by Mari N. Jensen and Adam Hinterthuer
FOR IMMEDIATE RELEASE – The decrease in fishery productivity in Africa’s largest lake is a consequence of global warming rather than just overfishing, according to a report to be published this week in the Proceedings of the National Academy of Sciences.
Local fishers on Lake Tanganyika. Photo credit: Saskia Marijnissen.
Lake Tanganyika, situated mainly between Tanzania and the Democratic Republic of the Congo, was already becoming warmer in the late 1800’s – 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 new finding helps illuminate why the lake’s fisheries are foundering, said the study’s lead author, Andrew S. Cohen, a geoscientist at the University of Arizona.
“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,” Cohen said. “We can see this decline in the numbers of fossil fish going down in parallel with the rise in water temperature.”
Ben Kraemer and his group of volunteer limnologists prepare to head out on Lake Tanganyika. Photo: Ben Kraemer
The fact that Lake Tanganyika’s fishery has been in decline since before commercial fishing began, says Ben Kraemer, a graduate student at the University of Wisconsin’s Center for Limnology, 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.
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.
“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. “This decline is seen in the sediment core records and is a major problem for the conservation of Lake Tanganyika’s many threatened species and unique ecosystems.”
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, Cohen said. 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 Pete McIntyre, a professor at the UW-Madison Center for Limnology and another of the paper’s co-authors. 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,” says McIntyre.
This shrinking habitat is reflected in cores the team of scientists took from Lake Tanganyika’s 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 cores from the bottom of the lake, 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.”
“We know this warming is going on in other lakes,” adds Cohen. “It has important implications for food and for ecosystems changing rapidly. We think that Lake Tanganyika is a bellwether for this process.”
The study’s authors are Andrew S. Cohen, University of Arizona, Elizabeth Gergurich, now at the University of Oklahoma in Norman; Benjamin Kraemer and Peter McIntyre of the University of Wisconsin, Madison; Michael McGlue of the University of Kentucky in Lexington; James M. Russell of Brown University in Providence, Rhode Island; Jack D. Simmons, now at Weston Solutions, Inc. in Austin, Texas; and Peter W. Swarzenski, now at the International Atomic Energy Agency of Monaco.
The National Science Foundation, the Lake Tanganyika Biodiversity Project, the U.S. Geological Survey, the Society of Exploration Geophysicists Foundation, the Packard Foundation and the Nature Conservancy funded the research.
* This release is a modified version of the original version written by Mari N. Jensen at the University of Arizona.
University of Wisconsin – Center for Limnology
University of Wisconsin – Center for Limnology
Andrew S. Cohen
University of Arizona
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