While Zimbabwe and Zambia squabble over fishing rights in the world’s biggest dam, an invasion of Australian crayfish and Nile tilapia is threatening to wreak havoc with Lake Kariba’s indigenous species.

Built in the 1950s, the 280km- long reservoir hosts an industry that has more than 1 100 commercial boats that hunt shoals of 7cm-long Tanganyika sardines, known locally as kapenta, at night.

While the fish were introduced in the 1960s, they haven’t harmed local species. But populations of Nile tilapia and Australian redclaw crayfish, which escaped from fish farms, may be causing more damage.

The boat fleet is more than double the recommended number on the lake, yet catches on the Zimbabwean side have plunged by more than half over the past two decades.

“The problem is unscrupulous fishing by some operators and the uneven numbers of fishing rigs between the two countries,” Zimbabwean Environment and Water Affairs Minister Saviour Kasukuwere said. “This is an urgent issue which must be addressed because the problem is overfishing. The other problem is the invasion of this crayfish, which also must be addressed as a matter of urgency.”

Under a 1999 agreement, Zimbabwe had the right to 55 percent of the boats on the lake, whose kapenta population could support 500 rigs, the Zimbabwe Parks and Wildlife Management Authority said in response to questions.

In 1999 there was a combined total of 605 rigs, according to a research paper by Loveness Madamombe of the Norwegian College of Fishery Science at the University of Tromso.

Zambia had 185 licensed boats in 2000, the UN Food and Agriculture Organisation said.

The kapenta, which are attracted using fluorescent lights then scooped up in round nets, are dried and sold in both countries as a cheap protein source.

Zambia now had 725 kapenta rigs on the lake, while Zimbabwe had 406, the Zimbabwe parks authority said.

Lake Kariba, on the Zambezi River, covers 5 580km2 and can hold 185 billion cubic metres of water, according to Water-Technology.net.

As recently as 2005, rigs could harvest between 600kg and 700kg of kapenta a night, while now 120kg to 150kg was the norm, Loveton Chareka, a sales representative at Leework Enterprise, a fishing company based near Kariba Town, said.

“Nothing seems to be improving,” Chareka said as he surveyed the catch, flanked by five workmates. “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.”

The total kapenta catch in Zimbabwe is forecast to fall to 8 500 tons this year from 8 746 tons last year, according to Zimbabwe Parks. That compared with a record catch of 19 957 tons in Zimbabwe in 1993 and a record combined catch of 28 843 tons that year, Madamombe said in the research paper.

Zambia is supposed to cut its number of rigs by 50 a year over the next decade while Zimbabwe will reduce its fleet by 13 a year, according to Zimbabwe Parks. The countries will conduct an acoustic survey later this year to assess the total population.

Still, enforcement of fishing rules is lax.

“People are fishing during the full moon and look what has happened now? The fish are gone,” said Stella Beka, who dries fish at Leework. “Back in the days the government and national parks used to enforce to make sure that there was no fishing when there was a full moon. These days nobody cares.”

Unlicensed boats without lights trawl during the full moon when the fish come to the surface.

While both sides had too many boats, the Zimbabwean side was better regulated, said Kennedy Siabuwowa, a hotel worker in Zambia who quit kapenta fishing because of the low catches. “They have good controls for fishing,” he said from Siavonga, on the northern shore of Kariba. “They normally follow the rules.”

Patrick Ngalande, Zambia’s director of fisheries, did not answer calls or immediately reply to e-mailed queries.

At the same time, Nile tilapia are outcompeting the indigenous Kariba bream, according to Zimbabwe Parks.

While the impact of the Australian redclaw crayfish, introduced at Zambian fish farms in 1992, is less clear, “it is a highly invasive species and can alter the ecosystem structure and processes of invaded waters”, scientists from the University of Zimbabwe said in a paper in the African Journal of Aquatic Science.

Since January, a number of kapenta fishing companies had closed, said Fridays Zondo, the vice-president of the General Agricultural Workers Plantations Union of Zimbabwe.

Kapenta Ventures had closed while Kapenta Fishing was now leasing its boats to employees, he said. Harrowel Distributors had switched from kapenta fishing to bream fishing, he said.

“The future for kapenta fishing is not looking good. We have 20 labour cases right now because some employers have filed for retrenchment. Some employers have given their boats to employees because they’ve given up.” – Bloomberg