Fewer fish species, overfishing
Away from the climatic conditions impacting fish life in Rwanda, the American scientists discovered that Rwanda also has a very small number of fish species.
Dr. Veverica explained: “Lake Kivu used to be [part of] the Nile [River] drainage, that used to drain up to Lake Edward [in Uganda]. But when the Virunga mountains formed – they are apparently relatively young – these changed the drainage of Lake Kivu to go to the south to Lake Tanganyika [in Burundi and Tanzania].”
This meant that few new species were naturally introduced in the lake due to its isolation, according to Dr. Veverica.
A checklist released in January 2001 by the Journal of East African Natural History, found 82 species belonging to 12 families in Rwanda’s waters, including the numerous lakes, Rusizi basin and Mukungwa river. It is the only available checklist so far.
To understand what it means by Rwanda having fewer fish species, we compared it to regional neighbours. Burundi, which is nearly the same size as Rwanda, had 112 fish species as of a 2012 checklist. In Rwanda’s northern neighbour Uganda, a government database shows that as of 2013, the country had at least 500 unique and endemic fish species in its waters – most of them in Lake Victoria and Lake Kyoga. In Tanzania, a 2002 study estimated more than 1,000 fish species, with up to 500 species found in the country’s coastal waters. As for Kenya, there were 206 species belonging to 38 families known from freshwaters alone, not counting its huge coastline.
Between 1951-1954, the Belgians colonial administration in Rwanda introduced three kinds of tilapia fish; Tilapia rendalli, Oreochromis macrochir and Tilapia rendalli from the Zambezi region in Southern Africa and Katanga (DR Congo). It turned out that some of them couldn’t thrive very well in some lakes. The Auburn University team found that another different species, the Nile Tilapia, survived better.
Among the many lakes in the Akagera park region, scientists found that Lake Ihema offered good breeding grounds for fish. The productivity was also increased by the presence of hippos that foraged and defecated in the lake, giving fish an ideal place to grow.
However, there is also a challenge in the Akagera region of over-fishing and using nets that take out much smaller fish – which eventually affects the growth in the lakes. The scientists have called this situation a “Tragedy of the commons”.
For the past nearly 10 years, the Rwanda National Police has deployed the Police Marine Unit to control overfishing and enforcement of rules such as the use of legal nets. However, local and central government officials admit that enforcement is politically difficult since people depend on the fish for their livelihoods.
The seasonal ban on fishing during August and October, for instance, is mainly enforced on Lake Kivu. However, illegal fishing has not been eradicated completely. For example, in Nyamasheke district, one of those bordering Lake Kivu, local officials said in January 2021 that for the previous year they had confiscated and burnt over 980 illegal fishing nets worth Rwf400 million ($400,000).
For years, there was no legal instrument dealing with various aspects of illegal fishing. A Ministerial Order regulating aquaculture and fisheries was finally put in place in November 2020 outlining a list of legal fishing equipment, fishing hours and sites, among other elements. It also details penalties.
Data released by the RAB in January 2021 as reported by The New Times showed that due to illegal fishing practices such as use of bad fish nets, fish production in Lake Kivu had decreased by more than 7,000 tonnes.
In addition to illegal fishing, the restrictions on movement during Covid-19 pandemic lockdowns also contributed to low fish production in 2020.
A shortage of oxygen
People were shocked and fishermen demoralized when tonnes of fish washed up dead on the shores of Lake Muhazi in early July. Something similar had also happened on Muhazi back in January 2021, killing more than 10,000 fish.
Lack of oxygen stems partly from the physiography of some of Rwanda’s lakes, or how they were “born.”
As deep waters contain no oxygen, Lakes Burera and Ruhondo in the north experience a mixing every year or what’s also called a “turnover,” where the oxygen-less waters come up and mix with the surface water, killing almost all the fish in those lakes. This is also what happened in Lake Muhazi, but caused by a different phenomenon. 
With regard to Lakes Burera and Ruhondo, Dr. Veverica explained; “It is not poison that kills [the fish], it’s the zeroing of oxygen.”
Rwandan researchers reporting in the 2020 Rwanda Journal of Engineering, Science, Technology and Environment also found that water in Lakes Burera and Ruhondo was “unusable for drinking” by humans.
When it comes to Lake Kivu, which is essentially Rwanda’s ‘fish basket’, it has unique features. A 2006 study by an expert committee comprising American and Danish teams describes how Lake Kivu is a nutrients sink, which basically means that the plankton (foods) which would be feeding the fish, is instead sinking too deep for the fish to reach.
“So those deep lakes like Kivu and Tanganyika are nice and clear, because they are nutrient sinks,” said Dr. Veverica, while commenting on the issue.
Lake Kivu is 485 meters deep – but there is no oxygen below 50 meters largely because the sunlight can’t penetrate to reach down there. This means fish cannot swim down to the areas where most of the nutrients are falling.
Due to its strange physiography and the methane gas that it contains, Lake Kivu also occasionally undergoes a phenomenon called “seiche” whereby the lake experiences violent storms that pull deep water up to the surface and push the surface water to the other side of the lake. As a result, multitudes of fish are killed. But according to Dr. Veverica, from Auburn University, the “seiche” phenomenon doesn’t come very often, and last took place in the 1980s.
While the deep waters are blocked below 250 metres, the surface waters within the first 60 metres are subject to a high level of seasonal mixing, considerably influencing the variations in this system, according to the team of Belgian researchers in their 2012 study on Lake Kivu.
These temperature changes during the different seasons contribute to an unstable environment for many fish to survive.
Researchers from the University of Rwanda’s fish research station say that although no in-depth research has yet been conducted, the phenomenon that happened on Lake Muhazi is worsening due to climate change. When temperatures increase, there is an increase in bacteria in lakes like Muhazi which influences decomposition of nutrients and organic matter. The problem is also aggravated due to agriculture, whereby nutrients deposited from fertilizers and waste stimulate the growth of algae, which consumes the oxygen. This process is called eutrophication.
The Rusumo Falls Dam factor
Several years ago, the governments of Rwanda, Tanzania and Burundi agreed to construct a hydropower dam at Rusumo falls along their common border. Jointly funded by the World Bank and the African Development Bank Group, the Chinese-built dam will generate 80 megawatts of electricity to be shared by the neighbors at a cost of $468 million. However, right from the time the idea was mooted, environmentalists had unease with the structure.
The Rusumo falls is one reason there are very few fish species in Rwanda – because it blocked the migration upstream.
“Fish tend to migrate upstream and they were blocked by those falls. So the fish that are in the Akagera region are kind of Lake Victoria species, but they didn’t ever move up all the way into the rest of Rwanda,” explained Dr. Veverica, from Auburn University.
When the dam was built, people feared it would further obstruct the migration of fish downstream, into the Akagera lakes and river system.
Now, with the dam in place and due for completion in early 2022, we sought for the project’s environmental impact assessment study. It was conducted by German-based Fichtner Group, and submitted in March 2013 to the Nile Basin Initiative, the regional grouping which is managing the dam project.
We shared the study with Auburn University’s Dr. Veverica for an independent review of the findings. She said that even if the dam was not there, the lakes in the Akagera region are not suited for major fish production.
“The lakes of the Bugesera [region] are very shallow and prone to silting in. They are therefore quite unproductive. There is also a listing of the total surface area of these lakes. If you add them up, it is about 20,000 hectares,” she said.
“At the low air temperatures in that area (average 20C) I expect the natural productivity of these lakes to be around 50 kilograms per hectare per year. That is only going to provide 1,000 tons [of fish], if the fishery is well-managed.”
Dr Veverica added that the backwaters of the dam will be another good opportunity to start fish farming there after the dam has been filled up.