Substitution of high-priced fish with low-priced species: Adulteration of common sole in German restaurants

High-priced fish and seafood prepared as restaurant dish is especially prone to fraudulent substitution due to the lack of morphologic characters and less stringent labelling requirements. Common sole (Solea solea) is considered one of the most valuable and tasty fish species on the European markets and fraudulent substitutions of common sole in restaurant stores have been reported by official German food authorities in the past (from 2005 to 2011). The aim of the present study was to assess the substitution of common sole prepared as dishes in restaurants and to identify species used as substitutes for common sole. Furthermore, it was investigated whether substitution takes place at the level of the restaurants or earlier within the food supply chain. Altogether, 47 common sole dishes were ordered incognito in 24 restaurants and 98 whole fish specimens or fish fillets were purchased from 35 different shops (including wholesale dealers and speciality markets). Species identities were determined by cytochrome b gene sequencing and isoelectric focusing of sarcoplasmic proteins, which are official German methods for fish species identification afforded by §64 of the German Food and Feed Code (LFGB). 50% of the restaurant samples were shown to be substituted by species of lower commercial values, such as catfish species (Pangasidae), Senegalese tonguesole (Cynoglossus senegalensis) or Portuguese sole (Synaptura lusitanica). As almost all samples from the retail were shown to be authentic - only one sample was identified as lemon sole (Microstomus kitt) - we presume that fraudulent substitution is performed by restaurant owners and staff rather than by persons responsible for fishing, manufacturing or retail.