A follow up: The potential impacts of Red Swamp crayfish in Western Cape waters

Since first being found in the canal systems of lower Olifants River in 2022 by CapeNature officials, invasive Red Swamp Crayfish (Procambarus clarkii) are being found further afield in the Western Cape at an alarming rate. This freshwater species is globally recognised as a seriously damaging invasive species, having been introduced for aquaculture and the ornamental pet trade.

Red Swamp Crayfish are generally recognisable through their dark red colouring and S-shaped claws. However, many different colour morphs exist in the pet trade, and they may be white, blue, pink, bright red or brown. Continental Africa has no native crayfish, therefore the threat posed by this species is considerable as the fragile freshwater systems and species of the Western Cape are not adapted to crayfish.

Crayfish are large bodied species which alter nutrient cycling through shredding organic matter and feeding across all trophic levels – including fish eggs, zooplankton, juvenile fish, amphibian larvae, macroinvertebrates, macrophytes and even each other. Invasion by crayfish has been linked to declines in popular recreational fisheries in Europe and they have the capacity to rewire entire ecologies when they establish. For example, amphibian tadpoles metamorphosise earlier in invaded locations to avoid predation and an increase in mosquito disease vectors is related to crayfish invasion as the risk of predation pressure reduces native dragonfly larvae consumption of mosquito larvae. Red Swamp Crayfish are also asymptomatic carriers of the water mould Aphanomyces astaci (“crayfish plague”) and the amphibian disease chytridiomycosis. They are burrowing species which destabilise riverbanks, cause damage to farming irrigation canals, and increase sedimentation.

It is essential that this species does not continue to establish in the Western Cape, as the ecosystem engineering capacity of Red Swamp Crayfish may alter the unique habitats permanently. Moreover, management of crayfish invasions is extremely costly. For example, in Canda, up to R2 billion / year is spent due to damage costs and control attempts for Red Swamp Crayfish. Please, do not breed, release or spread crayfish into the wild. Crayfish are extremely good escape artists and can survive out of water for up to 10 days if they stay wet enough. Please report any freshwater crayfish sightings to CapeNature (Dr Martine Jordaan, mjordaan@capenature.co.za) as soon as possible and catch any found in the wild.