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Invasive freshwater fishes in a South African RAMSAR wetland

Alexis Olds

C·I·B supported MSc student Alexis Olds.

The Wilderness Lakes system, situated in the Western Cape of South Africa, forms a major component of the SANParks managed Garden Route National Park. The lakes and interconnecting channels are part of a RAMSAR site. As a result of introductions into the catchment area, four alien fishes occur in the lakes system. Three of these, the common carp Cyprinus carpio, Mozambique tilapia Oreochromis mossambicus and largemouth bass Micropterus salmoides are popular angling species and the fourth, the mosquitofish Gambusia affinis was most likely introduced for mosquito control. These alien fishes are recognised as being among the worst invasive fish species worldwide. Elsewhere, they have been linked to a variety of impacts including competition with native biota, alterations of invertebrate and vertebrate communities through predation, habitat alteration and introduction of fish diseases. Understanding the status and establishment of these fishes in the Wilderness Lakes is therefore important for the development of strategies with which to manage these invasions.

Kerry-Ann van der Walt and Simon Calderwood

Rhodes students Kerry-Ann van der Walt and Simon Calderwood counting Mosquitofish during an annual BSc (Honours) field trip.

In an ongoing assessment of the fish fauna initiated in 2010 by the South African Institute for Aquatic Biodiversity and C·I·B-supported M.Sc. candidate Alexis Olds has been evaluating the presence and establishment of alien fishes in the system and has attempted to predict the probability of long term persistence of freshwater alien fishes in the saline Wilderness lakes. The research, implemented through a SANParks/South African Institute for Aquatic Biodiversity collaboration, has demonstrated that alien fishes are widespread and because of their tolerance of seawater salinities (35 g/kg), Mozambique tilapia and mosquitofish have established throughout the system. The less tolerant common carp are also widespread but appear to be in an early stage of invasion while largemouth bass appear to be restricted to freshwater inlets in the system. While these freshwater fishes are not often associated with estuarine systems, the research demonstrates that within the Wilderness Lakes there are always some freshwater refuge areas available to fish even when marine inflows increase salinities beyond the tolerance of largemouth bass and common carp. It is therefore likely that these fishes will persist in the lakes. As the source of invasion was from upstream introductions, the research emphasises the importance of a catchment wide approach to conservation planning.

Common carp

A common carp photographed in Rondevlei, Wilderness lakes.

The research has also had considerable training impacts with more than 30 BSc Honours students from Rhodes University learning through participating in the research over the last three years. Although To date, the research has resulted in one peer reviewed paper and has been presented at the South African Marine Science Symposium and at the Southern African Society for Aquatic Sciences conference where it was recognised the best MSc-student delivered oral presentation.

Olds, A.A., Smith, M.K.S., Weyl, O.L.F. and Russell, I.A. (2011) Alien invasive freshwater fishes in the Wilderness Lakes system, a wetland of international importance, Western Cape, South Africa. African Zoology 46: 179-184.

Olds, A.A., Weyl, O.L.F., Smith, M.K.S., and Russell, I.A. (2011) The potential impact of freshwater fish introductions on a South African wetland of international importance, the Wilderness lakes system, Western Cape. South African Society of Aquatic Science. Ithala Game Reserve, Kwa Zulu Natal, South Africa, 26-30 June 2011

Olds, A.A., Weyl, O.L.F., Smith, M.K.S. (2011) Distribution and abundance of alien invasive fish species in a South African RAMSAR wetland, the Wilderness Lakes South African Marine Science Symposium 2011. Estuarine, Coastal and Oceanic Ecosystems: Breaking Down the Boundaries. Grahamstown, 4-7 April 2011