The disappearing sardines

As Lake Tanganyika in Africa grows warmer, its massive schools of silvery fish get smaller. And nearby villagers say goodbye to their way of life.

Editor's note: Early Signs: Reports From a Warming Planet is a joint project of the U.C. Berkeley Graduate School of Journalism, Salon and NPR's "Living on Earth." The series runs Fridays through May 5 in Salon, and you can find radio versions of each story on "Living on Earth's" Web site. Read about how the series came into being here.

By Jori Lewis

Read more: Politics, News, Early Signs: Reports from a Warming Planet

March 24, 2006 | KALALANGABO, Tanzania -- Along the shores of Lake Tanganyika, there are two kinds of fishermen: the ones who fish for an abundant nocturnal sardine called dagaa and the ones who don't. Retired fisherman Myonge Seph has spent most of his life going out at night to trail that small fish, a silvery wonder the length of an index finger.

When the moon is not full, the fishermen of his village, Kalalangabo, paddle out a few hundred meters in search of a good spot to catch dagaa in their nets. They dangle kerosene lamps over the sides of their wooden boats to attract zooplankton, the dagaa's main food. It's a classic mouse trap. Lure the zooplankton and the dagaa will follow. The darker the night, the more they are seduced by the lights above. From the shore, Tanganyika at night looks like a city and the Kalalangabo fishermen are just smudges of light in the distance. But thousands of fishermen float on the waters of Africa's deepest lake all night, waiting.

For generations, the dagaa were so plentiful, it never occurred to anyone the abundance wouldn't last. Sure, there are cycles of plenty and scarcity, largely driven by the moon and the weather. But fishermen always filled their nets over and over again. "Oh, it was so good," said Seph, recalling the days when he was first learning the trade 30 years ago. "When we used to fish with our fathers, it was really good. There were so many dagaa. People could fish 5,000 tons. Tons! Back in those days, there was so much dagaa."

But now, Seph said, the catch is down. All along the Tanganyika basin, from Bujumbura in Burundi to Kalemie in the Congo to Mpulungu in Zambia, fishermen like Seph are beginning to say their lake is changing and with it, their way of life.

- - - - - - - - - - - -

Tanganyika is the longest freshwater lake in the world, the second deepest and the second most biologically diverse. It is also getting warmer. Over the past 80 years, temperatures in the region have increased by about 0.8 degrees Celsius. Studies in the journals Nature and Science assert that increasing air and water temperatures are having an adverse effect on the growth rate of algae on which many fish species, including dagaa, depend for food. And a loss of dagaa could have a serious effect in this poor region, where dishes featuring the small fish are served up every day in nearly every household.

"Global warming is having an impact on areas of the world we haven't looked at very carefully yet," said Bard College biologist Catherine O'Reilly, lead author of a Nature study on the Lake Tanganyika region. "Most of the work has been done in the Arctic or areas of high latitude. This is one of the first studies to show the magnitude of changes in an equatorial region and the whole ecosystem effect of global warming."

O'Reilly, who has been studying the lake's ecosystem for over a decade, explained that "algae form the base of the food web in the lake." So if "algae are growing more slowly and reproducing less frequently, that would mean there are overall less algae available." She said the lake's ecological imbalance, kick-started by increased temperatures in the region, could be putting the dagaa population at risk in a place where this little fish is the biggest thing going.

The lake, which holds nearly 18 percent of the world's freshwater, and its dagaa, are the region's best sources of protein. Some people have compared the lake to a well-stocked aquarium with an estimated 300,000 metric tons of fish; it's such a prolific fishery that many have compared it to the boundless plenty of the sea.

But as a tropical lake, Tanganyika's waters are almost permanently stratified. That means the water separates into layers according to temperature and density. The waters only really mix during the dry season, when strong winds blow across the lake and stir things up. A warming lakes means those layers are even more stubborn and require more energy to mix.

And if that deep water can't mix with the warm water on top because of increased temperatures or slowed wind speeds, the lake can't produce sufficient algae. And less algae means less zooplankton, the dagaa's main food, which could mean, eventually, less dagaa.

"If we look into the future and look at predicted temperature changes, the temperature changes for the next 80 years are twice as high as the temperature changes that this region has experienced in the past 80 years," said O'Reilly. "So the current trend toward less mixing is going to continue quite dramatically."

Then the entire lake will warm up and the mixing will resume. But that, O'Reilly said, could take hundreds of years. In the intervening period, there will be less food for the fish to eat and, perhaps, so few dagaa that fishermen will no longer even be able to depend on the cycles of scarcity and plenty.

Next page: As the scientists debate, the fishermen perform secret rituals