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
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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