is the most popular recreational angling species in the country and is important in
small-scale and subsistence fisheries (Weyl and Cowley 2015). Impacts on recipient
ecosystems are mainly associated with its impacts on water quality because bottomgrubbing during feeding suspends sediments, increasing nutrient availability and
turbidity (Lougheed et al. 1998). In addition, this species is responsible for introducing the most parasitic species into South Africa. Interestingly, while C. carpio is
also considered as the host fish that co-introduced the anchorworm L. cyprinacea, to
date no C. carpio has been reported to be infected by this parasite.
Hypophthalmichthys molitrix and C. idella were considered unable to reproduce
outside of captivity because they need to migrate up large rivers to spawn in flowing
water to allow eggs to float downstream and hatch prior to larvae settling in
floodplains (Skelton and Weyl 2011). Grass Carp Ctenopharyngodon idella, introduced from Malaysia in 1967, was stocked into ponds throughout South Africa for
the control of invasive aquatic plants. After its naturalisation and invasion of the
Vaal River system, this aggressive feeder on aquatic plants, has been demonstrated
to decrease the richness and abundance of native aquatic plants (Weyl and Martin
2016). Ctenopharyngodon idella was also responsible for the introduction and
spread of the tapeworm S. acheilognathi (Smit et al. 2017).
Similarly, H. molitrix imported in 1975 from Germany to the Marble Hall
experimental fish farm on the Olifants River, escaped, naturalised and spread into
the Limpopo River system (Lübcker et al. 2014; Ellender and Weyl 2014). As there
has been little ecological research directed at this species in South Africa (Lübcker et
al. 2016), their impact potential has yet to be determined. In North America however,
they have altered ecosystem structure and negatively affected commercial and
recreational fisheries and human safety (Kolar et al. 2007).
Tench Tinca tinca, a European fish species that can attain a weight of 5 kg was
introduced into the Western Cape in 1910 for angling but although widely stocked, it
currently only persists in the Breede River system in the Western Cape (Ellender and
Weyl 2014). Adults are omnivorous bottom feeders that grub through soft sediments
for insect larvae, worms, crustaceans and molluscs. There has not been any research
into the ecology of this species in South Africa which, as a result of dietary overlap,
has the potential to compete with native fishes and is likely to prey on native snails.
As its feeding behaviour is similar to that of common carp, it is also likely to
contribute to increased turbidity and nutrient cycling.
Cichlidae
Although several cichlid species have been introduced into the country, there is
currently only evidence for the establishment of the Blue Tilapia Oreochromis
aureus (Marr et al. 2018) and the Nile Tilapia Oreochromis niloticus (Ellender
and Weyl 2014).
Oreochromis niloticus is a medium-sized fish (max 4 kg) that, as a result of its
global importance in warm-water aquaculture, is one of the most introduced species
in the world (Ellender et al. 2014). It was widely spread in neighbouring Zimbabwe
and Mozambique for aquaculture in the 1980s, and its subsequent escape from
captivity and direct releases by anglers facilitated its invasion of the Inkomati and
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O. L. F. Weyl et al.
small-scale and subsistence fisheries (Weyl and Cowley 2015). Impacts on recipient
ecosystems are mainly associated with its impacts on water quality because bottomgrubbing during feeding suspends sediments, increasing nutrient availability and
turbidity (Lougheed et al. 1998). In addition, this species is responsible for introducing the most parasitic species into South Africa. Interestingly, while C. carpio is
also considered as the host fish that co-introduced the anchorworm L. cyprinacea, to
date no C. carpio has been reported to be infected by this parasite.
Hypophthalmichthys molitrix and C. idella were considered unable to reproduce
outside of captivity because they need to migrate up large rivers to spawn in flowing
water to allow eggs to float downstream and hatch prior to larvae settling in
floodplains (Skelton and Weyl 2011). Grass Carp Ctenopharyngodon idella, introduced from Malaysia in 1967, was stocked into ponds throughout South Africa for
the control of invasive aquatic plants. After its naturalisation and invasion of the
Vaal River system, this aggressive feeder on aquatic plants, has been demonstrated
to decrease the richness and abundance of native aquatic plants (Weyl and Martin
2016). Ctenopharyngodon idella was also responsible for the introduction and
spread of the tapeworm S. acheilognathi (Smit et al. 2017).
Similarly, H. molitrix imported in 1975 from Germany to the Marble Hall
experimental fish farm on the Olifants River, escaped, naturalised and spread into
the Limpopo River system (Lübcker et al. 2014; Ellender and Weyl 2014). As there
has been little ecological research directed at this species in South Africa (Lübcker et
al. 2016), their impact potential has yet to be determined. In North America however,
they have altered ecosystem structure and negatively affected commercial and
recreational fisheries and human safety (Kolar et al. 2007).
Tench Tinca tinca, a European fish species that can attain a weight of 5 kg was
introduced into the Western Cape in 1910 for angling but although widely stocked, it
currently only persists in the Breede River system in the Western Cape (Ellender and
Weyl 2014). Adults are omnivorous bottom feeders that grub through soft sediments
for insect larvae, worms, crustaceans and molluscs. There has not been any research
into the ecology of this species in South Africa which, as a result of dietary overlap,
has the potential to compete with native fishes and is likely to prey on native snails.
As its feeding behaviour is similar to that of common carp, it is also likely to
contribute to increased turbidity and nutrient cycling.
Cichlidae
Although several cichlid species have been introduced into the country, there is
currently only evidence for the establishment of the Blue Tilapia Oreochromis
aureus (Marr et al. 2018) and the Nile Tilapia Oreochromis niloticus (Ellender
and Weyl 2014).
Oreochromis niloticus is a medium-sized fish (max 4 kg) that, as a result of its
global importance in warm-water aquaculture, is one of the most introduced species
in the world (Ellender et al. 2014). It was widely spread in neighbouring Zimbabwe
and Mozambique for aquaculture in the 1980s, and its subsequent escape from
captivity and direct releases by anglers facilitated its invasion of the Inkomati and
172
O. L. F. Weyl et al.