Report Says that Global Warming Is Killing Off the Fish In Africa’s Deepest Lake

Fish are feeling the hurt of the Anthropocene and a changing climate in myriad ways. Our finned friends are digesting plastic at alarming rates, while overfishing is devastating fish populations and algae blooms caused by unusually warm water are displacing, poisoning, and starving the seafood species we love to eat.

But it isn't just our oceans that are feeling the heat: Fish and other aquatic species in Africa's deepest lake are dying off in part due to climate change, according to a new report.

Lake Tanganyika is an African Great Lake bordered by the Democratic Republic of the Congo, Burundi, Tanzania, and Zambia. It's the second-largest freshwater lake in the world, containing about 18 percent of the world's surface fresh water, and it's home to a fantastic diversity of species, many of which are only found in Lake Tanganyika. But thanks to global warming, some of those species' populations are in decline as the bottom of the food chain dies off. The algae that fish in Lake Tanganyika eat are disappearing as the lake's waters continue to warm.

"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 Andrew S. Cohen, a professor of geosciences at the University of Arizona who led an international research team studying the lake's fish population.

The report will be published online by the Proceedings of the National Academy of Sciences this week, and its findings don't look good for the surrounding countries that rely on the lake as a source of food. Fishermen pull around 200,000 tons of fish from the lake every year, and those fish provide around 60 percent of animal protein in the region, the research team says.

The fishermen aren't helping the situation, but the fish began dying long before the commercial fishermen arrived en masse in the 1950s. The lake began warming in the mid-1800s, slowly killing off its supply of algae.

Since the 1940s, species in the lake have lost 38 percent of their suitable habitat in the water. As the top of the lake heats up, the warm, oxygenated water on top mixes less and less with the colder waters at the bottom of the lake, where nutrients abound. With fewer nutrients available, less algae can grow on top, meaning less food for fish. A gradual depletion of oxygen at the bottom hurts species like mollusks.

"The warming surface waters cause large parts of the lake's floor to lose oxygen, killing off bottom-dwelling animals such as freshwater snails," Cohen said.

A commercial purse seine fleet of fishing boats—meaning boats that use dragnets—operated in Lake Tanganyika toward the end of the 20th century, but when the fish populations declined, it eventually was no longer worth the effort for fishermen, who switched to catamarans.

Other lakes around the world are similarly warming, and the evidence laid forth detailing the decline of fish populations in Lake Tanganyika is something of a wakeup call that shows the extent to which global warming affects freshwater fisheries.

"We're showing the rising temperatures and declines in fish food are resulting in a decrease in fish production–less fish for someone to eat. It's a food security finding," Cohen said. "We think that Lake Tanganyika is a bellwether for this process."