On the morning of June 8, Mr Joseph Ochieng was inspecting his fish cages at Kisaka Beach in Lambwe West location when he noticed fish floating in the water with their mouths wide open – a sign that they were suffocating.
The water had turned dark and had an unusual odour.
Mr Ochieng could do nothing to help his fish from dying as the dark matter on the water surface was spreading quickly.
News about what happened in the cages soon spread across Lambwe as fishermen and residents also came across dead fish washing up on the shore.
"We thought the water was contaminated by raw sewerage because it was unusually dark. Fish was washing up on the shore," Mr Ochieng said.
When the farmer saw all his fish dead, he decided to report the matter to the fisheries department, where he was told why his fish had died.
The deaths of fish in cages in Lambwe were inevitable, said Michael Omondi, Homa Bay County aquaculture business development programme officer.
He said such deaths had also been reported in some cages in Siaya and Kisumu and the occurrence moved like a wave across the lake from last month.
The main factor behind the mass deaths is a sudden change in the chemical composition of water. What happened to fish in Kisaka is called black water, a natural occurrence in water bodies where the level of oxygen is reduced due to chemical activities.
Mr Omondi said the phenomenon also occurs when fish farmers fail to manage the environment properly, leading to contamination of the surroundings.
"Scientists believe black water occurs when organic matter decomposes at the bottom of the lake before rising to the surface. Decomposition uses oxygen, which is drawn from the water, leaving fish with less air," he said.
Hyacinth has disappeared from the lake surface in Homa Bay and it is believed it is decomposing on the lake bed.
Decomposition that consumes oxygen can also come from fish feeds and waste from fish that drops from cages to the lake bed.
Scientists are yet to pinpoint the exact cause of black water, and fishermen are yet to understand it.
But Safina Musa, a research scientist at the Kenya Marine Research Institute, said: "Black water is often in Lake Victoria but does not have a definite pattern.
“The main cause is upwelling, which is bottom water that has almost no oxygen [and] comes on top and displaces oxygenated waters at the surface then lushes across certain cages, depriving fish of oxygen."
For Mr Ochieng, the deaths in his two cages cost him Sh3.6 million, as he was planning to sell his 24,000 fish at an average of Sh200 per piece.
"My first harvest was scheduled for August before another one in December. I was looking for a market and I was sure of getting between Sh150 and Sh300 for every fish," he said.
Farmers planning to venture into commercial fishing were advised to be on the lookout for black water.
Mr Omondi said cage fishing is a new concept in Lake Victoria and all its challenges are not yet known.
"A precautionary measure is to have an oximeter to check on the oxygen concentration in water," he said.
"Fish need to eat less when the oxygen level is low because during feeding, metabolism requires a lot of oxygen, which leads to death in case of deficiency."
Back water occurring in Lake Victoria reduced after the incident in Kisaka but is an occurrence that will soon catch more fish farmers unawares.
Mr Omondi also assured cage farmers in Suba, who are major suppliers of fish, not to worry about black water, because the water there has plenty of oxygen.
