Walter Nyamukondiwa Kariba Bureau

THE population of invasive Australian red claw crayfish in Lake Kariba has levelled out or is even declining, cheering up the fishermen who had seen catches of bream and kapenta fall, as crayfish ate the fish eggs and small fish, as well as damaging shorelines.

The species, scientifically known as cherax quadricarinatus, was first seen in 2010, after some escaped from cages on the Zambian shore.

In interviews, fishermen said they used to catch at least a tray of the crayfish on fishnets each night, but the frequency has drastically declined and now crayfish are hardly seen at some points where they used to be seen in the lake.

But not all fishermen are convinced that the crayfish were a menace.

Mr John Mugande, who keeps some crayfish in aquariums to see what they actually eat and how they behave, reckons lower fish catches cannot be laid at the door of the invaders.

“I discovered that they are not fast eaters and that they are faster when retreating than when they are going forward, which makes it difficult for them to catch fish such as the bream or even the tiger fish as initially suspected,” he said. “They are getting fewer and fewer in the lake. Even those who used to set traps report that they are fewer now and they have even stopped setting them.

“They are rare now. People in Nyakatanda and Makuyu fishing camps no longer find them in their nets.”

Sources from the University of Zimbabwe Lake Research Station said there were instances when they would not find the crayfish in some basins of Lake Kariba, but found them in others.

The crayfish is a delicacy for some people, but fishermen say harvesting on a commercial basis was not viable without a strong market.

“With the level of catches, it will be suicidal to do it on a commercial basis,” said one of the fisherman. “The catches and market are simply not enough to sustain a commercial venture.”

Zimbabwe Parks and Wildlife Management Authority (ZimParks) has been providing licences for the harvesting of the crayfish so that they can control the population, but it seems the population is in decline due to factors that have not yet been established.

A multi-sectoral taskforce comprising the Zambezi River Authority, the University of Zimbabwe Lake Kariba Research Station and the ZimParks has been established to carry out investigations into the feeding and breeding habits of the invasive crayfish in Lake Kariba and the relationship of the invaders with other species.

Progressive decline of fish and kapenta catches by fishermen in the lake have triggered other researches into what is causing the reduction.

Preliminary findings of the research have shown that the crayfish type might not have significant impact on other fish.

From a peak of between 30 000 and 54 000 tonnes of kapenta in 1998, harvests have declined to around 7 000 tonnes a year.

Kapenta is a non-native species of lake fish, introduced into Kariba to fill an ecological niche that the river fish of the Zambezi could not fill.

The introduction was considered successful.