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Study Examines Purity of Norwegian Shellfish

NORWAY - The level of undesirable substances and microorganisms in shellfish from Norwegian waters was examined by the National Institute of Nutrition and Seafood Research (NIFES) in 2007. The study shows a generally low level of undesirable substances in these species, with the possible exception of horse mussels and oysters. The microbiological quality was found to be mainly good.


On behalf of the Norwegian Food Safety Authority, NIFES annually carries out microbiological examinations to determine the level of E. coli, enterococci and Salmonella, as well as chemical analyses of pollutants (metals, DDT, PCB, dioxins, polybrominated flame retardants and PAH) in shellfish. The analyses are a part of the Shellfish Monitoring Programme. The results from 2007 showed generally low levels of undesirable substances in several species of bivalve mollusks and one species of crab, with the possible exception of horse mussels and oysters. The microbiological quality was found to be mainly good.

Examinations were carried out on pooled samples. The pooled sample is a representative selection from a number of individuals of the same species sampled simultaneously at the same locality. In this monitoring programme, a pooled sample corresponds to up to 450 grams of the soft parts of several individuals. The number of grams depends on which microbiological and chemical parameters are to be tested.

Mainly good microbiological quality

Shellfish can absorb intestinal bacteria such as E. coli, enterococci and Salmonella if they are found in the water where the shellfish grow. Analyses of E. coli and  enterococci are used to identify faecal pollution, and thus possible health risks.  

Amount of E. coli identified forms the basis of the EU’s classification of shellfish harvesting areas. Shellfish from a class A area can go directly to consumption, while shellfish from class B and C areas must go through a cleaning process first. The level of E. coli was less than 230/100 grams in 289 (87%) of the 334 examined pooled samples. This corresponds to the EU’s classification of shellfish harvesting areas as class A areas. The concentrations in 43 samples corresponded to class B areas, while two samples had concentrations corresponding to class C areas.  

There are more than 2500 variants of Salmonella bacteria. Many of these may cause infections in humans, and foodstuffs should therefore not contain Salmonella bacteria. Of four analysed samples of horse mussels, one was shown to contain Salmonella Infantis. Salmonella Infantis can cause infection in humans, but is not very common in Norway. Horse mussels are usually steamed at 70-80ºC, which is sufficient to kill these bacteria.   

Undesirable substances in blue mussels (Mytilus edulis)

For analyses of undesirable substances, blue mussels were sampled mainly from farm sites, from West Finnmark in the north to the county of Østfold in the south. 65 pooled samples were taken from 33 localities in spring 2007 and from 31 localities in autumn 2007.

For the undesirable substances covered by this report, the results for blue mussels showed levels below the EU’s upper limits, where such limits exist. However, some of the metals showed considerable variations between regions. This is illustrated by two of the blue mussel samples from the fjords of Hardanger and Sogn og Fjordane in the autumn of 2007, where unusually high levels of arsenic were detected; 8.3 and 19 mg/kg wet weight, respectively, of which 2.3 and 3.8 mg/kg were inorganic arsenic. Even though the level was unusually high, this is not considered to threaten the food safety. For further information, see the article ”High level of inorganic arsenic in blue mussels from some areas”.

Undesirable substances in great scallops (Pecten maximus)

Six pooled samples of the muscle and roe of scallops were taken from wild populations in the Frøya area and from a cultivation area in the county of Hordaland. Generally, the results showed low concentrations of the analysed undesirable substances in muscle and roe of great scallops, confirming the findings of previous years.

Undesirable substances in horse mussels (Modiolus modiolus)

Three pooled samples of horse mussels were taken from wild populations in the county of Finnmark and the county of Hordaland. Analyses showed that all three samples had cadmium concentrations above the EU’s upper limit of 1.0 mg/kg wet weight. In two of the three samples the concentrations of lead were above the EU`s upper limit of 1.5 mg/kg wet weight. The relatively high values observed were most likely due to the fact that the entire soft parts were analysed without removing the kidney. Previous studies show that lead and cadmium in the horse mussel mainly accumulate in the kidney (see link to article).  The food safety authority has therefore adviced people to remove the kidney before steaming horse mussels. When the kidney is removed before the horse mussel is steamed, the concentration of lead and cadmium in the remaining soft parts is low and does not threaten food safety. 

Reference is made to the article ”Cadmium and lead in bivalves – importance for food safety

Undesirable substances in European flat oysters (Ostrea edulis)

Twelve pooled samples of oysters were taken from eight different cultivation sites from the county of Møre og Romsdal to the county of Aust-Agder. Five of the pooled samples of oysters contained levels of cadmium above the EU’s upper limit of 1.0 mg/kg wet weight. Norwegian oyster farmers have experienced problems with harvesting being banned due to the level of cadmium in oysters just exceeding the EU’s upper limit. Flat oysters appear to be particularly susceptible to elevated levels of cadmium in the water.

Oysters are considered a delicacy product, which most people only eat occasionally. The consumption of oysters is therefore not normally considered a food safety problem. Thus, a somewhat higher upper limit for cadmium in oysters could have been considered.

Undesirable substances in the common cockle (Cerastoderma edule)

One pooled sample of the common cockle was taken from a wild population in the county of Nordland. The results showed low levels of all of the undesirable substances analysed for.

Undesirable substances in the common crab (Cancer pagurus)

Seven samples of claw meat and five samples of brown meat were taken of wild common crab caught in the Bergen area. The level of cadmium in the brown meat was above the EU’s upper limit of 0.5 mg/kg wet weight for crustaceans. Brown meat from common crab is however excempted from this limit. Analyses of claw meat showed low concentrations of all metals.

Purpose of the Shellfish Monitoring Programme

The purpose of the National Monitoring Programme for Shellfish Production is to ensure that shellfish harvested for consumption are not harvested in areas polluted by microorganisms or undesirable substances and that the shellfish do not contain faecal material in concentrations that exceed stipulated upper limits. The programme also includes monitoring of wild shellfish, so that the public can harvest them locally. 

How can shellfish contain undesirable substances?

Bivalve mollusks are filter feeders, feeding on particles from the water. Thereby they readily absorb undesirable substances or microorganisms from the water or from the particles they eat. Bivalent molluks can accumulate algal toxins, environmental contaminants (such as metals and organic environmental toxins) and microorganisms.

The report does not include surveillance of algal toxins. For further information about algal toxins in mussels please refer to: http://matportalen.no/Emner/Giftig_blaaskjell

the Fish Site Editor

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