Fish are becoming more scarce in Africa's oldest and deepest lake, Lake Tanganyika, because of climate warming, not just overfishing, US researchers said this week.

The study on Lake Tanganyika, which covers parts of Tanzania, Burundi and the Democratic Republic of Congo, relied on sediment samples drilled from the lake bed and going back 1,500 years to analyze the changing biodiversity.

Researchers found that the lake has been warming since the 1800s, leading to a decline in lake algae which fish feed on, and decreasing numbers of fish.

Large-scale commercial fishing began on Lake Tanganyika in the 1950s, and some have raised concern that overfishing may be threatening the lake's fish.

The lake provides 60 percent of the animal protein to feed people in the region, and yields some 200,000 tons of fish annually.

"Some people say the problem for the Lake Tanganyika fishery is 'too many fishing boats,' but our work shows the decline in fish has been going on since the 19th century," said lead author Andrew Cohen, a professor of geosciences at the University of Arizona.

"We can see this decline in the numbers of fossil fish going down in parallel with the rise in water temperature."

Overfishing is still partly to blame for the reduction in catch, Cohen acknowledged.

But the warming of the lake predates that, and has reduced the suitable habitat for fish and molluscs by 38 percent since 1946, said the study in the August 8 edition of the Proceedings of the National Academy of Sciences.

Lake Tanganyika is known for diverse species, many of which are unique to the lake, with hundreds of species found nowhere else, Cohen said.

As it warms, large parts of the freshwater lake's floor are losing oxygen and killing off bottom-dwellers such as snails.

"We know this warming is going on in other lakes," Cohen said. 

"It has important implications for food and for ecosystems changing rapidly."