Exploring the possible reasons for fish fraud in China based on results from monitoring sardine products sold on Chinese markets using DNA barcoding and real time PCR

Sardine is the common name for several small-sized pelagic species from Clupeiformes, representing a resource of great importance in the global fishery. Great efforts have been made to utilise these species as dried, smoked, and restructured fish products. However, in most of these products, it is quite challenging to identify the individual species as the external features are lost during processing, paving the way for species mislabelling. In this study, DNA barcoding (max, using about 650 bp, described as FDB; mini, of about 192 bp, described as MDB) was used for species identification of 139 specimens taken from 48 sardine products (canned and dried seasoning) randomly collected from local markets in Nanjing, China. Moreover, species specific primers were designed for Sardina pilchardus, with the aim to screen the species of S. pilchardus in mixed products. Results highlighted a success rate of amplification from 38.1% for FDB to 97.9% for MDB. Only one sample failed the Sanger-sequencing, and species-specific real time PCR confirmed the existence of S. pilchardus in the product. A maximum species identity in the range of 98-100% was obtained for all readable sequences and 11 species/genera were identified, belonging to 5 orders (Scorpaeniformes, Perciformes, Clupeiformes, Aulopiformes, Scombriformes). Significant legislative and managerial shortcomings and incentives to facilitate the market access of certain species, together with public indifference, represent the main reasons for fish fraud in China.