Crayfish infestation a concern

The redclaw crayfish is native to northern Australia and Papua New Guinea, but has been introduced around the world for aquaculture and the aquarium trade.

Continental Africa had no native freshwater crayfish, with the ecological equivalent being freshwater crabs.

However, in February 2016, crayfish were found in the Kruger National Park (KNP), just below Van Graan Dam on the Crocodile River and the border of the park.

A scientist of freshwater ecology at SA National Parks, Dr Dumisani Khosa, said these crayfish had previously been recorded upstream of the Crocodile River, and in high abundance throughout the Komati River, including at the confluence with the Crocodile River, so it is possible that they dispersed into Kruger from there.

“Following this 2016 discovery, crayfish invasion was recognised as a major threat to the functioning of KNP rivers, requiring careful monitoring and control.

“Almost seven years later, in May 2023, we set crayfish traps in the Crocodile River at Crocodile Bridge, approximately 50 kilometres downstream of Van Graan Dam, to establish how far crayfish had dispersed.

“Three redclaw crayfish were captured, suggesting a downstream spread of about 7km per year,” he said.

Khoza said crayfish invasions were a serious conservation and management concern as they negatively impacted aquatic biota, including fish, crabs and both invertebrate and macrophyte biodiversity.

Redclaw crayfish consume high numbers of fish fry and plant material, regardless of temperature, in comparison to native crabs.

They also negatively affect fisheries through scavenging and destruction of fishing gear.

In Lake Kariba, they cause losses to Zimbabwean fisheries of up to $512 352 (about R1 million) a year.

As with many invasive species, removal from a system is highly unlikely once the species is established, and preventing and slowing spread is crucial during the early stages of an invasion.