(Karssing et al. 2012; Rivers-Moore et al. 2013; Jackson et al. 2016). Oncorhynchus
mykiss and S. trutta have also been implicated in the decline of populations of
Hadromophryne natalensis (Natal Cascade Frog) in a streams in the uKhahlamba
Drakensberg Park (Karssing et al. 2012). The two trout species have aslo been
implicated in the extirpation of the endangered fish Amatolacypris trevelyani (Border Barb) in headwater streams of the Keiskamma River in the Eastern Cape. In the
Breede River system, Shelton et al. (2015b) demonstrated that the abundance (mean
density and biomass) of native fish species—Pseudobarbus burchelli (Breede River
Redfin), Sandelia capensis (Cape Kurper) and Galaxias zebratus (Cape Galaxias),
were 5–40 times lower where O. mykiss was present, and that invertebrate species
assemblages in streams with O. mykiss were distinctly different from those found in
streams without O. mykiss. In addition, Shelton et al. (2015a) observed that O.
mykiss had a weaker predation effect on aquatic invertebrates relative to the native
fishes that it had replaced. As a result, there was also evidence of cascading effects,
with algal biomass being significantly lower when O. mykiss was present because of
higher abundance of herbivorous invertebrates relative to uninvaded sites. In the
Drakensberg, and the Keiskamma River system, O. mykiss and S. trutta also share
emerging insects as a food resource with riparian spiders, causing a decline in spider
abundance at invaded sites (Jackson et al. 2016).
Black bass is the collective name for species of the genus Micropterus which, in
South Africa, includes M. salmoides, M. dolomieu, M. punctulatus (Spotted Bass)
and M. floridanus (Florida Bass) (Weyl et al. 2017b). As warm water species, Black
Bass were introduced for sport fishing in the lower reaches of rivers and impoundments (see Khosa et al. 2019). Their impacts are similar to those documented for O.
mykiss and S. trutta, and include the alteration of invertebrate communities (Weyl et
al. 2010) and local extirpations and fragmentation of native fish populations (e.g.
Kimberg et al. 2014; van der Walt et al. 2016; Ellender et al. 2018). A quantitative
assessment of aquatic invertebrates in the Wit River, Eastern Cape, found that M.
salmoides changed relative abundance and community structure (Weyl et al. 2010).
In sections with M. salmoides, several members of large or conspicuous taxa
(Odonata, Hemiptera and Coleoptera) were significantly reduced or even absent,
while members of cryptic/inconspicuous taxa (Trichoptera, Leptoceridae, Mollusca,
and Physidae) were significantly more abundant. In the headwaters of the Swartkops
River system in the Eastern Cape, for example, M. salmoides predation has
fragmented populations of the now endangered Pseudobarbus afer (Eastern Cape
Redfin) (Ellender et al. 2018). Similarly, in the Olifants River system in the Western
Cape, M. dolomieu and M. punctulatus invasions have restricted native Cedercypris
calidus (Clanwilliam Redfin) and Pseudobarbus phlegethon (Fiery Redfin) to headwater reaches above physical barriers to invasion (van der Walt et al. 2016) and in
the Marico River in the North West, M. salmoides and M. punctulatus have depleted
mainstream Enteromius motebensis (Marico Barb) populations (Kimberg et al.
2014).
Trout and black bass often act synergistically on native fish populations. Trout are
cold water species that are established in cooler headwater reaches of rivers (Ellender
et al. 2016; Shelton et al. 2018), from which they exclude native fishes through
496
T. A. Zengeya et al.
mykiss and S. trutta have also been implicated in the decline of populations of
Hadromophryne natalensis (Natal Cascade Frog) in a streams in the uKhahlamba
Drakensberg Park (Karssing et al. 2012). The two trout species have aslo been
implicated in the extirpation of the endangered fish Amatolacypris trevelyani (Border Barb) in headwater streams of the Keiskamma River in the Eastern Cape. In the
Breede River system, Shelton et al. (2015b) demonstrated that the abundance (mean
density and biomass) of native fish species—Pseudobarbus burchelli (Breede River
Redfin), Sandelia capensis (Cape Kurper) and Galaxias zebratus (Cape Galaxias),
were 5–40 times lower where O. mykiss was present, and that invertebrate species
assemblages in streams with O. mykiss were distinctly different from those found in
streams without O. mykiss. In addition, Shelton et al. (2015a) observed that O.
mykiss had a weaker predation effect on aquatic invertebrates relative to the native
fishes that it had replaced. As a result, there was also evidence of cascading effects,
with algal biomass being significantly lower when O. mykiss was present because of
higher abundance of herbivorous invertebrates relative to uninvaded sites. In the
Drakensberg, and the Keiskamma River system, O. mykiss and S. trutta also share
emerging insects as a food resource with riparian spiders, causing a decline in spider
abundance at invaded sites (Jackson et al. 2016).
Black bass is the collective name for species of the genus Micropterus which, in
South Africa, includes M. salmoides, M. dolomieu, M. punctulatus (Spotted Bass)
and M. floridanus (Florida Bass) (Weyl et al. 2017b). As warm water species, Black
Bass were introduced for sport fishing in the lower reaches of rivers and impoundments (see Khosa et al. 2019). Their impacts are similar to those documented for O.
mykiss and S. trutta, and include the alteration of invertebrate communities (Weyl et
al. 2010) and local extirpations and fragmentation of native fish populations (e.g.
Kimberg et al. 2014; van der Walt et al. 2016; Ellender et al. 2018). A quantitative
assessment of aquatic invertebrates in the Wit River, Eastern Cape, found that M.
salmoides changed relative abundance and community structure (Weyl et al. 2010).
In sections with M. salmoides, several members of large or conspicuous taxa
(Odonata, Hemiptera and Coleoptera) were significantly reduced or even absent,
while members of cryptic/inconspicuous taxa (Trichoptera, Leptoceridae, Mollusca,
and Physidae) were significantly more abundant. In the headwaters of the Swartkops
River system in the Eastern Cape, for example, M. salmoides predation has
fragmented populations of the now endangered Pseudobarbus afer (Eastern Cape
Redfin) (Ellender et al. 2018). Similarly, in the Olifants River system in the Western
Cape, M. dolomieu and M. punctulatus invasions have restricted native Cedercypris
calidus (Clanwilliam Redfin) and Pseudobarbus phlegethon (Fiery Redfin) to headwater reaches above physical barriers to invasion (van der Walt et al. 2016) and in
the Marico River in the North West, M. salmoides and M. punctulatus have depleted
mainstream Enteromius motebensis (Marico Barb) populations (Kimberg et al.
2014).
Trout and black bass often act synergistically on native fish populations. Trout are
cold water species that are established in cooler headwater reaches of rivers (Ellender
et al. 2016; Shelton et al. 2018), from which they exclude native fishes through
496
T. A. Zengeya et al.