Global warming depletes Lake Tanganyika fish stocks

Climate change has been the key cause of the decline of fisheries in east Africa's Lake Tanganyika rather than just overfishing, a new study has found.

Boats on Lake Tanganyika. Monusco/Abel Kavanagh

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Global warming responsible for decline in fish in Lake Tanganyika

Clea Broadhurst

The lake's fish are a critical part of the diet of neighbouring countries, Burundi, the Democratic Republic of Congo, Tanzania and Zambia, providing up to 60 percent of the animal protein consumed in the region and it also is an important biodiversity hotspot.

There have been growing concerns about the impact of overfishing, land use changes and changes in climate on this key ecosystem.

But the study published in the Proceedings of the National Academy of Sciences shows that the abundance of fish began declining in the 1800s when the lake's temperature began increasing.

Large-scale commercial fishing in the area did not start until the 1950s, so that is why the rising surface temperature is more likely to blame for the decline in fish productivity.

The researchers examined samples of cores and fossils from the bottom of the lake. They found that fish numbers have been dropping as global temperatures have risen.

"The issue is, if it happened before the start of industrialisation in the 1950s, you'd have evidence of a strong decline, however, this predates that," Peter W. Swarzenski, a chemical oceanographer and one of the researchers, told RFI. "So there's no doubt that overfishing has had an impact in the last 60 or so years but we're looking at the longer term climate impact on fish numbers per say.

"The stratification of the lake has something to do with oxygen concentration that the fish need and so as the stratification moves up and down in the lake, the size of the lake obviously changes as well and so does the amount of oxygen-rich water for the fish."

As a result, fewer nutrients from the bottom reach the top, making less algae, which serve as food for the fishes.

Key finding

This finding means scientists have to look for different ways to prevent further decline.

"We've been watching this happen for the past several decades in the lake and we've been knowing that we've been nearing some kind of critical tipping point, we just don't know exactly when it's going to happen but I think it's going to be pretty soon, within the next decade for sure," Catherine M O’Reilly, an assistant professor of geology at the Illinois State University, told RFI.

"What we're seeing on the lake now, the fishermen are starting to catch juveniles and, once you start to catch juveniles, the fish population will start to decrease very dramatically and so in a relatively short time period, we could end up with fish stocks that, through a combination of overfishing and this climate change impact, could become very small."

The obvious solution to prevent this decline would be to reverse climate change but immediate action is also needed.

Fisheries management will have to be much more controlled and is going to have to incorporate the impact of climate change in its models.

Fishermen are going to have to be even more restrained about how much they catch.

"The lake on average produces 200,000 tonnes of fish a year, for millions of people in the area," Pierre-Denis Plisnier, an agronomic engineer specialised in the Great Lakes, says. "So that can have quite an impact on the food production for a lot of people. What can be done, if there is a problem of fish production, is to, when it is possible, increase aquaculture, fish culture in some areas of the lake and, depending on the conditions, it can be possible.

"The other thing is also to increase regulations on fishing so that's it's done with the most appropriate methods so that, in addition to global warming, you don't also have the problem of overexploitation that may occur in some areas of the lake."

If nothing is done in the coming years, it will not only have a huge impact on the ecosystem of the lake, but also on livelihoods of people around the lake.