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The development of citizen science has brought together scientific expertise and volunteer involvement to answer both scientific and societal questions. In this study, a consortium of citizens, journalist, scientists and non governmental organisations reports the first measure of the market-wide rate of fish mislabelling in France.We collected in fishmonger shops, supermarkets and restaurants and sequenced 390 samples of fish either in fillets or prepared meals, which is the largest dataset assembled to date in an European country.The overall substitution rate is one of the lowest observed for comparable surveys with large sampling in Europe. Remarkably, we detected no case of species mislabelling among the frozen fillets or in industrially prepared meals. We also investigated most of the mislabelling cases detected directly from the sellers. A number of them admitted that the substitution took place at the end of the supply chain.The rate of mislabelling does not differ between species (3.7 %, ci 2.2-6.4%), except for bluefin tuna. Despite a very small sample size (n=6), this species stands in sharp contrast with the low substitution rate observed for the other species (rate between 36 and 99%). This study shows that even in countries where species substitution rate is low, citizen science can enhance the management of natural resources and provide important insights for regulation policies.
Changes made since version 1: Extension of the paragraph about bluefin tuna in the discussion; cchanges in the figure S1 also renamed figure 4; addition of missing references in the bibliography; formatting of the references in the main text to fit the requirements of a PeerJ submission; a few cosmetic corrections in the main text.