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THE ruthless cherax quadricarinatus crayfish, which has spread in Lake Kariba’s waters, could be multiplying at a faster rate than previously thought, fresh reports indicated. Results from a University of Zimbabwe (UZ) investigation have painted a gloomy picture for commercial fishing in the lake. The UZ report, titled Invasive Australian crayfish Cherax quadricarinatus in the Sanyati Basin of Lake Kariba: a preliminary survey, indicates that the invasive predator is now also in the Bumi Basin, about 80 kilometres west of the resort town.

The report was done in 2012. The Financial Gazette, which first wrote the story on the spreading crayfish, obtained it this week. There is a high possibility that the crayfish scourge might have spread to other basins in the past 24 months. The report confirms fears of a potential devastation in the industry, whose annual turnover plunged to about US$10 million last year, from about US$20 million in 1993.

The red claw Australian crayfish, introduced in the lake after escaping from fish farms in Zambia in 2002, could be inflicting more damage than previously thought, as its troops hunt shoals of the three-inch long Tanzania sardines, commonly known as kapenta, the dominant source of fish protein in Zimbabwe.

Crayfish eat almost anything that it finds, including plants, invertebrates, snails, small fish, fish eggs and even its own offspring. The alien species have been breeding out of control, devouring food sources of all fish breeds, including breams, whose population has also been extensively decimated.

Over the past 10 years, it has outpaced other aquatic populations, knocking out weaker species and piling pressure on the delicate ecosystem, which has to adapt to its new aggressive occupiers that have no natural predators. Even crocodiles do not eat crayfish. Now, ecologists are extremely worried that the alien omnivore could have spread into other dams across Zimbabwe and migrated downstream of Lake Kariba where they could destroy fisheries.

“The possible introduction of this species into other Zimbabwean waters is a matter of concern and there are already unconfirmed reports that it has been introduced into other waters in the country,” the report indicated.

“Its potential to migrate downstream from Lake Kariba into the Zambezi River is also of major concern, as it may spread further in the region… it had established feral populations at Siavonga on the Zambian shore of the Sanyati Basin by 2008… at that time. It is now known to be widely distributed in the Sanyati Basin, where anglers have caught it along the southern shores, and it has also been taken further west at Elephant Point in the neighbouring Bumi Basin. However, it has not yet been recorded in the Binga Basin,” the UZ report said.

The invasive fish eater’s exploding population, which could destabilise the reservoir’s decades-old ecosystem, has sent the biggest shivers down the spines of government and the kapenta fishing industry. There are 1 100 commercial fishing rigs on the Zambian and Zimbabwean waters, providing jobs to over 7 000 workers. The maximum carrying capacity of Lake Kariba is 500 rigs, which means it is already overfished.

Crayfish’s arrival has been exerting extra pressure on the troubled waters. It is a crisis that has led to the closure of several fishing companies, triggered the loss of 2 000 jobs in Zimbabwe, and left researchers scrounging for solutions. Researchers’ findings were depressing.

“Crayfish are known to become sexually mature at 45 to 50 grammes. Most crayfish caught during the present study were above this range, showing their potential to multiply extensively… it is a highly invasive species and can alter the ecosystem structure and processes of invaded waters… some countries have discouraged its introduction due to these aerse ecological impacts,” the report said.

The Parks and Wildlife Management Authority of Zimbabwe (PWMAZ) says kapenta output plunged to 8 746 tonnes last year, from 19 957 tonnes in 1993. Output is expected to drop to 8 500 tonnes this year. There are 725 kapenta rigs on the Zimbabwean side, against a carrying capacity of 406. “The resource is being harvested at unsustainable levels,” says the PWMAZ.

The two countries — Zimbabwe and Zambia — have conducted a biometric survey to see how many fishing rigs would be required to sustainably exploit kapenta. They have agreed to trim the rig population to 500, with Zimbabwe operating 275. Zambia would be allowed to operate 225. These ratios are based on the surface area of the lake occupied by each country.

“Both countries have committed to a phased reduction over 10 years starting 2014. Zimbabwe will reduce at a rate of 13 rigs per year and Zambia will reduce at a rate of 50 rigs per year,” added the PWMAZ.

“People are fishing during the full moon and look what has happened now? The fish are gone,” said Stella Beka who works as a fish drier.

“Government used to enforce laws to make sure that there was no fishing when there was a full moon. These days nobody cares.”

In Kariba, about nine years ago, fishermen were still bringing as much as 700 kilogrammes per night. This has declined to 120 kilogrammes. “Nothing seems to be improving,” said Loveton Chareka, a kapenta salesman.

“There is no more kapenta to talk about here there are too many boats, too much fishing not just on the Zimbabwean side, but even on the Zambian side.” Even the ruling ZANU-PF party says it is worried that the aggressive killer species could destroy the industry.

“The fishing industry has also been affected by the rogue alien fish which threatens to destroy the kapenta industry unless something is done quickly,” ZANU-PF said in a Central Committee report after its 14th National People’s Conference in December 2013. The plight of Zimbabwe’s kapenta has captured headlines before.

But its troubled story is by no means unique. Other species are also on the brink of being wiped out by crayfish unless interventions are made to arrest its breeding. Identifying a troubled fishery is not difficult. Experts compare the average size of mature stock over a period of time. In the case of kapenta, fishermen say they have been noticing shrinkage in its size.

The more daring fishermen have had to be restrained from fishing in breeding areas as they battle to improve tonnages. While overfishing and crayfish have affected the health of kapenta stock, researchers say natural climatic changes have also had an impact.

Source : Financial Gazette