Researchers and the fishing fraternity have been thrown into a spin as they try to unravel the mystery behind the recent fish deaths in Lake Naivasha.
The phenomenon which was spotted last week led to an indefinite ban on fishing in the lake. The district public health officer, Mr Samuel King’ori, said commercial fishing will only resume after the cause of deaths has been established.
“We want to find out the cause of the deaths before commercial fishing can resume,” he said.
Dead fish have been floating in droves in the water.
“We are greatly worried by the phenomenon and we don’t know what to attribute it to,” fisherman Geoffrey Mwirikia, said.
Customers were quick to note the anomaly and immediately stopped buying or eating fish from the lake, fearing they might be contaminated with toxic materials.
“We are counting losses since different species of fish started dying and eventual closure of the lake,” said Mr Mwirikia.
According to a research officer with Kenya Marine and Fisheries Research Institute, Mr George Morara, the initial investigations indicate very low levels of oxygen.
“The dead fish have their gills and mouth wide open, indicating they might have died due to suffocation,” he said.
Most of the fish died at night or early in the morning, with very large sizes of fish, especially the spawning (females), being the most affected, according to the Aquatic Ecology expert.
The researcher said a lot of organic materials had been swept by raging storms into the lake, adding that the decomposition process demanded a lot of oxygen.
Some of the characteristics witnessed included the fish jumping out of the water, indicating lack of enough oxygen.
“All the species had been affected but the most hit are large sized fish,” he added.
The dry spell and increased human activities around the lake, he said, had affected vegetation which acts as the buffer zone, leading to siltation.
He said oxygen depletion refers to low levels of dissolved oxygen which may result in fish mortality, he told the Nation.
Mr Morara said a concentration of five milligrams per litre of dissolved oxygen is recommended for fish survival.
The researcher said mortality usually occurs if the concentration level is less than three milligrams per litre.
“The number of fish that die during an oxygen depletion event is determined by how low the dissolved oxygen gets and how long it stays down," says the researcher.
Usually larger fish are affected by low oxygen levels as opposed to smaller fish. Dissolved oxygen level might be affected by the water being too warm. Cold waters tend to hold more oxygen.
The researcher said that fertilizer runoffs from farms can make aquatic plants grow faster.
“When the increased numbers of aquatic plants eventually die, they decompose and support high amounts of bacteria which use large amounts of oxygen,” he adds.
Ecologically, common carp, the most affected, feed from the sediments on the bed of the lake.
At the moment, Mr Morara said there were no clear indications of chemical contamination.
“Although most of the dead fish had been eaten by birds, no massive deaths of waterfowl were reported with exception of the four pelicans reported on the northern part of the lake,” said the researcher.
However results on analyses of water, sediment and tissues (fish and birds) will clarify whether or not chemicals killed the fish.
He said there were no new cases of fish deaths in the past few days.
Mr Morara estimated the dead fish to be between 700 and 1,000. He said further laboratory tests were being undertaken to determine the real cause of the deaths.
The fisheries institute took samples for pesticide analysis to the Government Chemist. Four samples were taken for full chemical analysis and four for pesticide analysis including discharge point at River Malewa, which carries loads from Naivasha town.
Early in the week, officials from National Environmental Management Authority, led by the Rift Valley Provincial Director of the Environment, Mr Edward Juma Masakha, spent the week collecting soil and water samples from some of the flower farms surrounding the lake.
Speaking to journalists before embarking on the tour, he said the officials would confirm if the farms around the Lake have environment audits or whether they have been licensed by Nema.
“The team will visit several flower farms to ascertain their activities in the week and the chemical they use like pesticides before laboratory analysis are completed,” he said.
Lake Naivasha is a fresh water lake lying North West of Nairobi, outside the town of Naivasha and is part of the Great Rift Valley.
The name derives from the local Maasai name Nai’posha, meaning “rough water” because of sudden storms. The lake has an estimated surface area of 139 km², and is surrounded by a swamp which covers an area of 64 square km, but this can vary largely depending on rainfall.