A warming climate and not local fishermen is to blame for the falling fish harvests in Lake Tanganyika, according to new research findings published in the British journal Nature.
The shortfall threatens the diets of the lake's shoreline countries of Burundi, Tanzania, Zambia and the Democratic Republic of Congo.
Has 18% of the world's fresh water
Yields 200,000 tonnes of fish annually
Sardines down by about 50% since 1970s
The study was carried out by a team of researchers led by Catherine O'Reilly of New York's Vassar College.
Lake Tanganyika, the second largest lake in the world, yields 200,000 tonnes of fish a year, an important source of food and revenue for the shoreline nations.
The scientists found out that the harvest of sardines, the lake's main commercial fish, has declined by as much as 50% since the 1970s.
This was caused mainly by increased climate warming - air temperatures over the lake have increased by about 1.5 degrees Celsius, while wind speeds have diminished.
With the temperature of deeper water rising less dramatically, this has resulted in less mixing of the layers, and algal growth has dropped by 20%.
This in turn has led to reduced food for several important fish species, such as sardines.
As temperatures increase, the decline in the lake's productivity is expected to continue, according to Ms O'Reilly and her team of researchers.
Scientists have already predicted that central Africa's Great Lakes region, where Lake Tanganyika is situated, may face a temperature increase of up to 1.7 degrees Celsius over the next 80 years, suggesting a greater shortfall in fish harvests.
"The human implications of such subtle, but progressive environmental changes are potentially dire in this densely populated region of the world, where large lakes are essential natural resources for regional economies," the researchers say.
They say the study shows that global climate change has had a greater impact on Lake Tanganyika than local human activity, such as farming.
Lake Tanganyika, 650 kilometres long and 50 km wide, contains 18% of the planet's fresh water.
The researchers say that other lakes might be undergoing similar changes.