Invasive Alien Species (IAS) are second only to habitat destruction in their adverse effects on native biodiversity. Several Invasive Alien Species are now present in the lower Kafue basin where they are causing significant problems. The worst are described here:
Mimosa Mimosa pigra
Mimosa pigra, a native to South America, is an erect prickly shrub that can form dense impenetrable thickets 3-6 m high. In the 1960s it was sparsely distributed on the Kafue Flats (van Rensberg, in FAO, 1968) although none was found at Lochinvar National Park in the early 1970s (Douthwaite and Lavieren, 1977). Van Rensberg concluded in his survey report that “the floods alone prevent bush encroachment”. The first infestation at Lochinvar was found in 1982 following completion of the Kafue hydropower project and regulation of the flood. By 2005 at least 29 km2 of the floodplain in the Park had been invaded, forcing lechwe and wading birds out of the Park. As a result, the tourist camp closed with an estimated loss of 50,000 US$ in annual receipts.
A donor-funded pilot control project successfully cleared 800 ha of Mimosa during 2007-2009, but no government funding was available to continue the work once the project ended and cleared areas were reinvaded. Mimosa is now found all across the Flats, with huge stands immediately downstream of Itezhi-tezhi and near Namwala. Infestations are spreading in Blue Lagoon National Park threatening the most important grazing areas of lechwe whose population has fallen to the lowest level ever recorded.
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Red-claw Crayfish Cherax quadricarinatus
A native of Australia, Red-claw crayfish were brought to Zambia in 1992, first to a fish farm near Livingstone and later to Kafue Fisheries, from where some may have escaped during a flood. Further unauthorised introductions appear to have been made to a number of sites. In 2001 they were discovered in Lochinvar National Park, 185 km upstream of Kafue Fisheries, and by 2008 they were abundant in Lake Itezhi-tezhi and the eastern basin of Lake Kariba. The species is now spreading rapidly in the Zambezi river basin through both natural means and further introductions. In 2014 Red-claw crayfish were introduced to the upper Zambezi river above Victoria Falls enabling them to reach the pristine Okavango Delta.
Very little work has been done on the ecological impacts of Red-claw crayfish invasions, but freshwater crayfish generally are high-impact invaders that can act as keystone ecosystem engineers, disrupting the food chain at multiple levels and damaging artisanal fisheries. Trustee Ben Tyser found that crayfish at Namwala now comprise a significant food source for four predatory fish species. Local fishermen regard them as a nuisance as they damage a high proportion of the fish caught in gill nets and no rural market for crayfish exists.
To learn more of the work of the Trust on Red-claw crayfish click HERE.
Water hyacinth Eichhornia crassipes
A native of South America, water hyacinth is a free-floating aquatic plant that generally grows to 0.5 m in height but sometimes to nearly 1 m. It can form dense floating mats that prevent navigation and fishing; block water supply intakes for irrigation, industry, hydropower and municipalities, and provide a breeding ground for waterborne diseases including malaria and bilharzia, impacting badly on the health and livelihoods of local communities.
No water hyacinth was found at Lochinvar in the early 1970s but in the mid 1970s it became a problem at the eastern end of the floodplain following completion of the Kafue Gorge dam. The infestation was attributed to the impact of dam construction, increased eutrophication and low rainfall, and this led to permanent mats and blockages being formed. Three biocontrol insects were released in the ‘70s but the problem persisted (Hill, 1997). In 1998, the government declared a National Disaster due to weeds clogging up about 90 km of river channel between Mazabuka town and the Kafue Gorge dam (Zambia Weekly, 2013). Vast mats of floating weeds, including water hyacinth, hippo grass and papyrus, covered the entire breadth of the river, blocking the intakes for Lusaka’s municipal water supply and the turbines at the Kafue Gorge hydropower station. It further threatened the stability of the Kafue road and rail bridges and halted fishing activities. Despite several initiatives, involving mechanical removal by hand and dredging machine, spraying with Glyphosate and 2-4D, and biological control with four imported agents (two weevils, one moth and one bug), the problem was only resolved by closing Nitrogen Chemicals of Zambia, and introducing pollution control measures at Zambia Sugar and Kafue Fisheries. Today the area is relatively free of water hyacinth and other aquatic weeds.
Nile tilapia Oreochromis niloticus
The Nile tilapia occurs, as its name suggests, in the Nile river basin, as well as in other river basins in northern and western Africa. The species escaped to the Kafue river from the Nakambala Sugar estate fish farm after its introduction there in 1983 (FAO, 1992). Concern was expressed that it would have negative impacts on the local fish species either through competition and/or hybridisation with indigenous species but the effects have not been assessed. Nevertheless, its rapid increase coincided with the collapse of the Kafue flats fishery. Experimental gillnet surveys on the Chunga lagoon for example recorded 3 kg of fish caught per set in the mid 1980s compared to only 0.2 kg per set in the year 2000 (Tweddle et al., 2015).
Douthwaite, R.J. and Lavieren, L.P. van. 1977. A Description of the Vegetation of Lochinvar National Park Zambia. NCSR/TR 34. National Council for Scientific Research, Lusaka.
FAO 1968. Multipurpose Survey of the Kafue River Basin Zambia. 7 vols. FAO/SF:35/ZAM. FAO, Rome. [Vol. 4: Ecology of the Kafue Flats; Vol. 5: Wildlife, Fisheries and Livestock Production].
FAO, 1992. The aquatic animal species introduced or translocated in Zambia. http://www.fao.org/docrep/005/ad005e/AD005E02.htm#ch2.1 (accessed 21/02/16)
Hill, M. 1997. Water hyacinth in Zambia: restoring the balance on the Kafue River.
Plant Protection News. 47: 11-13
Tweddle, D., Cowx, I.G., Peel, R.A. and O.L. F. Weyl, 2015. Challenges in fisheries management in the Zambezi, one of the great rivers of Africa. Fisheries Management and Ecology,22: 99-111.
Zambia Weekly, 17 October 2013. Beware of the water hyacinth!