Chasing reports of invasive Australian crayfish found in Zambian rivers and discovering other invasive crayfish from the USA in the Cederberg’s waterways, Matt Kennedy chats to the experts about how this might play out. Specialists Joe Cutler (of The Wilderness Project) and Josie South (of Leeds University) explain how invasive freshwater crayfish can alter an ecosystem in many negative ways, but their impact on native species has not yet been quantified.

Invasives, non-natives, aliens. When a species is introduced to an ecosystem that has never seen the likes of it before, the consequences can be unpredictable. Sometimes the effects are relatively minor, like that of alien trout in South African rivers. Trout occupy a niche in the ecosystem that was otherwise (mostly) unoccupied and have minor impacts on native species.

They also fill a gap in the food web, promoting energy and nutrient cycles. Some invasives, on the other hand, can be devastating for the habitat into which they are introduced. Smallmouth, spotted and largemouth bass, for example, prey on all sorts of indigenous fish species in rivers throughout South Africa, and present a real threat to these populations.  

Well, what’s the big deal? Survival of the fittest right? The problem is these native ecosystems and their inhabitants have evolved and stabilised over millennia. When a new species is introduced, the natives often don’t have a natural way of dealing with the threat.

Whether they’re transported in the ballast water of ships (e.g. mussels), introduced for recreation (e.g. bass, trout, hyacinth) or just passengers lurking on global travellers (that itch you brought back from Bali), the invasion of alien species tends to link back to one common denominator – humans.   

“Zambians also have to contend with redclaw crayfish that have spread through over 100km of the Kafue system.”

THE KAFUE RIVER 

Aquatic non-native invasive species are threat enough alone but, when combined with a lack of legislation, ill-managed conservation and general ignorance, disaster can be imminent. Serving as a tried and tested food protein source in other parts of the world, fisheries venturists thought it would be a wise idea to start farming freshwater crayfish in parts of Southern Africa where protein sources can be limited.

The plan was either to sell locally, or export on a bigger scale. At least this is the case for how the redclaw crayfish Cherax quadricarinatus, native to Northern Australia and Papua New Guinea, made its way to southern Africa’s waterways.  

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Continue reading about southern Africa’s freshwater crayfish invasion in The Mission issue 50 below