WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2013 61 (CONTINUED ON PAGE 63) therapeutic, levels at which they tend to be found in seafood. Genotoxic carcinogens, such as the nitrofurans and some organochlorine pesticides, are another set of chemicals for which it can be argued that insufficient data exists for setting a safe level. Standard testing has shown that these chemicals continue to be carcinogenic as they are dosed at lower and lower levels, so that it is impossible to define a level at which the carcinogenic potential is zero. However, there must be a point at which cancers attributable to the carcinogen become insignificant compared to background cancer levels. Current techniques do not allow this level to be defined. The Regulator’s Dilemma and Zero Tolerance Consumers worldwide expect national governments to protect their health through regulation. The Treaty on the Functioning of the European Union (2009), for example, specifically requires “a high level of human health protection” to be ensured in all policies and activities. When toxicity data is available on a substance, the regulator takes risk advice from scientific experts and writes laws that prohibit the presence of harmful levels of the substance. When the data is insufficient, how does the regulator legislate to protect the health of the consumer? The first step is to ban use of the substance, inasmuch as this should in practice eliminate the substance from all legally produced goods. The next step, setting a safe maximum residue limit (MRL) in the food to account for illegal use of the substance, is a problem because the data to set a safe limit are not available. The regulator may argue that, because the use of the substance is unacceptable, there should be no residue in the product, so it would seem reasonable to set the level at zero; this would satisfy the “high level of human health protection” criterion. This zero tolerance principle is applied by the European Commission and other bodies worldwide. The problems with zero tolerance in food are well documented (Heberer et al. 2007, Tran et al. 2012), enforcement among them. The enforcement of zero levels depends very much on the techniques and equipment used to detect a substance. The levels at which substances can be detected are now very low and continue to fall as analytical science develops. With increasing sensitivity of detection, levels are being measured that appear to be background environmental levels or in any case cannot be attributed to inadvertent or malicious use of chemicals. Examples of this include the presence of semicarbazide (a breakdown product of the banned nitrofuran drug nitrofurazone) in laboratory-bred and wild-caught crustaceans (Table 2) and new findings of antibiotic residues in several farmed and wild-caught species of fish.1 Protecting Human Health Imagine a situation where a residue is found in a consignment of frozen shrimp at a very low level, so low that modern equipment can just barely measure it. If zero tolerance applies or if the level exceeds the MRL, the legal status of the product is clear: it is illegal. In the EU the consignment must be destroyed. This does not provide evidence that the chemical was applied illegally during aquaculture, although this is generally assumed. The findings of “natural” background levels of contaminants described above indicate that this assumption may be wrong. In some cases, there will be enough toxicological data to estimate how harmful to human health it would be to release the consignment, especially if the level exceeds the MRL. This is especially useful if the consignment has been released for consumption and a decision has to be made to manage the risk, such as issuing consumer advice or recalling the product. In other cases, especially where there is not enough toxicity data and an MRL has not been set, the potential for harm of the contaminated product remains uncertain. National authorities find cases such as this extremely hard to address. Faced with an unquantifiable risk, consumers fear the worst and may put pressure on authorities to withdraw the product. On the other hand, authorities are unable to balance the ‘unknown’ risk to consumers with the costs of a product recall. Because it is accepted that current risk assessment mechanisms cannot deal with this type of situation, efforts are being made to find alternative methods of TABLE 2. Levels of semicarbazide (SEM) measured in the shell and meat of farmed and wild-caught crustaceans (van Poucke et al. 2011). Species Origin SEM in meat SEM in shell (µg/kg) (µg/kg) Litopenaeus vannamei Farmed in Vietnam, Thailand and Ecuador Absent (<0.5) 2.1–2.6 Penaeus monodon Farmed in India (Gujarat) and Indonesia Absent (<0.5) 1.5–2.3 Nephrops norvegicus Caught in Scotland Absent (<0.5) Absent (<0.5) Scylla serrata Caught in Vietnam and Madagascar Absent (<0.5) 4.1–12.6 Portunus pelagicus Caught in China and Vietnam Absent (<0.5) 3.3–4.1 A level close to zero or even ‘undetectable’ is no guarantee of safety. Regulators and the aquaculture industry have a collective responsibility to evaluate the safety of their product and this includes evaluating the risk to human health of contaminants and residues in the product when they are present, whatever the level.
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