Heavy pollutants and discharge of raw sewage into Lake Victoria are the causes of the recent mass fish deaths, the Kenya Marine and Fisheries Research Institute (KMFRI) has established.
The dumping led to an increase in population of algae and water hyacinth in the lake. The revelation comes days after Kisumu locals accused water firms of discharging raw sewage into River Wigwa that empties into the lake.
The research followed the reporting of floating of fish on Lake Victoria.
KMFRI Assistant Director Christopher Aura said the fish died from shock following movement of deoxygenated cold water currents, which replaced the top warm lake water with oxygen, a condition referred to as potted upwelling.
“Currents found their way into cages with minimal circulation, leading to shock and death of the fish,” he said.
Dr Aura added that pollution has led to an algae coverage of 6,000 hectares while hyacinth covers at 4,000 hectares in the lake.
He said extreme high temperatures facilitate fast decomposition of algae and aquatic plants.
“Our team sampled water from where factories discharge waste into the lake. We found out that the lake has been experiencing extreme cold and warm temperatures due to climate change,” he said, adding that the deoxygenated water finds its way into the cages with minimal water circulation, thereby causing fish to die.
Dr Aura said potted upwelling is common in shallow parts of the lake, thus the need for potential fish farmers to consult relevant authorities before embarking on the practice.
Ammonia and hydrogen
He also advised the affected farmers not to dump the dead fish into the lake as this may lead to increased pollution, further encouraging the growth of algae.
“The dead fish should be incinerated or buried away from the lake as directed by the National Environment Management Authority,” Dr Aura said.
He cautioned against eating the dead fish, saying the decomposed algae produces ammonia and hydrogen sulphide, which can corrode one’s stomach and other internal organs.