The news item last week about the fish kill in a cage belonging to the aquaculture youth cooperative on Lake Muhazi got a young member of my family wondering whether it was linked to a similar incident on Lake Victoria a couple of weeks before.

I vaguely knew Rwanda is part of the catchment area for the Lake Vitoria Basin and therefore a likely if remote, possibility.

To hide my embarrassment at not having a ready answer, I suggested, like murder detectives, we do a little sleuthing. It would be a good opportunity as any to learn from something practical.

We found that one might think such a link possible, given the lake basin. Nyabugogo River, for instance, flows from Lake Muhazi passing Kigali before emptying in Lake Rweru and Akagera River in the north on its way to Lake Victoria.

One might imagine some kind of poison spreading upriver. But, even with our inexperience, this did not seem a likely answer.

The kills on Lake Victoria happened sometime early this month. Hundreds of dead fish were found floating or washed up at landing sites in Mwanza in Tanzania, Homa Bay in Kenya and Entebbe, Uganda.

A similar occurrence had been observed on the beaches of Lake Kyoga and at the outlet of River Nile, both in Uganda.

Add to these Lake Muhazi. Are the fish incidents related?

When experts from Rwanda’s Ministry of Agriculture and Animal Resources (Minagri) visited the area around Lake Muhazi to find the cause, they zeroed it on the recent heavy rains.

They determined that runoff from the rain that was also laden with soil erosion must have been of such magnitude that the disturbance it caused on the lake resulted in a drop in oxygen levels, subsequently suffocating the Tilapia.

It turns out that this is a well-known phenomenon known as hypoxia. One way it comes about is when water at the top which usually has more oxygen gets mixed with water with low levels at the bottom of a lake, often leading to a fatal drop in oxygen concentration.

This is the same observation by Lake Victoria experts. The most affected fish in the landing sites was the Nile Perch, which thrives in water with high oxygen levels.

However, in addition to beached Nile Perch, one site also reported Tilapia and the Silver cyprinid (locally known as dagaa, omena, injanga or mukene, depending on what the tiny silverfish are called on your side of Lake Victoria).

This particular detail is perhaps not so crucial, except to observe that the large quantities reported were said to be unusual.

This might be linked to the recent storms in the lakes, in addition to high tides and strong winds especially whipping Lakes Victoria and Kyoga, leading to rapid mixing of the waters.

According to the experts, fish kill is an annual phenomenon that, on Lake Victoria, is usually associated with weather patterns in October and November.

That the phenomenon occurred in January, months after expected schedule, suggests it an example of the changing weather patterns in the global warming the world is experiencing.

Though the news item about the Lake Muhazi incident did not hint at fish kills being a regular occurrence in the lake, it is still bound to happen again. 

One reads this from the Minagri advice to the cooperative to specifically place fish cages between 6.6 metres and 8.2 metres below the water.

More generally, as our investigation revealed, various scenarios can lead to low levels of oxygen.

It can come about during windy days making the water get cloudy and suspended with thick matter, especially in shallow lakes.

Floating vegetation covering fishing grounds, coupled with floods, can also be a cause. Decomposing vegetation in the water can also lead to a depleted supply of oxygen, though various other factors can lead to hypoxia.

Having reached this point, the young relative decided we had solved the mystery.

But it is something he and I will continue to try and better understand.