Invasive crayfish threaten Okavango Delta

© The Ecological Society of America www.frontiersinecology.org Invasive crayfish threaten Okavango Delta No indigenous freshwater crayfish exist in continental Africa, but several North American and Australian species have been introduced since the 1970s. The invasive Australian redclaw crayfish (Cherax quadricarinatus) is now spreading rapidly in southern Africa, especially in Zambia. Introductions have resulted from their use in aquaculture, so that the species was first reported in the middle Zambezi system in the Kafue River in 2001, and later in Lake Kariba in 2008 (Figure 1). The Zambezi River basin is the fourth largest river basin in Africa and contains 11 Ramsar wetlands of international importance, as well as the UNESCO World Heritage Sites of Lake Malawi and Victoria Falls. It is also periodically connected to the Okavango Delta, another World Heritage Site. Crayfish have now spread through much of the middle Zambezi, where the river ecosystem has unfortunately been transformed by flood regulation associated with dam construction for hydroelectric power generation, and where a number of alien vertebrates – for instance, Nile tilapia (Oreochromis niloticus) and Lake Tanganyika sardine (Limnothrissa miodon) – are now present (Tweddle 2010). However, the flows of the upper Zambezi and the Okavango Delta are unregulated and their associated habitats are in pristine condition, with no invasive aquatic animals reported in the Delta, one of the largest intact wetlands in the world. In the upper Zambezi, the only record of an alien animal is for Nile tilapia in offchannel aquaculture ponds (Alonso and Nordin 2003; Tweddle 2010). Until recently, Victoria Falls prevented the spread of crayfish from Lake Kariba into the upper Zambezi, but in November 2014 a new population was reported spreading on the Barotseland floodplains after their introduction near Mongu (Figure 1). The extent of this population is currently unknown, but there is now an extremely high risk of it spreading to the Okavango Delta via the seasonal Selinda Spillway, which connects the Okavango to the Linyanti and Chobe rivers, and ultimately to the upper Zambezi River. While very little work has focused on the ecological impacts of redclaw crayfish invasions, freshwater crayfish are highimpact invaders that can act as keystone ecosystem engineers, disrupting the trophic chain at multiple levels (Twardochleb et al. 2013). The potential impact o f cray f i sh invas ions includes the destruction of aquatic macrophytes, disruption of macro inver teb ra te communi t i e s , competition with native invertebrates, predation on fish and amphibian eggs, and disturbance of fish breeding habitats, resulting in overall alterations to ecosystem functioning (eg Rodríguez et al. 2005), along with damage to artisanal fisheries. This is of particular importance in the Zambezi River basin where fisheries – an important source of livelihoods – are WRITE BACK WRITE BACK WRITE BACK