The lake has experienced unprecedented warming during the last century, reaching a surface temperature of 26 degrees Celsius (78.8 Fahrenheit) when it was last measured in 2003, the highest level in 1,500 years, according to a paper published in the journal Nature Geoscience in May.
"Our data show a consistent relationship between lake surface temperature and productivity such as that of fish stocks," the study's lead author, Jessica Tierney of Brown University in the United States, said in a statement. "As the lake gets warmer, we expect productivity to decline, and we expect that it will affect the fishing industry."
While some researchers say shrinking stocks are due to over-fishing, the paper's authors said the warming of the lake is undoubtedly making things worse.
Lake Tanganyika - one of Africa's Great Lakes - is bordered by Tanzania, Burundi, Democratic Republic of Congo and Zambia. As well as being the world's longest lake, it is the second oldest, and the second deepest after Siberia's Lake Baikal.
Climate change models show a general warming trend in the region, which would cause even greater warming of Lake Tanganyika's surface waters, according to the study funded by the U.S. National Science Foundation.
SMALLER CATCHES, FALLING INCOMES
In and around the city of Kigoma, almost 870 km west of the Tanzanian capital Dar es Salaam, 70 percent of the region's one million residents depend on the lake for food, water and their livelihoods.
Fishing is the major source of income for almost 40 percent of men, and has allowed them to earn enough to send their children to school. A simple fisherman spends around 10,000 shillings ($7) on general household outgoings per week.
Juma Agundi, 37, who has been fishing in the lake for the last 10 years, said he and his colleagues used to be satisfied with what they caught whenever they went out on a hired motor boat. They would catch different types of fish, with sardines being the most common and lucrative.
But dwindling fishing stocks have slashed Agundi's income by around three quarters Â? from up to 40,000 shillings a day ($27) to 10,000 or less.
That has had severe consequences for his family, who depend on him for food, school fees and other daily necessities. The father of three has two children in Muungano primary school and one in Mwanannci secondary school.
"I am not sure whether I will manage to pay for their school fees next year, especially the one in secondary school, because I depend on fishing activities to get money for school and other needs," he said, adding that school expenses amount to 100,000 shillings ($67) a year.
CHILDREN'S HEALTH SUFFERS
A 2001 report by the Lake Tanganyika Biodiversity Project says fishing is an essential activity for local people's diets, noting that 25 to 40 percent of the protein consumed by the 10 million or so people who live around the lake comes from fish.
Up to 200,000 tonnes of sardines and four other fish species are harvested annually from Lake Tanganyika.
Agundi's wife, Mariam Ukwise, 26, says she used to prepare two fish each for her children's lunch and dinner. They would eat as much as they wanted, but now they sometimes have only one fish, or nothing but sardines. Ukwise says they are less healthy than before.
Fisherman Hamis Shaban, 37, who lives in Katonga village, told AlertNet he spends around three to four hours searching for fish on the lake these days, whereas before it took just 30 to 40 minutes to get a good haul. He now relies on financial support from relatives and can no longer afford to pay school fees for his three children.
Yet, despite the reduced income from fishing, boat owners are still charging fishermen the same amount of money per trip Â? around 15,000 shillings ($10).
The chairman of the Fisher's Equipment Owners Association, Mada Musa, said his members may stop hiring their vessels out to fishermen because they can't cover the cost of the petrol used on ever-longer fishing trips.
They plan to use their boats for other purposes, meaning fishermen will have to rely on the few among them who have their own boats.
NEW SKILLS
Early this year, the government launched a development project for Lake Tanganyika, aimed at educating fishermen and finding ways for them to overcome the growing challenges they face.
Programme manager Harrison Mkotagu said the initiative, which covers Kigoma and Rukwa among other regions, will help fishermen form associations, allowing them to take out loans from the government to buy fishing equipment, thereby reducing the cost of longer trips.
The project also plans to train fisherman in skills that will enable them to venture into other types of small business. Shaban, like many of his colleagues, is keen to try his hand at something besides fishing to maintain his income at a decent level.
Agriculture is one attractive option, he said, because farmers are entitled to free fertiliser under a government programme. The fishermen would also like financial institutions to lend them money so they can start other activities like selling palm oil.
Manager Mkotagu said he hoped the programme would lead to a more sustainable way of life for the wider region, not just Kigoma's fishermen.
"It is a big challenge not only to them and their families, but also to other people around, because fish are used for eating, as well as a source of income," he explained.
Evance Ng'ingo is an environmental reporter based in Dar es Salaam.
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