22.5.5 Multi-species Extirpation Projects
Many invasive and extra-limital freshwater fish species have been co-introduced and
have been the subject of multi-species extirpation efforts, either for biodiversity
restoration or improving ecosystem function. Paardevlei wetland outside Cape Town
is an example where a large number of invasive species, including C. carpio (Common
Carp), Micropterus spp. (Black Bass), Tinca tinca (Tench) and Lepomis macrochirus
(Bluegill) had been stocked since the 1920s (Impson et al. 2005). The dominance of
C. carpio over time resulted in declining water quality, algal blooms and altered
ecosystem function. In 2005, the wetland was treated by aerial spraying of rotenone
and over 35 tons of fish, mostly C. carpio, were removed after treatment (Impson et al.
2005). The project, including a coordinated stakeholder information campaign, was
designed and implemented by an independent consulting firm and funded by the
landowner. There was negligible stakeholder opposition, with the exception of concerns around animal welfare. The fish have not been detected since the termination of
monitoring in 2014, so the operation appears to have succeeded.
Die Oog is a small urban wetland in Cape Town that forms part of a formal
conservation area managed by the City of Cape Town. The wetland is home to alien
fish species such as C. carpio and Gambusia affinis (Mosquito fish) and some extra
limital species like Tilapia sparrmanii (Banded Tilapia) and Oreochromis
mossambicus (Mozambique Tilapia). Ecological functioning of the wetland deteriorated, mostly as a result of the large C. carpio population. Attempts to remove the
C. carpio by draining of the wetland were unsuccessful and rotenone treatment was
initiated in 2005. This successfully removed all the alien fish species and restored
ecosystem functioning of the wetland. Total costs are not known, but the treatment
cost was reduced through partial drainage of the wetland, thereby reducing the
volume of water to be treated. The fish have not been detected since and the wetland
is in a good condition and supports a number of native bird and amphibian species,
including the Endangered Sclerophrys pantherina (Western Leopard Toad; Measey
et al. 2012).
An off-stream dam in the Lourens River catchment outside Cape Town was
treated with rotenone in 2005 to remove M. salmoides, T. sparrmanii and
C. carpio. The dam is part of the Helderberg Nature Reserve, managed by City of
Cape Town. CapeNature implemented the project, and removal of the invasive
species allowed for the establishment of a refuge population of native fish from
the Lourens River. The extirpation was successful.
22.6 Amphibians
Two invasive amphibian species have been identified for control operations in the
greater Cape Town area: Xenopus laevis (African Clawed Frog) and Sclerophrys
gutturalis (Guttural Toad). Both species pose threats to endemic native species. The
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645
Many invasive and extra-limital freshwater fish species have been co-introduced and
have been the subject of multi-species extirpation efforts, either for biodiversity
restoration or improving ecosystem function. Paardevlei wetland outside Cape Town
is an example where a large number of invasive species, including C. carpio (Common
Carp), Micropterus spp. (Black Bass), Tinca tinca (Tench) and Lepomis macrochirus
(Bluegill) had been stocked since the 1920s (Impson et al. 2005). The dominance of
C. carpio over time resulted in declining water quality, algal blooms and altered
ecosystem function. In 2005, the wetland was treated by aerial spraying of rotenone
and over 35 tons of fish, mostly C. carpio, were removed after treatment (Impson et al.
2005). The project, including a coordinated stakeholder information campaign, was
designed and implemented by an independent consulting firm and funded by the
landowner. There was negligible stakeholder opposition, with the exception of concerns around animal welfare. The fish have not been detected since the termination of
monitoring in 2014, so the operation appears to have succeeded.
Die Oog is a small urban wetland in Cape Town that forms part of a formal
conservation area managed by the City of Cape Town. The wetland is home to alien
fish species such as C. carpio and Gambusia affinis (Mosquito fish) and some extra
limital species like Tilapia sparrmanii (Banded Tilapia) and Oreochromis
mossambicus (Mozambique Tilapia). Ecological functioning of the wetland deteriorated, mostly as a result of the large C. carpio population. Attempts to remove the
C. carpio by draining of the wetland were unsuccessful and rotenone treatment was
initiated in 2005. This successfully removed all the alien fish species and restored
ecosystem functioning of the wetland. Total costs are not known, but the treatment
cost was reduced through partial drainage of the wetland, thereby reducing the
volume of water to be treated. The fish have not been detected since and the wetland
is in a good condition and supports a number of native bird and amphibian species,
including the Endangered Sclerophrys pantherina (Western Leopard Toad; Measey
et al. 2012).
An off-stream dam in the Lourens River catchment outside Cape Town was
treated with rotenone in 2005 to remove M. salmoides, T. sparrmanii and
C. carpio. The dam is part of the Helderberg Nature Reserve, managed by City of
Cape Town. CapeNature implemented the project, and removal of the invasive
species allowed for the establishment of a refuge population of native fish from
the Lourens River. The extirpation was successful.
22.6 Amphibians
Two invasive amphibian species have been identified for control operations in the
greater Cape Town area: Xenopus laevis (African Clawed Frog) and Sclerophrys
gutturalis (Guttural Toad). Both species pose threats to endemic native species. The
22 Alien and Invasive Animal Control Projects
645