Risks various chemicals pose to fish, your health

Chemical fishing is illegal in many countries, but it continues to be one of the widely used methods of catching fish.

The system is used because of its quick ability to capture large quantities of fish to meet high demand, weak law enforcement to curb the vice and unavailability of simple, fast and cost-effective chemical confirmatory test methods.

The technique poses serious dangers to fishermen, consumers and the environment due to its effect on non-target organisms and indiscriminate killing of the target population.

Various chemicals are used to catch large quantities of fish in a short time. These include sodium/potassium cyanide, gunpowder, pesticides such as Dichlorodiphenyltrichloroethane (DDT), carbide, powdered detergents, petrol and diesel and plant extracts like Rotenone.

Chemical fishing is destructive. In the case of cyanide, it discolours and destroys coral reefs while DDT has the ability to accumulate in fish, threatening consumers’ health.

In one case, a farm manager reported to one of the veterinary laboratories that all their over 5,000 fish in a pond had died overnight.

Investigators confirmed the massive deaths, with all the fish floating on one part of the pond, which is about 50x100m2 and was used to rear tilapia for five years.

The clinical history of the case was that the fish were active and healthy on Sunday through Monday but were found dead on Tuesday morning.

Further, the pond used to draw water from a borehole and water had not been pumped over the weekend due to a pump breakdown. Lack of oxygen (anoxia) and poisoning were suspected to be the cause of the deaths.

Investigators were also eager to know if the deaths were accidental or intentional.

The farm manager submitted some dead fish, fish feed and water from the pond for analysis. The investigation team took similar samples.

The team also carried out some uncommon tests like pouring the pond water on grass to test for herbicides. It was further noted that houseflies were all over the dead fish, implying that whatever had killed the fish was not an insecticide. The greenish colouration of the water was an indication of algal growth.

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The grass was not altered by the pond water, an indication that herbicides were not involved in the death of the fish. There were no significant findings from the post-mortem examination.

The aflatoxin B1 level in the feed was 43.72ppb, which is above the maximum allowable limit of 10ppb in animal feeds but lower than the tolerable levels of 100ppb for tilapia.

Aflatoxin therefore was ruled out as the cause of death. The dissolved oxygen (DO) in the water was between 0.25mgl-1 and 3.25 mgl1 but tilapia can tolerate DO levels of as low as 0.3 mgl-1.

Thus, DO levels might have contributed to the deaths but unlikely to have caused a 100 per cent mortality. No organophosphates were detected in the water, but cyanide levels of between 2.48mg/100ml and 4.43mg/100mls (24.8-44.3mg/l) in water were detected.

Cyanide levels of 0.01-0.2mg/l are normally lethal to fish. Although the investigation did not identify the culprit, it was concluded that having used a chemical that is normally used for fishing, an overdose of cyanide was the cause of the massive deaths.

The survey served as a reminder that fish require high water quality with parameters such as DO, turbidity, colour intensity, temperature, pH and others affecting their growth and survival.

Materials that are not normally harmful like feeds and fertilisers affect water quality and hence the well-being of fish.

Regulatory agents and food safety advocacy bodies need to mount regular surveillance on the use of chemicals in fish, not only cyanide but also others like sodium metabisufite, which has been found to be used as a preservative in other meat in the country.

Dr Mwirigi is a livestock specialist; [email protected]

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Aquatic resources plenty in Kenya

Kenya is endowed with various aquatic resources that include lakes, ponds and rivers.
Species of fish available in Kenya include trout, tilapia and mudfish.
Before setting up a pond, ensure the land has a slope of about 11 per cent.
Steeply sloped land is not suitable for building ponds as the water may flow away.