Part of the fish imported into the region from China contains traces of heavy metal, harmful to human health, laboratory investigations commissioned by The EastAfrican show.
Samples of the fish, purchased from a wholesale dealer at Nairobi's Gikomba market, had lead, mercury, copper and arsenic, albeit in levels termed permissible by the World Health Organisation standards.
Statistics available in Tanzania and Kenya show there is a high consumption of fish imported from China, which is generally cheaper than the local fish from lakes and rivers, and farms in the region, making it more popular in restaurants, food kiosks and homes.
The EastAfrican took the samples of fish to the University of Nairobi's laboratory for a residue and drug analysis.
The results confirmed residues of 0.04 ppm of lead, 0.005 ppm of mercury, >0.001 ppm of arsenic and 1.2 ppm of copper, indicating possible contamination of the water ponds used to farm the fish.
According to the WHO, the permissible limits are 0.5 ppm for lead and mercury, 1ppm for arsenic and 30 ppm for copper.
In an ideal situation, the metals should not be detected in the first place.
"The results show that these fish have permissible limits, but it is still worrying that their presence can still be detected in them.
"Long-term exposure to these metals for the human body, through frequent consumption of such food, can have a disastrous effect, and therefore their presence and long term effects in the human body poses serious health risks," said Prof James Mbaria, the head of the Department the University of Nairobi's Public Health, Pharmacology and Toxicology.
The presence of the heavy metals in the imported fish means that they are exposed to either the use of petrol powered water pumps or pesticide application apparatus, leading to contamination of their ponds.
"Heavy metals can cause serious health hazards, and any potential dietary exposure to lead or mercury possesses possible risk to human health," Prof Mbaria said.
Kenya recently turned to China to meet its fish consumption demand, which has seen imports double in the past two years to $20.1 million in 2017, from $10.2 million the previous year.
According to the Kenya National Bureau of Statistics, Kenya spent $22.17 million on fish imports in the first 11 months of 2017, with the Chinese fish market accounting for more than 90 per cent of these imports.
The Chinese imports, which stood at $6.24 million in 2015, came at a time production was falling over concerns of dwindling stocks in lakes.
Frozen fish that included tilapia and mackerel are the most imported fish stock from China, with more than 19,000 tonnes worth more than $18 million. This category included mostly frozen and chilled tilapia.
Tanzania, which controls more than half of Lake Victoria, also saw a 23 per cent increase in its fish imports from the Asian nation, to stand at $8 million as at the end of last year, having doubled from $3.6 million in 2014.
Dar es Salaam imported frozen pacific mackerel, Indian mackerel, chub, frozen sardines, tuna tilapia.
The tilapia had the highest value per tonne at $2,300 followed by the pacific mackerel at $1,002 per tonne.
In 2017, Dar imported more than 12, 000 tonnes of the mackerel fish species.
The EastAfrican commissioned the laboratory investigation following unsubstantiated health concerns raised over Chinese fish, after several countries, including the US recently, called for tighter controls in the enforcement of safety and health checks by Chinese authorities over their fish products.
The tests were to determine the levels of drug residues, including streptomycin, sulfadimidine, oxytetracycline, and penicillin; as well as pesticide residues.
The experts also tested the fish for lead, copper, mercury and arsenic. The test did not detect any drug or pesticide residue in the sampled fish.
In Kenya, all imports must be inspected and tested by the Kenya Bureau of Standards so as to meet certification.
We asked KEBS what standards it employs, how frequently this fish is subjected to tests, and whether the agency has flagged such concerns.
However, the agency did not respond to our questions by the time we went to press, despite promising to do so.
However, the presence of toxic metals is not limited to the imported fish.
Two years ago, a study by the University of Nairobi, led by Dr Isaac Omwenga, tested 213 fish samples from 60 ponds in Kiambu and Machakos, and found them to be contaminated with banned agricultural chemicals, with some having the the potential to cause cancer.
Human poisoning from Aldrin and Dieldrin is characterised by major body convulsions.
Heptachlor is highly toxic to humans and can be absorbed through the skin, lungs and the food tract.
These chemicals are banned in most countries and in Kenya by the Pest Control Products Board.
The study led by Dr Omwenga, showed Lindane and DDT as the most prevalent in all the samples analysed.
"While the contamination did not breach international health safety standards, it is an extremely worrying trend," Dr Laetitia W. Kanja of the University of Nairobi and one of the study authors reported.
The department that conducted the laboratory tests on behalf of The EastAfrican in June 2013 had its researchers test for lead and copper in fish and soil sediments in farmed fish in Kirinyaga, in the central region, and found higher levels of lead exposure, which they said could have been the result of the agricultural methods of fish farming practised in the area.
"Heavy metals that may be found in fish include mercury, cadmium, arsenic, chromium, thallium, and lead which have a tendency to bio accumulate in the food chain and can be highly toxic to humans even at low concentrations.
"On absorption, pollutants are transported in the blood stream to either the bone or liver for transformation and storage," the researchers said in their report published in the Journal of Applied Science and Environmental Sanitation.