Is it Climate or Overfishing? Paleolake Records Can Explain

Boulder, Colo., USA: Fishing in Lake Tanganyika—a 670-kilometer-long lake in Africa, surrounded by Congo, Zambia, Tanzania, and Burundi—is an important industry. With more than 260 species of fish, millions of people depend on that abundance for their livelihoods.

But the number of fish are decreasing. At the moment, fishery managers are uncertain whether the decrease is because of overfishing or a change in climate.

While some investigations have been done in the northern part of the lake, paleolimnologist Tumaini Kamulali at the University of Arizona notes that conditions in the giant lake are diverse. The temperature, stratification, and nutrients all differ between the northern and southern regions, so data from one location might not be representative of the entire lake.

Kamulali and his colleagues are hoping to clear the waters by looking at the paleolake record in southern Lake Tanganyika. They hope to get a broader picture of how the lake behaved in past climates and better predict what it might be like in the future.

The Whole Picture

Commercial fishing in Lake Tanganyika started in the 1970s. Kamulali notes that since that time, there has been a decrease in fish. To understand if the decrease is because of overfishing or climate, the team is concentrating on conditions before the 1970s to get an idea of lake conditions in the past.

“If we see a trend that the productivity of the lake has been decreasing since before the onset of commercial fishing, then we can tell that climate change has been in effect,” he explains. “But if we don’t see any change before commercial fishing, then we can tell that okay, it’s overfishing.”

Most of the previous studies into this issue have been in the northern, warmer part of the lake. But because Lake Tanganyika is so long, it actually spans two separate regions of climate, says Kamulanlu. He notes that their goal was to look at the whole lake to get the big picture.

Signs of Life

Today, mixing doesn’t occur in the 1400-meter-deep lake—in fact, the waters are anoxic below about 300 to 400 meters deep, according to Kamulali. The team wanted to know if in the past the waters did mix and oxygenate the bottom of the lake, distributing nutrients throughout the water column.

The team took gravity cores from the lake bottom, at 420 and 680 meters deep. The cores were carbon dated to give ages of sediment, and then the team looked at multiple paleo records to reconstruct the lake history.

“We did diatoms, as a primary producers; we did ostracods, these are bottom dwellers; then we did mollusks, and we also counted fish bones,” he says, adding that they also did geochemical analyses to determine if conditions were once oxygenated.

They found a shift from large to small diatoms, which indicates a shift from more nutrient-rich conditions to less. They also found invertebrates, ostracods and mollusks, that were living at the bottom of the lake, more than 600 meters deep. “That tells us there was oxygen at the bottom of the lake,” says Kamulali, adding that oxygenated water is only found at about 300 meters deep today.

The team concluded that climate conditions were different in the past, leading to oxygenated water and nutrient cycling. Kamulali notes that both overfishing and climate are affecting the fish supply, and fishing managers should be aware of the conditions when setting fishing thresholds.

Tumaini M. Kamulali
University of Arizona

Paper no. 210-14: Paleoecological Analysis of Holocene Sediment Cores from the Southern Basin of Lake Tanganyika: Implications for the Future of the Fishery in One of Africa’s Largest Lakes
Thursday, 29 Oct., 4:45 pm.

Session 210: T167. Sedimentary Records of Neogene and Quaternary Environmental Change from Eastern Africa:

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For Immediate Release
28 October 2020
GSA Release No. 20-35

Kea Giles

Contributed by Sarah Derouin

Nighttime fishermen, Kigoma, Tanzania
Nighttime fishermen, Kigoma, Tanzania. Photo by Tumaini M. Kamulali.

analyzing diatoms
Tumaini M. Kamulali analyzing Lake Tanganyika diatoms at the Univ. of Arizona. Photo by Jeffrey Stone.

sampling cores
Tumaini M. Kamulali subsampling Lake Tanganyika cores. Photo by Julia Manobianco.