Global Warming

Not Fishing, Blamed in Decline of Fisheries of Africa’s Largest Lake

by Mari N. Jensen and Adam Hinterthuer, Limnology News – Number 25, Fall 2016

The decrease in fishery productivity in Africa’s largest lake is a consequence of global warming rather than just overfishing, according to a report published in August in the Proceedings of the National Academy of Sciences.

Situated mainly between Tanzania and the Democratic Republic of the Congo, Lake Tanganyika was already becoming warmer in the late 1800s – the same time that abundance of fish began declining, the team found. The lake’s algae – fish food – also started decreasing at that time. However, large-scale commercial fishing did not begin on Lake Tanganyika until the 1950s.

The fact that Lake Tanganyika’s fishery has been in decline since before commercial fishing began, says Ben Kraemer (PhD 2016, McIntyre), a co-author of the paper, is at the “heart of this study.” Lake Tanganyika yields up to 200,000 tons of fish annually and provides about 60 percent of the animal protein for the region’s population.

Ben Kraemer and his group of volunteer limnologists prepare to head out on Lake tanganyika. Photo: Ben Kraemer

Kraemer has spent a large portion of the last several years in Tanzania researching temperature changes and fishery impacts. “The fish are not just a huge protein source for people, they’re also a huge part of the livelihood and income of people involved in the fishing trade,” he says.
While Kraemer and the paper’s other authors acknowledge that overfishing is one cause of the reduction in catch, they suggest sustainable management of the fishery requires taking into account the overarching problem that as the climate warms, algae – which is the basis for the lake’s food web – will decrease.

And it won’t just be fish food that decreases, but fish habitat as well. In fact, the warming of the lake has reduced the suitable habitat for many species by 38 percent since the 1940s, the team found.

Local fishers on Lake Tanganyika. Photo credit: Saskia Marijnissen

In tropical lakes, increases in water temperature reduce the mixing between the oxygenated top layer of the lake and the nutrient-rich but oxygen-free bottom layer of the lake. Fewer nutrients in the top layer mean less algae and therefore less food for fish.
Those rising temperatures also mean less space for fish, says CFL faculty member and another co-author of the report, Pete McIntyre. In fact, based on instrumental records of oxygen in the lake water, the study’s researchers calculated that since 1946 the amount of oxygenated lake-bottom habitat decreased by 38 percent.

That’s because, unlike temperate lakes in North America, the oxygen in a deep tropical lake like Lake Tanganyika doesn’t go all the way down to the bottom. Instead, says McIntyre, there’s a “floor” within the water column and, beneath that floor, there is no more oxygen in the lake. Over the last 150 years, that floor has been rising in Lake Tanganyika.

“Whether you’re a snail living on the bottom, or a fish swimming in the middle of the lake, you have less oxygenated habitat to operate in than you used to,” he says.

This shrinking habitat is reflected in cores of bottom sediments. The remains of fish, algae, molluscs and small arthropods are preserved in the annual layers of sediment deposited in the bottom of Lake Tanganyika. By examining these cores, the team reconstructed a decade-by-decade profile of the lake’s biological history going back 1,500 years.

The team found that as the lake’s temperature increased, the amount of fish bits, algae and molluscs in the layers of sediment decreased.
You can think of the story playing out in Lake Tanganyika like a play, says McIntyre, “It’s not that the cast is changing, it’s that the amount of stage they have to work with is being reduced. That means fewer fish for people to catch and less habitat to support viable populations of the amazing diversity of life in Lake Tanganyika.”