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Debate rages on effects of farmed fishes


• Studies claim farmers are feeding fisheries with banned substances, toxic chemicals

Until now, several studies have shown that fish is a very important part of a healthy diet. Fishes are the major sources of healthy long-chain omega-3 fats, high in protein, rich in other nutrients such as vitamin D and selenium, and low in saturated fat.Also, scientists have demonstrated that the consumption of fat in fish is of importance because they contribute to the reduction of cardiovascular diseases and lead to improvement in learning ability.

Other researches indicate that fish also lower blood pressure and heart rates, improve blood vessel function, at higher doses, lower triglycerides and may ease inflammation.These benefits have raised the demand for fish meals. Most people now have fish farms (also called fisheries or aquaculture) to provide food for consumption, employment and financial income, and a food source when other sources are out season.

But fresh concerns have emerged on the health implications of eating farmed fishes.A mini review of “Safety and Quality Concerns Associated with Fish Production” noted that with the increased use of veterinary drugs in food production, there is global concern about the consumption of low levels of antimicrobial residues in aquatic foods and the effects of these residues on human health. According to the World Health Organisation (WHO), the potential hazards associated with the presence of antimicrobial drug residues and Genetically Modified Foods/Organisms (GMOs) in edible tissues of products from aquaculture include allergies, increased toxicity, changes in colonization patterns of human-gut flora and acquisition of drug resistance in pathogens in the human body, increased cancer risks and changed nutrient levels.


The mini review published in the Journal of Fisheries and Aquatic Science by O.M. Edun established current conditions and practices in aquatic animal production that are harmful to human health.Blood meal and blood products in fish feed: The European Union permitted the use of blood meal from non-ruminants in fish feed (EC, 2001b). This regulation is conditional on the products being based on pure pig or poultry blood. Pigs have a low retention of heavy metals and other undesirable substances such as dioxins. Today what is being used in most fish farm is ruminant (cow) blood, which may harbour a lot of parasites, pathogens and heavy metals.

Veterinary drugs in fish feed and fish: Antibiotics are commonly used in aquaculture worldwide to treat infection caused by a variety of bacterial pathogens of fish. They are commonly used as in-feed medications or surface coated onto feed pellets and dispersed in water. The use of antibiotics in fish farming is associated with new hazards in fishery products that are not encountered in wild captured species (EC, 2001a). The main hazards are antibiotic residues and the development of antimicrobial resistance in bacterial that may be transferred to consumers of farmed fish (EC, 2001b). Fish feed scare highlights challenges of aquaculture boom. More recently, several US States banned certain catfish imports from overseas after tests on frozen fillets showed that some contain blacklisted antibiotics. These incidents illustrate the importance of ensuring product safety in fish farming.

Pathogens in fish: It must be emphasized that it is nearly always possible to detect a range of human pathogenic bacteria on any fish that has not received any bactericidal treatment. It is common for these pathogens that some growth in the fish products is required to produce disease in humans (EC, 2001b). In contrast, the presence of pathogens from the animal/human reservoir is a serious safety concern for products to be eaten without (further) cooking. Growth of pathogen in raw fish to be cooked is similarly of little safety concern. An example is Salmonella. Salmonella manifests itself clinically either as the enteric fever syndrome caused by typhoid or paratyphoid strains or as the non-typhoid dependent gastroenteritis with symptoms appearing eight-72 hours after exposure to the pathogen.

Mycotoxin in fish feed and smoked fish: Moulds perform a double role in feeds. Some are necessary to obtain different feeds. On the other occasions moulds act as spoilage microorganisms. The moulds themselves do not represent a hazard for consumers, but some secondary metabolites that they produce during their growth (mycotoxins) could be dangerous for human health. A lot of agricultural commodities are susceptible.

Genetically modified organisms (GMOs): These days, we come across more and more feed types and individual products in our daily life, for example, genetically modified, such products have to be carefully examined from the safety point of view. The associated risk is considered to be the unexpected consequences of inserting foreign Deoxyribonucleic Acid (DNA)/genetic material into genome of a plant or animal. The potential hazards associated with GMOs in edible tissues of products from aquaculture include allergies, increased toxicity, increased cancer risks and changed nutrient levels.

But a professor of nutrition at University of Nigeria Nsukka (UNN) and former President of Nutrition Society of Nigeria (NSN), Ngozi Nnam, disagrees. The consultant nutritionist told The Guardian on Tuesday: “We advocate that people should have fish pond as a source of Omega 3 fatty acid, which is very beneficial for health. But I have not thought of some of these issues raised. We need to establish what is used as the feed for these fishes and we can use such to substantiate these claims. You cannot say homegrown fishes are not good until you establish the concerns.”

Also, scientists had last year questioned the health benefits of oily fishes especially catfish despite the fact that the dishes are now in popular demand across the country. Hotels, restaurants, beer parlours now offer catfish, popularly called ‘point and kill.’ Point and kill means ‘make a choice from live fishes to be prepared at the spot’ with spiced delicacies.

Is eating catfish dangerous to health? An unpublished study claims that consumption of catfish could increase the chances of developing cardiovascular diseases. According to the study, catfish contains omega-6 fatty acids, which could increase the level of inflammation in the body and inflammation is the underlying cause of cardiovascular diseases, certain cancers and diabetes.

However, several studies have shown that although catfish also contains omega-3 fatty acids, which has a lot of health benefits, the ratio of omega-6 is far greater than omega-3 fatty acids. Researchers, however, recommend that catfish should be consumed in moderation to prevent health complications.

Also, catfishes farmed with hormonal feeds are laden with steroids and some fattening chemicals. There are also concerns that the use of antibiotics in fish farming is fuelling antibiotics resistance in humans who consume them. Scientists had in 2015 raised alert that dosing livestock with antibiotics is putting human health at risk. They said farmers should thread with caution in their use of the drugs because bacteria are becoming immune to treatment. They cautioned that within 30 years antibiotic resistant superbugs would claim more lives than cancer.

However, Council of Fellows of Fisheries Society of Nigeria (FISON), last year dismissed suggestions that regular intake of catfish is dangerous to health.Executive Director, Nigerian Institute for Oceanography and Marine Research (NIOMR) and Secretary Council of Fellows, FISON, Dr. Gbola Akande, told journalists: “The story of catfish being bad has taken the dimension of a myth. The story is mythical in the sense that it was invented from imaginary and fictitious perspectives, basically unproven and should be regarded as false. We planned to put the record straight by portraying what has led to this latest campaign against catfish, further reinforce the good qualities of catfish and continue to promote catfish as a good and healthy candidate that it has been known for.”

Akande said catfish oils contain monounsaturated fatty acid (MUFA) and polyunsaturated fatty acids (PUFA) and fatty acid composition varies not only from species to species, but often to an even greater extent from one fish to another of the same species as environment, feeding, maturity and sex of fish can also affect its fats content.

He continued: “A balance of the essential fatty acids, omega-3 (n-3) and omega-6 (n-6) PUFAs are critical to healthy living. While n-6 can block or promote inflammation, it only does so in response to the amount of omega-3 received by the body. Recommended ratios in diets that provide health benefits include n-6: n-3 is less than five by WHO and PUFA: SFA greater than 0.4 (WHO).”


Akande said that catfish from the wild and farmed raised, fall within the recommended n-6 to n-3 ratios as proposed by WHO, thereby cannot inflict any damage on the body system.Also, researchers at the Department of Chemistry, University of Uyo, Akwa Ibom State, have shown that irrespective of the sources of fish, they are good bio-accumulators of organic and inorganic pollutants, especially trace metals. Trace metals gain access into the aquatic system from natural and anthropogenic sources and get distributed in the water body, suspended solids and benthic sediments during the course of their transportation.

The researchers in a study published in the Journal of Applied Life Sciences International found that trace metals in fish represent a potential risk, not only to the fish themselves but also to piscivorous birds, mammals and even humans. Metals may enter fish either directly through the digestive tract due to consumption of contaminated water and food, or non-dietary routes across permeable membranes such as gills.

Cultured fishes may absorb dissolved elements and trace metals from its feeding diets, pesticides, antibiotics and the surrounding water leading to their accumulation in various tissues in significant amounts thereby exhibiting toxicological effects at target organs. Water quality variables and chemical elements are important determinants of metal availability and toxicity. It is also dependent on the availability of the metal in the ionized form. Water quality is usually influenced by factors such as hardness, pH, dissolved oxygen, temperature, salinity, interactions with trace metal salts and other particles such as suspended solid.

The study is titled “Human Risk Assessment of Consuming Farm Raised Fish in Uyo, Nigeria.”
The researchers speculate that all fish feeds contain measurable levels of some contaminants. Thus, one of the primary concerns regarding contaminants in fish feed is the possible human health impact.

The researchers noted: “The fish species collected from these farms was Clarias gariepinus. The fish feeds (multi-feed and coppen) as well as the water samples from the ponds were also analysed. All samples were collected from two commercial fish farms located at different areas in Uyo. These samples were digested and analyzed for the level of six trace metals namely lead (Pub), nickel (Ni), cadmium (Cd), chromium (Cr), cobalt (Co) and zinc (Zn) using atomic absorption spectrophotometer. Target hazard quotient as well as hazard index were calculated for the analysed trace metals. Physicochemical properties of the water samples collected were analysed using standard methods.”

The results showed that in general, the highest concentrations of trace metals were present in the feeds except zinc while the lowest concentrations were observed in all the water samples. From the results, it was observed that lead, cadmium and chromium exceeded the maximum permissible limits set by World Health Organisation (WHO) and other international regulatory agencies in water while cobalt and chromium was above permissible limits in fish and feed. From the results, it was revealed that the target hazard quotients (THQ) for individual trace metals and hazard index (HI) values based on Ni, Pb, Cd, Cr and Zn were all less than one. The physical parameters of the water samples obtained from the ponds indicated that all the parameters were within permissible limits except for pH and conductivity.

The researchers concluded: “From the results obtained from the target hazard quotients and hazard index calculations, the study established that health risk associated with the intake of these metals via consumption of these fishes was insignificant. However, continuous monitoring of farm raised fishes as well as the feeds given to these fishes is necessary in other to reduce or completely avoid contamination by trace metals.”

Also, a new study warns published last week warned salmon in supermarkets across the United States (US) and the United Kingdom (UK) may contain banned toxic chemicals linked with developmental problems in children.

As a result, since 2004 the US and most of Europe have been working to eliminate a certain chemical called polybrominated diphenyl ether (PBDE) from all waters – of both farmed and wild fish – because they can disrupt hormones and cause developmental effects in the people who consume them.

However, a new study by the University of Pittsburgh, US, has found evidence of PBDEs in food fed to farmed salmon – even in those in supposedly PBDE-free environments.The chemicals were detected at such high concentrations that lead author Dr. Carla Ng warns it could be reaching our plates. “The international food trade system is becoming increasingly global in nature and this applies to animal feed as well,” Dr. Ng said.“Fish farming operations may import their feed or feed ingredients from a number of countries, including those without advanced food safety regulations.”Ng, assistant professor of civil and environmental engineering at the University of Pittsburgh’s Swanson School of Engineering, said: “The United States and much of Europe banned several PBDEs in 2004 because of environmental and public health concerns.

“PBDEs can act as endocrine disruptors and cause developmental effects. Children are particularly vulnerable.”Restrictions were placed on PDBEs in 2004. But, despite restrictions on their use, PBDEs were classed as ‘persistent organic pollutants’ at the Stockholm Convention, an international environmental treaty, in 2009.Ng’s paper said they continue to be found in areas that process large amounts of electronic waste and with poor recycling regulation such as China, Thailand and Vietnam.

As a result, salmon grown in environments free of PBDEs could still contain dangerous amounts of the chemical as a result, according to the findings.Farmers, Ng warns, could be using feed that contains a type of synthetic flame retardant imported from countries ‘without advanced food safety regulations’. Her paper, which presented new models on how the chemical enters food chains, shows it could also affect feed for cattle and sheep as well.


Conventional models to predict human exposure to pollutants mostly look people’s risk from their local environment.But Ng’s model takes into account factors to find ‘the best predictor’ of PBDEs in farmed salmon.These included pollutants inhaled through gills, how the fish metabolized and eliminated pollutants, and the concentration of pollutants in the feed.She said: “We found that feed is relatively less important in areas that already have high concentrations of pollutants in the environment.

“However, in otherwise clean and well-regulated environments, contaminated feed can be thousands of times more significant than the location of the farm for determining the PBDE content of salmon fillets.”She added the model could be applied in other fish with large global markets such as tilapia or red snapper and to predict pollutant content in livestock or feeds produced in contamination ‘hot spots.’

The mini review recommended: “However, the principal steps to avoid problems are awareness of all the potential vulnerable points in production and handling of feed and raw materials and taking of appropriate action to address these. Having identified the main threats and risks through pathogenic, chemical or naturally occurring contaminants and through industry practices, it should be noted that a raft of legislation should be in place. Aim should be increased safeguards against bad practice leading to feed contamination. Inspection of imported feed ingredients is also very necessary.“Hazard Analysis and Critical Control Point (HACCP) system for feed processing like those already in place for animal processing would make feed manufacturers spell out where contamination may occur during processing, then build in procedures to prevent it.”

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