Extreme temperatures coupled with low oxygen levels attributed to pollution are to blame for mass fish deaths recently experienced in Lake Victoria.
The report by the Ministry of Mining, Blue Economy and Maritime Affairs last month estimated losses by Kisumu and Homa Bay fish farmers at Sh927,969,000.
The multi-agency team was tasked with investigating the fish deaths - a phenomena of sudden and unexpected death of a number of fish or acquatic organisms over a short period of time - in November last year following reports by farmers in October and November.
Farmers reported dead fish floating on water with mouths open. And now the official report has blamed low oxygen levels caused by pollutants as a major cause.
"Low oxygen level is the main cause of fish kills either in cages or in the wild - emanating from the decomposition of organic matter/ pollutants due to increased nutrient enrichment within the lake or from land," the report noted.
The report linked pollution to the decomposition of water hyacinth and fertilisers from agricultural farms and river discharges as major causes leading to reduced oxygen levels.
The increased amount of decomposing matter, the report says, utilises the dissolved oxygen in the water, leading to low oxygen levels.
Effects of climate change have also been blamed for speeding up the rate of decomposition of water hyacinth, fertilisers and other river discharges within the lake.
"Extreme temperatures (more than 26 degrees Celcius) during the day and at night (very cold) are effects of climate change recorded in the lake waters from the months of September to November 2022. High temperature levels have further sped up the decomposition of organic matter," said the report.
While instances of deep, cold water rising toward the surface, also known as up-welling, occurs in Lake Victoria mostly around February/ March or September/ November, the pattern of wind and water circulation has changed and now occurs many times.
"In this case, low oxygen water is dispersed over a distance and the scope of which could be at the cage site that suffocates the fish which may have no room to escape. However, up-welling alone could not kill the fish, unless the water experiencing turnover have low dissolved oxygen levels," says the report.
With the changing phenomenon, the report cautions farmers to undertake remedial measures such as planning for maturity of fish on time and procuring cold chain facilities
The fish kills in the two counties affected 135 cage farmers in 10 beaches, with the highest losses experienced by Kentila and Lake Aqua farms at Ogal beach.
Losses from fish kills in Kisumu was estimated at Sh884,814,000 while Homa Bay County recorded Sh43,155,000.
Records from the task-force indicate that 747 cages were reported to have experienced fish kills in the two counties although the task-force revealed that of all the cage farmers affected, only 14 had Nema license, particularly in Ogal beach in Kisumu while those in Homa Bay had concession with Beach Management Units.
"Other licenses by the regulatory bodies such as Kenya Fisheries Service and National Land Commission were lacking in both counties. This makes it difficult to determine efficacy of management to address such threats," said the report.
It was revealed that the absence of proper and sufficient data to help understand and assess the lake ecosystem and its related impacts resulted in decisions being made based on estimations, which are unlikely to address the impact in optimal manner.
The report also noted that with increased pollution, there was need to redouble conservation efforts, enforcement and adherence to regulations for a better lake ecosystem.
It recommends establishment of a national standing committee on cage aquaculture and strengthening coordination among state actors.
