Market policies have profound implications for consumers as well as for the management of resources. One of the major concerns in fish trading is species mislabelling: the commercial name used does not correspond to the product, most often because the product is in fact a cheaper or a more easily available species. Substitution rates depend heavily on species, some often being sold mislabelled while others rarely or never mislabelled. Rates also vary largely depending on countries. In this study, we analyse the first market-wide dataset collected for France, the largest sea food market in Europe, for fish species substitution. We sequenced and analysed 371 samples bearing 55 commercial species names, collected in fishmonger shops, supermarkets and restaurants; the largest dataset assembled to date in an European country. Sampling included fish fillets, both fresh and frozen, and prepared meals. We found a total of 14 cases of mislabelling in five species: bluefin tuna, cod, yellowfin tuna, sole and seabream, setting the overall substitution rate at 3.7% CI [2.2–6.4], one of the lowest observed for comparable surveys with large sampling. We detected no case of species mislabelling among the frozen fillets or in industrially prepared meals, and all the substitutions were observed in products sold in fishmongers shops or restaurants. The rate of mislabelling does not differ between species, except for bluefin tuna. Despite a very small sample size (n = 6), the rate observed for this species (83.3% CI [36–99]) stands in sharp contrast with the low substitution rate observed for the other substituted species. In agreement with studies from other countries, this work shows that fish mislabelling can vary greatly within a country depending on the species. It further suggests that more efforts should be directed to the control of high value species like bluefin tuna.

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 sampling covers 55 different commercial namesThe 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.

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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.

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.