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Dark secret of the lake - lake Victoria


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Dark secret of the lake

New Scientist vol 181 issue 2436 - 28 February 2004, page 40

The fabulous diversity of Lake Victoria's cichlid fish is under siege, and not just from alien predators. Time is running out to tackle another villain that has been lurking unsuspected in the shadows, as Sarah DeWeerdt reports

AT FIRST, Ole Seehausen was overjoyed to find the small fish thrashing about in his net last spring. The blue-black cichlid with a saffron tail fin looked like one of the 200 species that biologists had watched go extinct over the past two decades in East Africa's Lake Victoria. It was as if Yssichromis piceatus had returned from the dead. But as evolutionary biologist Seehausen studied its features more closely, his joy evaporated. The little fish looked like a hybrid of two of the lake's cichlid species. It was the latest evidence that without quick action by conservationists, Lake Victoria's fantastic diversity of cichlids is doomed.

If Seehausen and his colleagues at the University of Hull, UK, are correct, conservationists fighting to save the lake's remaining 300 or so species of cichlids have been ignoring a dangerous enemy. They will need to shift their focus quickly before the surviving species fade away. And they'll have to act fast to prevent the same fate befalling the even richer diversity of native fish in Africa's other Great Lakes.

You can read the classic story of the evolution of Lake Victoria's cichlids - technically, a subgroup of the cichlid family called haplochromines - in almost any ecology textbook. Until a few decades ago, the lake was home to more than 500 cichlid species, most of which had evolved within the past 15,000 years - one of the world's most impressive examples of exuberant diversification. In that evolutionary blink of an eye, the fish expanded into every slice of habitat available in the lake. They took on a profusion of colours, forms and ways of life, including some improbably bizarre ones. Some sifted detritus from the muck at the bottom of the lake, some scraped algae from rocks, and others gulped tiny zooplankton in the lake's open waters. One particularly strange group, the paedophages, ate the eggs or larvae of other cichlids by engulfing the heads of mouth-brooding females with their thick, rubbery lips and sucking out the young.

But then - so the story goes - a sinister newcomer began to destroy all that diversity. The Nile perch, one of several species introduced to Lake Victoria in the 1950s and 1960s to compensate for the overfishing of native food fish, can grow to 2 metres in length and weigh 100 kilograms - and it eats cichlids. After the perch had lain low for a couple of decades, its population exploded during the 1970s and early 1980s. Over the same period, cichlid populations plummeted. In less than a decade, at least 40 per cent of the lake's known cichlid species had disappeared altogether.

It was one of the largest and most rapid episodes of vertebrate mass extinction ever witnessed. The story of conservation tragedy that has been told and retold ever since is so simple as to be almost allegorical: the bright, delicate beauty of nature done in by the huge, gape-mouthed monster of human meddling.

But recently, this tidy story has grown messier. By the early 1990s, fisheries biologists were discovering the first signs that overfishing had begun to knock back Nile perch populations in Lake Victoria. Over the past decade, the total perch catch has declined, the average size of perch caught has decreased, and fishers are catching more and more immature fish - all classic signs of overfishing.

According to the standard story, this should have reduced predation pressure and allowed what was left of the lake's bright little cichlids to bounce back. And it has - but with a twist. Some species that had become very rare have indeed rebounded, and a few thought to be extinct have been rediscovered. But more often, what comes back is not the original cichlid species but intermediate, presumably hybrid forms.

At the same time, scientists were learning surprising things about a seemingly unrelated subject, a form of water pollution known as eutrophication. This is essentially over-nourishment, most often as a result of an influx of nutrients from deforestation, farming, urban sewage and other human activities. The excess nutrients trigger massive blooms of algae and phytoplankton, and the water turns a murky, unappealing greenish-brown.

In Lake Victoria, the worst signs of eutrophication appeared about the same time as the Nile perch upsurge, in the early 1980s. Among these signs were blooms so large that some of the algal species could even be identified by an observer standing on the shore. At first, scientists assumed Lake Victoria had become eutrophic gradually, as deforestation of the lake basin, conversion of natural vegetation to intensive agriculture, and population growth increased through the 20th century.

But in the early 1990s, when limnologist Robert Hecky of the University of Waterloo in Ontario, Canada, and his colleagues analysed a core of sediment from the bottom of the lake, they came to a different conclusion. Although nutrient levels in the lake did indeed begin to increase early in the century, the explosive growth of algae and other characteristic signs of full-blown eutrophication happened abruptly, and not until the late 1960s to early 1970s - suspiciously close to the cichlid crash.

A few years later, Seehausen and his colleagues began linking the new hybrid cichlids to eutrophication. They had found that female cichlids are very picky when it comes to choosing mates, and use the visual cues of bright male colouring to identify suitors of their own species (New Scientist, 14 June 2003, p 36). And both sexes rely on vision to find and select food. In short, the change in water quality would have wreaked havoc with cichlid biology.

"When eutrophication turns the lights off, ecological and species diversity erode rapidly," Seehausen and his colleagues wrote in a 1997 paper in Science. They reported that parts of the lake where the water was murky had on average just one species of cichlid per genus, while parts with clear water teemed with up to six. They also showed that in clear areas of the lake, males of one particular cichlid species were bright blue, while males of another species were bright red. But in turbid waters, males of both species were dull brown, and the red species appeared to be hybridising with a closely related blue one. In the lab, under light conditions that simulated eutrophic water, the species chose mates indiscriminately.

That was strong circumstantial evidence that eutrophication played a role in the cichlids' demise. But it wasn't definitive. Because little was known about how these factors actually played out in the lake, many scientists remained sceptical.

New research is banishing their scepticism. In one study, not yet published, Seehausen and his collaborators analysed the DNA of the red and blue fish to confirm that the two species are hybridising. In clear waters, there's no evidence of gene flow. Apparently females of one species always choose to mate with blue males, and females of the other species always choose red. But as visibility declines, gene flow between the two species increases, and the fish in the murkiest waters share a single, undifferentiated gene pool.

In another study, Seehausen's group has collaborated with Hecky's lab to investigate the diets of the fish. This work, also unpublished, shows that loss of the two species' highly specific feeding habits goes hand in hand with the dulling of their colour. In clear waters, the blue species forages along the bottom among the rocks near islands in the lake, while the red one feeds higher in the water column, above the rocks. As the water becomes more turbid, the two species' dietary habits overlap more and more, until in the murkiest areas they are indistinguishable. What were once two distinct ecological niches merge into just one.

The more researchers looked, the more they realised that these two villains - the Nile perch and eutrophication - would work together, each magnifying the effect of the other to diminish cichlid diversity. For example, eutrophication gives the perch an advantage in capturing cichlid prey, because perch have better vision in dim light. And predation also promotes hybridisation by reducing cichlid numbers and making appropriate mates scarce and hard to find. Meanwhile, fewer cichlids eating algae means more algal blooms and murkier water.

For Seehausen, all the pieces of the puzzle are beginning to fall into place, and he can now sketch out a flow chart of these interactions (see Graphic). What was once a simple, big-fish-eat-little-fish story has now become a complex series of interlocking, circular relationships among Nile perch, cichlids and their environment. "It's the interaction between the Nile perch and eutrophication which is making the loss so dramatic," Seehausen says.

With a more accurate picture of what went wrong in the lake, scientists now have a better idea of how to fix the problem. Fortunately, most of the major ecological types or "guilds" of cichlids - the crab-eaters, the rock-scrapers, the snail-crushers, and so on - are still present in the lake itself or surrounding satellite ponds. And all the main branches of the evolutionary tree are still represented in the region. Several types of cichlids that scientists thought were gone have come back, including detritus feeders and predators on small cichlids, and one genus of snail-crusher that had vanished is now seen again. These remnants could become the seeds of a true, broad resurgence of cichlid diversity.

But the new understanding heightens the urgency of rescue efforts. Eutrophication continues to worsen in Lake Victoria as the growth of the Nile perch fishery and processing industry has drawn more people to towns along the lakeshore. Meanwhile, the expensive nets and other gear required to fish for Nile perch are causing some traditional, small-scale fishers to switch to farming the land around the lake with increasing intensity. That leads to more deforestation, more soil erosion and more agricultural run-off.

And below the surface of that increasingly murky water, scientists believe, hybridisation continues to contribute to the collapse of cichlid diversity. As gene pools mix, the gene combinations that drive idiosyncratic feeding habits such as scale-scraping and parasite-picking become mixed. Hybrid cichlids tend to be ecological generalists, and generalists have many different ways of feeding but are not particularly good at any one. If the trend continues, what remains of the lake's 500-plus raucously diverse species could become a dull handful. "The ecological complexity of the system would be lost," Seehausen says.

Extrapolating from observed relationships between water transparency and the number of species in different parts of the lake, he estimates that if eutrophication continues at the current rate, within 50 years about 60 per cent of the lake's remaining cichlid diversity could be lost. In the worst-case scenario, Lake Victoria could end up with as few as 10 cichlid species.

In other words, Lake Victoria is at a turning point in its story. In the coming years, the foundations could be laid for a true recovery of species diversity in the lake - or its evolutionary treasure could be lost forever.

The governments of the three lakeshore nations, Kenya, Uganda and Tanzania, have recently adopted fishing regulations that they hope will strike a balance enabling both perch and cichlids to thrive. But with stocks of Nile perch already severely depleted, there may be little more that can be done on the fisheries front. "In order to increase diversity beyond what we have now, we have to improve water clarity," says Seehausen.

To reduce excess nutrients in the lake, the towns along the shore will need modern sewage treatment plants, with sufficient capacity to cope with future population growth. Farmers throughout the basin will have to change their practices so that less nitrogen and phosphorus end up in the lake. The papyrus swamps and grassy wetlands surrounding the lake need to be protected and restored, to act as natural filters. And forests need to be replanted along the basin's lakes and streams, to control erosion.

Like the solutions to many environmental problems, however, this one is simple from a scientific point of view but enormously difficult in practice. The logistics of undertaking an extensive restoration effort in a lake the size of Switzerland are complicated by international borders and scarce resources. "Eutrophication is difficult to reverse, and we haven't accomplished that in a large tropical lake before," adds Hecky.

But there is some cause for optimism. A number of small reforestation projects using native trees have been carried out in all three lakeshore nations. And many of the changes in farming practice that would benefit the lake are also in the farmer's interest. "If the farmers of Africa knew how much phosphorus they were losing from their soils, they would be happy to stop," Hecky says. He has seen these changes take place on individual farms, and scientists are now working to spread the practices more widely.

Many of the small-scale activities that have taken place so far were part of the Lake Victoria Environmental Management Project, a recently concluded, five-year restoration effort funded in part by the World Bank's International Development Association, the UN Global Environment Facility and the European Union. A second, 15-year phase of the project is currently being discussed and could provide the opportunity to scale up reforestation and other efforts.

The good news is that the water of Lake Victoria might not need to be returned to a "pristine" state. Extrapolating once again from observed relationships between transparency and species richness, Seehausen estimates that if nutrient levels were reduced to what they were in the early 1970s, the corresponding increase in water transparency would allow many more cichlid species to persist.

The threat from eutrophication reaches beyond Lake Victoria, though. "The same thing could happen in other lakes in Africa," Hecky says. Lake Victoria is the shallowest of the African Great Lakes, and the effects of eutrophication spread quickly and are amplified through the system. Until now, Lakes Tanganyika and Malawi have been buffered against increasing nutrients by their deep waters, naturally low nutrient levels and fjord-like coastlines that confine problems to particular areas of the lake. Their cichlid populations are still largely intact: Lake Tanganyika has between 160 and 180 species, and Lake Malawi around 600.

But scientists have begun to show that hybridisation is occurring in these two lakes, albeit on a much more limited scale. "Hybridisation in cichlid species flocks seems to emerge as a more general phenomenon than just something happening in Lake Victoria," Seehausen says. So the other lakes run the risk of a similar collapse of cichlid diversity.

That's exactly what Hecky is afraid of. "I've been concerned that if people think it's just Nile perch [causing the problem], they won't realise the risk eutrophication represents to the other great lakes," he says. Prevention is the only option. "We have a tremendous challenge trying to reverse eutrophication in one great lake, and we cannot contemplate trying to do it for three." Unless steps are taken to prevent nutrient build-up, scientists' nets will continue to document the loss of the lake's ecological treasures.

Definition –

Eutrophication:- Having waters rich in mineral and organic nutrients that promote a proliferation of plant life, especially algae, which reduces the dissolved oxygen content and often causes the extinction of other organisms.

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Guest Panga

awesome article biggrin.gif

thanks for sharing it as i havent seen it before either.

im sure there will be many that havent seen it yet either that wouldnt mind reading it.

Cheers Troy biggrin.gif

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Thanks for sharing that article

The lakes cichlids look like they are on a colision coarse to extinction

A spin off question to the the article is

How many victorian cichlids are available to the hobby in Australia


I keep the crimson tide

Lets make a list of what we know that is here and a second could be a wish list

And hopefully someone reads the wish list and sources some fish for the hobby

I will be looking at adding another vic to my range in the near future



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Guest Panga


you can add me to the listing as i keep these :

Astatotilapia brownae

Pundamilia nyererei

Astatotilapia latifasciata

Cheers Troy biggrin.gif

Ps. Do you have any fry available of Astatotilapia latifasciata Craig ? I am interested in puchasing some if you have. tongue.gif

My wish list is getting some more Astatotilapia latifasciata for my colony blush.gif

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Guest Panga

No probs Ducksta.

I hope they breed well for you and you get many mouthfuls tongue.gif

Cheers Troy biggrin.gif

Ps. I got your mail Craig and i have sent one back cool.gif

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O.K Time to play a little...

I don't think this article is as scary as people make out. If anything providing the process of eutrophication can be overcome, by better farming practices and improved fishing methods. What is actually occuring in Lake Victoria is the very process that brought about the cihclid species flock in central africa!


Even though as dedicated hobbyists we see the diminishing of species as negative, by the remaining species interbreeding they are expressing their genetic urge to prolong their species. Even if that means making a new species.

Many cichlid books talk about the speciation of many many species occuring in a fragment of the time evolution was thought to require for speciation.

Lets save what we've got, but there could be whole new groups of fish coming along for us...

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Unfortunatle on the new scientist web site it dosent have the authors name the only credit is "New Scientist vol 181 issue 2436 - 28 February 2004, page 40"

Sorry for the lack of info.

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Thanks for posting the article. I look forward to reading it when I find a quiet minute. I saw the summary recently, but didn't have access to the full article. This is great.

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Hi Craig,

Firstly, H. burtoni are a Lake Tanganyikan inhabitant. Secondly, there are H. obliquidens and H. lividus in WA......well they are labelled as such at LFS unsure.gif .


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Hi Merjo

Thanks for the info the burtoni as I am told are also found in tributaries that run out of lake Victoria as well "but i may be wrong"

I would love to see some photos of the other two you mention and if they are the real mccoy get some over here to the east coast as they arent available here anymore



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