Fish contaminants
Introduction
Several organic and inorganic compounds can find their way into fish and seafood. These compounds can be divided into three major groups:
- Inorganic chemicals: arsenic, cadmium, lead, mercury, selenium, copper, zinc and iron.
- Organic compounds: polychlorinated biphenyls (PCBs), dioxins and insecticides (chlorinated hydrocarbons). This is a very diverse group with a wide range of industrial uses and a chemical stability that allows them to accumulate and persist in the environment.
- Processing-related compounds: sulphites (used in shrimp processing), polyphosphates, nitrosamines and residues of drugs used in aquaculture (e.g. antibiotics or hormones).
Many of the inorganic chemicals are essential for life at low concentration but become toxic at high concentration. While minerals such as copper, selenium, iron and zinc are essential micronutrients for fish and shellfish, other elements such as mercury, cadmium and lead show no known essential function in life and are toxic even at low concentrations when ingested over a long period. These elements are present in the aquatic environment as a result of natural phenomena such as marine volcanism and geological and geothermal events, but are also caused by anthropogenic pollution arising from intensive metallurgy and mining, waste disposal and incineration, and acidic rain caused by industrial pollution. This is in contrast with organic compounds, most of which are of anthropogenic origin brought to the aquatic environment by humans.
Increasing amounts of chemicals may also be found in predatory species as a result of biomagnification, which is the concentration of the chemicals in higher levels of the food chain. Similarly, they may be present as a result of bioaccumulation, when chemicals in the body tissues accumulate over the life span of the individual. In this case, a large (i.e. older) fish will have a higher content of the chemical concerned than a small (younger) fish of the same species. The presence of chemical contaminants in seafood is therefore highly dependent on geographic location, species and fish size, feeding patterns, solubility of chemicals and their persistence in the environment.
Risks from fish contaminants
But what are the risks to human health caused by these contaminants as a result of consuming fish and seafood?
Several studies indicate that in the open seas, which are still almost unaffected by pollution, fish mostly carry only the natural burden of these inorganic chemicals. However, in heavily polluted areas, in waters that have insufficient exchange with the world’s oceans (e.g. the Baltic Sea and the Mediterranean Sea), in estuaries, in rivers and especially in locations that are close to industrial sites, these elements can be found at concentrations that exceed the natural load.
Likewise, several studies have concluded that levels of these chemicals in fish intended for human consumption are low and probably below levels likely to affect human health. Nevertheless, they can be of potential concern for populations for whom fish constitutes a major part of the diet and for pregnant and nursing women and young children who consume substantial quantities of oily fish. These concerns can only be clarified if updated and focused risk assessments are conducted.
While scientists and other experts recognize that certain of these elements are present naturally in fish and seafood, some consumers regard their presence even at minimal levels as a hazard to health. Consequently, food scares can be easily started and further amplified if communication is mismanaged – particularly given the growing speed of communication and information dissemination facilitated by the Internet. A number of such scares concerning fish contaminants have recently led to significant negative impacts on fish trade flows.
Example 1: Mercury in fish
In 2003, the Codex Joint Expert Committee on Food Additives (JECFA), administered by FAO and the World Health Organization (WHO), revised the guideline for mercury in fish to 1.6 micrograms of methyl mercury intake per day per kilogram of body weight, nearly half the original guideline of 3.3 micrograms. At the same time, the JECFA review emphasized that people should continue to eat a normal diet of fish, pointing out its many health benefits. Included in its considerations was the recently released Seychelles Islands study, which analyzed mother and child pairs and fish consumption for almost ten years. That study determined that high levels of fish consumption led to no adverse effect to a foetus or child’s neurodevelopment.
In order to translate the recommended weekly intake of mercury into national maximum mercury levels in fish it is necessary to take into account consumption patterns, other sources of mercury intake and other relevant information. However, public pressure often leads to consumer confusion between the maximal allowable levels necessary to protect human health and the limits recommended to protect the environment. The latter require that appropriate actions be taken consistently and for a significant period of time in order to reduce the environmental burden of the contaminant. In the case of mercury, for example, proper energy policies would be required to reduce reliance on coal-fired power stations and the reduction of waste incineration; these two factors combined account for 70 percent of new, human-made mercury emissions to the atmosphere.
Unfortunately, a number of media articles and public health warnings exacerbated the pre-existing consumer confusion and sent out conflicting messages regarding the health benefits of fish and seafood and the mercury risks from fish to the point that local authorities in California, the United States, instructed grocery retailers to display signs cautioning consumers about the dangers of mercury in fish and threatened to sue those that did not abide.
Since then, the Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) in the United States have released a consumer advisory document along the lines of the recent JECFA guidelines but stressing that fish and shellfish are an important part of a healthy diet. Despite this measure, the tuna industry considers that the damage already inflicted will be difficult to repair.
Example 2: Organic pollutants in salmon
A recent study published in the magazine Science reported on “Global assessment of organic contaminants in farmed salmon”.1 Concentrations of 14 chlororganic compounds in farmed and wild salmon were examined. Each of these compounds is thought to cause cancer. The study revealed that all the substances tested were present in higher concentrations in farmed salmon than wild salmon. This applied in particular to fish produced on European farms. Although the levels found were consistent with results from earlier surveys and official controls, the researchers concluded hastily that consumers should tightly limit their consumption of farmed salmon and suggested that anyone who does not want to additionally increase the risk of getting cancer should restrict consumption of one portion of farmed salmon to a maximum of once every two months.
On the basis of the identified concentrations of toxic substances, the authors of the study then calculated the portion sizes for wild and farmed salmon that could be consumed without increasing the risk of cancer. The recommended quantities fluctuate strongly depending on the salmon’s origins. Whereas, for example, eight portions (227 g) of salmon from Kodiak (Alaska) could be consumed per month, consumers should not eat more than one portion of Chilean farmed salmon per month, no more than one portion of Norwegian farmed salmon every two months, or one portion of farmed salmon from Scotland or the Faeroe Islands no more than every four to five months.
It is these calculations that caused a big stir. The model used for the calculations is highly disputed among scientists and is not specifically intended for calculations on commercially produced fish; it had been developed by the EPA to estimate how much of their catches could be eaten by anglers who regularly fished in contaminated inland waters. By contrast, commercial products should be evaluated according to the FDA criteria. To refute the model, researchers calculated that on the basis of the PCB contamination levels cited in the study, after 70 years of regular consumption of 200 g of salmon per week the risk of developing cancer for the high-risk group (pregnant women, children, nursing mothers) would be one-hundred-thousandth higher – equal to a rise in risk of 0.0001 percent. By comparison, the risk of dying of a cardiovascular disease by eliminating fish completely from the diet can be as high as 30 percent2.
It is therefore understandable that the recommendations made by the authors of the Science study to limit salmon consumption met with strong objections in Europe, the United States and elsewhere. Food control and health authorities reacted by announcing that its findings did not raise new food safety issues as the levels were consistent with results from other surveys and official controls. They encouraged consumers to continue eating salmon and other fish, the health benefits of which had been proven beyond all doubt in over 5 000 scientific studies. Unfortunately, the study had already alarmed the consuming public, and retail orders of farmed fish fell by 20–30 percent in countries such as Ireland, Norway and Scotland. A great deal of time and effort were required to restore consumer confidence.
Conclusion
Globalization and further liberalization of the world fish trade, while offering many benefits and opportunities, also present new safety and quality challenges. Fish safety regulators have been applying a host of control measures, from mandating the use of the Hazard Analysis and Critical Control Point (HACCP) system to increasing testing, with varying degrees of success. Improved risk-based scientific tools must be adopted so that the fish safety standards reflect the most current and effective scientific methods available to protect public health.
In establishing maximum levels of fish pollutants, regulators need to ensure the highest level of consumer health protection, but they must also take into account the reality of the current background contamination of the environment in order not to endanger the food supply. Concurrently, strategies must be adopted to reduce gradually the background contamination of the environment and lower progressively the maximum levels in feed and foods to follow this downward trend. In addition, consumer information and awareness programmes will be necessary in order to improve transparency and consumer education.
Progress in this area will require enhanced international cooperation in promoting scientific collaboration, harmonization, equivalency schemes and standard-setting mechanisms that are based on scientific principles. The World Trade Organization’s Agreements on Sanitary and Phytosanitary Measures and Technical Barriers to Trade, together with the benchmarking role of the Codex Alimentarius Commission, provide an international platform in this respect. Meeting these challenges will be of the utmost importance for fish trade, both in developed and developing countries, particularly as the latter contribute more than 50 percent (in value) of international fish trade.
1 R.A. Hites, J.A. Foran, D.O. Carpenter, M.C. Hamilton, B.A. Knuth and S.J. Schwager. 2004. Global assessment of organic contaminants in farmed salmon. Science, 303(5665): 226–229.
2 Does farmed salmon cause cancer? Eurofish, 2004/1: 62–65