Invasive alien fish removal from the Cape Floristic Region

Posted by: Saving Water SA (Cape Town, South Africa) – partnered with Water Rhapsody conservation systems – 20 November 2010

The Rondegat River in the Cape Floristic Region (CFR) will be the first to undergo rehabilitation through the removal of alien invasive fishes. Monitoring the efficacy of the removal and recovery of vertebrate and invertebrate communities after this action is the focus of the current Water Research Commission (WRC)-funded study conducted by the South African Institute for Aquatic Biodiversity (SAIAB), in collaboration with CapeNature and the Cape Action for People and the Environment (CAPE).

Rotenone treatment will commence in the Rondegat River in Feb 2011

The CFR is one of the world’s most unique and endangered bioregions, and is severely threatened by the presence of invasive alien fishes, with the impacts ranging from predation, arguably the most serious threat, to competition and hybridisation.

“Rotenone treatment has, therefore, been chosen by the project team as the method to be used for eradicating alien fishes in a pilot rehabilitation study of four rivers in the CFR” says Dr Olaf Weyl, Senior Aquatic Biologist and project leader for the study at SAIAB.

”Preserving the biodiversity of the rivers of the CFR is part of Cape Nature’s obligation under the National Environmental Management: Biodiversity Act (NEMBA, Act 10 of 2004)” says Weyl.

In the Rondegat River, rotenone treatment is scheduled to commence during February, 2011. The other chosen sites that will follow thereafter are, Krom River (Eastern Cape) and Suurvlei (Cedarberg).

Weyl adds: “Whilst alien fish removal by rotenone has been demonstrated to be an effective management tool, it has also been surrounded by controversy owing to both known and unknown impacts on various aquatic organisms”.

The main source of controversy has been the highly variable impact of rotenone on communities of invertebrates. These impacts can range from minimal to severe, making it difficult to draw broad conclusions about the danger being posed to the aquatic environment.

Rotenone has been used to remove invasive fish from reservoirs and streams in the United States, Britain, Australia and New Zealand. In all of these cases, alien fishes were successfully eradicated from the treated water body. In one case, native fish were successfully re-introduced to a treated reservoir after the removal of the alien fishes.

In South Africa, concerns were raised during the public consultation phase of the comprehensive EIA carried out in connection with proposed alien fish eradication in the rivers of the CFR. Concerns, for the most part, have been fuelled by the lack of knowledge of rotenone effects on South African rivers.

This knowledge gap underlines the need to assess the pros and cons of rotenone by accurately quantifying treatment effects on non-target organisms, along the way to deciding whether or not rotenone use is a feasible and environmentally-sound fish conservation tool for South Africa.

The current WRC study has been approved by relevant conservation authorities and is aimed at addressing this knowledge gap. The Rondegat River rehabilitation pilot study represents an important opportunity to meet the need for quantifying the impacts of rotenone treatment, as a river rehabilitation method, on a suitably wide range of native aquatic organisms.

Successful river rehabilitation will depend, firstly, on the ability of rotenone to completely eradicate alien fish species, and, secondly, on the ability of invertebrates, native fish and frogs to re-colonise the river after treatment.

For more information contact: Bonani Madikizela, Research Manager: Water-Linked Ecosystems Tel: 012 330 9021 E-mail:

Source: WRC

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