailfin Pterygoplichthys
disjunctivus. The occurrence of the P. sanitwongsei in the Breede River is considered incidental, based on the lack of evidence for reproduction (Mäkinen et al. 2013).
Pterygoplichthys disjunctivus, an armoured catfish native to the Amazon River in
Bolivia and Brazil is an important species in the pet trade that has colonised the
Mthlatuze and Nseleni Rivers in KwaZulu-Natal after escaping from captivity (Jones
et al. 2013). Its impacts are yet to be evaluated in South Africa, but elsewhere include
siltation and shoreline instability resulting from the burrows constructed by breeding
males into which females lay eggs; and potential displacement of native fishes (Jones
et al. 2013; Hill et al. 2015).
6.3 Conclusion
South Africa’s geographic position and diverse landscape provides opportunities for
establishment of both temperate and tropical freshwater species. This is illustrated in
Fig. 6.7, which shows the extent of suitable habitat for S. trutta, a typical cold-water
fish, a warm-water tilapia O. niloticus, and M. salmoides, which has a wide temperature tolerance (Fig. 6.7). Consequently, few introductions have failed and most of
the freshwater biota that have naturalised have also become invasive (Table 6.1).
While many taxa are considered to have the potential for high impacts (e.g. Nunes
et al. 2017; Marr et al. 2017), evidence for actual impacts in South Africa is scant
6 Alien Freshwater Fauna in South Africa
175