Global Warming to Blame for Declining Fishery Productivity in Lake Tanganyika

A new study revealed that the declining fishery productivity at Lake Tanganyika is caused by global warming rather than just overfishing.

The study, published in the journal Proceedings of the National Academy of Sciences, showed that the abundance of fish begun declining in 1800s when the lake is becoming warmer. On the other hand, large-scale commercial fishing in the area did not begin until the 1950s, making the increasing surface temperature likely to blame for the decline in fish productivity.

"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," explained Andrew S. Cohen, a Geosciences professor at University of Arizona and lead author of the study, in a statement. "We can see this decline in the numbers of fossil fish going down in parallel with the rise in water temperature."

For the study, the researchers reconstructed a decade-by-decade profile of Lake Tanganyika's biological history by examining the preserved remains of fish, algae, mollusks and small arthropods in the annual layers of sediment deposited in the bottom of the lake.

The researchers discovered that as the lake warms, the amount of preserved remains in the layers of sediment decreases. Furthermore, the researchers found that the amount of oxygenated lake-bottom habitat has decreased by 38 percent since 1946.

Tropical lakes undergo turnovers or the seasonal mixing between the oxygenated top layer of the lake and the nutrient-rich but oxygen-free bottom layer of the lake. However, increasing water temperatures reduce the mixing capacity of the lake, making it unable to reach as far down into the lake.

As a result, fewer nutrients from the bottom reaches top, making less algae, which serves as food for the fishes. At the same time, the oxygenated top becomes shallower, reducing habitat for bottom-dwellers like arthropods and mollusks.

Lake Tanganyika is considered to be second largest freshwater lake in the world, second deepest and the world's longest freshwater lake. The lake is shared among four countries-Tanzania, Democratic Republic of Congo, Burundi and Zambia.