Limpopo River systems in South Africa. Impacts of invasions include decreased
abundance of native congeners resulting from habitat and trophic overlaps, competition for spawning sites, and hybridisation (Ellender et al. 2014). In South Africa,
hybridisation and potential loss of genetic integrity with native Mozambique tilapia
Oreochromis mossambicus are the main concerns regarding its invasions (Ellender
and Weyl 2014).
Oreochromis aureus was imported (as “Tilapia nilotica”) for experimental purposes from Israel to the Jonkershoek Hatchery near Stellenbosch in 1959 and
released into farm dams in the Lourens and Eerste River catchments in 1961 and
1962 to evaluate its potential to survive the Western Cape winter (Marr et al. 2018).
Its persistence in the Eerste River catchment was recently confirmed using morphological and genetic identification methods (Marr et al. 2018). Impacts on native biota
are likely to be similar to those reported for O. niloticus, including hybridisation
with, and loss of genetic integrity by, native O. mossambicus (Marr et al. 2018). The
case of the O. aureus is interesting because it demonstrates the potential for the
persistence of other introduced fish species that are presumed to have failed. These
include the Red-bellied Tilapia Tilapia zilli, Threespot Tilapia Oreochromis
andersonii and Nembwe Serranochromis robustus (Ellender and Weyl 2014).
Percidae
European Perch Perca fluviatilis was introduced in 1915 from England for angling in
impoundments. Although this species favours slow flowing parts of rivers and stillwater habitats in lakes and dams and can tolerate brackish water environments,
naturalised populations are limited to a few small dams around the country (Ellender
and Weyl 2014). It is not considered as an invasive threat.
Poecilidae
The fish family Poecilidae includes Mosquitofish Gambusia affinis, Guppy Poecilia
reticulata, Southern Platyfish Xiphophorus maculatus and Green Swordtail
Xiphophorus helleri. Poecilids are small (<10 cm), live-bearing fishes that mature
within months of birth. Early maturity together with an ability for females to “store”
sperm and produce multiple broods in a season, results in rapidly growing population sizes (Sloterdijk et al. 2015; Howell et al. 2013). This, coupled with aggressive
behaviour and generalist diet, has resulted in G. affinis and P. reticulata being
considered among the world’s worst invasive species (Lowe et al. 2000).
Gambusia affinis was introduced into South Africa in 1936 to control mosquitoes;
and was subsequently released into many watersheds for this purpose and as prey for
introduced gamefishes (de Moor and Bruton 1988). Current distribution includes
most of the southern drainages from the Great Fish River to the Berg River, as well
as parts of the Limpopo and Mvoti River systems. Gambusia affinis are omnivorous,
feeding on a variety of prey that includes small invertebrates, fish eggs and larvae,
including cannibalism, as well as on vegetative material and detritus. There has been
no research into the impacts of G. affinis on native biota in South Africa, but their
diet often overlaps with that of native fishes and there is potential for competition
with native fishes when resources are limited (Pyke 2008). Experimental work by
Cuthbert et al. (2018), however, also highlighted that G. affinis select non-mosquito
6 Alien Freshwater Fauna in South Africa
173
abundance of native congeners resulting from habitat and trophic overlaps, competition for spawning sites, and hybridisation (Ellender et al. 2014). In South Africa,
hybridisation and potential loss of genetic integrity with native Mozambique tilapia
Oreochromis mossambicus are the main concerns regarding its invasions (Ellender
and Weyl 2014).
Oreochromis aureus was imported (as “Tilapia nilotica”) for experimental purposes from Israel to the Jonkershoek Hatchery near Stellenbosch in 1959 and
released into farm dams in the Lourens and Eerste River catchments in 1961 and
1962 to evaluate its potential to survive the Western Cape winter (Marr et al. 2018).
Its persistence in the Eerste River catchment was recently confirmed using morphological and genetic identification methods (Marr et al. 2018). Impacts on native biota
are likely to be similar to those reported for O. niloticus, including hybridisation
with, and loss of genetic integrity by, native O. mossambicus (Marr et al. 2018). The
case of the O. aureus is interesting because it demonstrates the potential for the
persistence of other introduced fish species that are presumed to have failed. These
include the Red-bellied Tilapia Tilapia zilli, Threespot Tilapia Oreochromis
andersonii and Nembwe Serranochromis robustus (Ellender and Weyl 2014).
Percidae
European Perch Perca fluviatilis was introduced in 1915 from England for angling in
impoundments. Although this species favours slow flowing parts of rivers and stillwater habitats in lakes and dams and can tolerate brackish water environments,
naturalised populations are limited to a few small dams around the country (Ellender
and Weyl 2014). It is not considered as an invasive threat.
Poecilidae
The fish family Poecilidae includes Mosquitofish Gambusia affinis, Guppy Poecilia
reticulata, Southern Platyfish Xiphophorus maculatus and Green Swordtail
Xiphophorus helleri. Poecilids are small (<10 cm), live-bearing fishes that mature
within months of birth. Early maturity together with an ability for females to “store”
sperm and produce multiple broods in a season, results in rapidly growing population sizes (Sloterdijk et al. 2015; Howell et al. 2013). This, coupled with aggressive
behaviour and generalist diet, has resulted in G. affinis and P. reticulata being
considered among the world’s worst invasive species (Lowe et al. 2000).
Gambusia affinis was introduced into South Africa in 1936 to control mosquitoes;
and was subsequently released into many watersheds for this purpose and as prey for
introduced gamefishes (de Moor and Bruton 1988). Current distribution includes
most of the southern drainages from the Great Fish River to the Berg River, as well
as parts of the Limpopo and Mvoti River systems. Gambusia affinis are omnivorous,
feeding on a variety of prey that includes small invertebrates, fish eggs and larvae,
including cannibalism, as well as on vegetative material and detritus. There has been
no research into the impacts of G. affinis on native biota in South Africa, but their
diet often overlaps with that of native fishes and there is potential for competition
with native fishes when resources are limited (Pyke 2008). Experimental work by
Cuthbert et al. (2018), however, also highlighted that G. affinis select non-mosquito
6 Alien Freshwater Fauna in South Africa
173