The toxicity of heavy metals such as lead, cadmium, and mercury is well known. Their concentrations in feed and food are monitored and regulated by a wide variety of governmental agencies. For cadmium, levels are typically found to be highest in foods such as shellfish, oysters, salmon, and fungi.16 Food safety laws for aquaculture in Norway are harmonized with the EU, where measures to monitor certain substances and residues in live animals and animal products were developed in 1996 as Council Directive 96/23/EC.17 This directive states that for farmed finfish, such as salmon, the minimum number of samples to be collected each year must be at least 1 per 100 tons of annual production and that samples taken at the farm level should be taken from a minimum of 10% of the registered sites.
In 2005, Norway had 870 registered sites that produced over 570,000 tons of farmed Atlantic salmon. However, there is a lack of compliance for Norwegian farmed salmon producers as less than half the registered farms needed (33 out of 87 needed) were sampled from.18 For the minimum sampling requirements of 1 per 100 tons, Norway has sampled only 125-375 salmon per year for cadmium and lead since 1998.19 The laboratory primarily responsible for contaminant testing in seafood, the National Institute of Nutrition and Seafood Research (NIFES), only tested 12 farmed salmon in 2004. When the contaminated pre-mix ingredients from China were being processed into feed, in November of 2004, the European Free Trade Authority (EFTA) surveillance inspection team in Norway concluded that "The emphasis on food safety and human health was not as clearly stated as with regard to control on feed for marine species." And, "Finally, follow up visits to check on corrective actions were not always documented. Consequently, establishments could keep their approval in spite of not complying with the feed production requirements laid down in Council Directive 95/69/EC."20
One of the reasons Norwegian authorities refuted the heavy metal concentrations the Russians found in farmed salmon was that it is believed that farmed rainbow trout and Atlantic salmon could not possibly accumulate such levels without observing mortality or bone deformities. For fish in general, the deposition of lead and cadmium from either water or feed borne exposure is usually highest in the gills, kidney, and liver.16 However, environmental variables such as temperature, pH, chemical composition, and variations in the production of metallothionein have been shown to affect both the uptake and the toxic impact of heavy metals in fish. Also, there exist dramatic differences in fish mortality when the exposure is from either fresh or saltwater, or from feeds. For cadmium, coho salmon have been shown to have a LD50 of 1,500 ug/L when raised in saltwater compared to 6.2 ug/L in freshwater.16 In freshwater, it is estimated that Atlantic salmon can tolerate 7.9 ug/L compared to 1.3 ug/L in rainbow trout.16 Rainbow trout have greater tolerance to cadmium toxicity from dietary exposure compared to waterborne exposure such that feed exposures of 500 mg/kg for 30 days can be tolerated with no mortality or growth retardation.21,22 In Atlantic salmon, one study showed no adverse morphology or reduced growth after freshwater parr (pre-smolt) were exposed to cadmium levels in feed of 250 mg/kg for 4 months.23
With regards to heavy metal uptake from commercial feed pellets, assimilation efficiencies of 15% for rainbow trout and up to 6% for Atlantic salmon have been reported for cadmium.24,25 These results indicate that with a feed contamination of 11-17 mg/kg, the uptake in farmed rainbow trout could result in 1.7-2.5 mg/kg and the levels in farmed salmon could be 0.7-1.0 mg/kg. Thus, the research from Norway with farmed salmon appears to support the Russian claims of finding 0.7 mg/kg of cadmium in the farmed salmon. There has been some discussion that only the internal organs, and not the filet, could accumulate such high levels. However, the current scientific literature lacks studies that examine tissue distribution at equilibrium or complete elimination kinetics in Atlantic salmon fed cadmium contaminated feed. The one study that does exist for Atlantic salmon, shows in freshwater parr (not adult salmon) a nonlinear uptake of cadmium over 4 months with levels in feed of 0.5 to 25 mg/kg and no examination of elimination kinetics.26 Therefore, no quantitative predictions on filet concentrations can currently be made after dietary exposure in farmed salmon. If whole fish were exported and analyzed, or there was organ contamination to fillets on either the Norwegian or Russian side during fish processing or sample analysis, this could easily explain the levels observed by Russian veterinarians.
While 2005 had several feed and food accidents with cadmium, it was the year Norway recommended, and the EU enforced, an increase in the maximum limit of cadmium in feed from 0.5 to 1.0 mg/kg.27,28 As surveillance programs had observed cadmium levels of 0.94 mg/kg in Norwegian fish feed previously, principle scientists at NIFES stated in a 2003 report that "...the uptake of cadmium from feed in salmon is very low (2-6%)", and "Norway has recommended that this limit be increased to 1.0 mg/kg feed, on the basis of results generated."25 These authors point to the results from their own experiments (the aforementioned 4 month study) and these results are again used in the opinion of the Norwegian Scientific Committee for Food Safety to increase the cadmium level in feed, except the values have been curiously changed "The retention of dietary cadmium is low (1-5%)..."28 While a 1% change may not seem significant, it is the difference between having your fish on the market or not as the EU maximum limit for cadmium in farmed fish is 0.05 mg/kg. A 6% dietary accumulation from 1 mg/kg in feed will result in 0.06 mg/kg in fish, exceeding the EU limit while a 5% uptake will not.
As the low-cost and high-volume production of Norwegian farmed salmon may have producers excited, the lack of testing before feed and food enter the food chain, and the lack of scientific knowledge and integrity should have it's world-wide consumers concerned.
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Claudette Bethune, Ph.D.. Norwegian farmed salmon production raises global concern. EnvironmentalChemistry.com. May 31, 2006. Accessed on-line: 5/28/2017