and a range of processing avenues, all of which poses the logistic challenges of a long and complex supply chain. Secondly, "tuna" is one of the general "umbrella" terms under which many species with diverse biological traits continue to be traded, especially in geographic areas exporting high volumes of seafood to Europe \[[@pone.0196641.ref033]\]. Collectively, the opportunities for species substitution, be it for deliberately maximising financial gain or through sheer logistic errors/mismanagement, remain high; so does the exposure of consumers to a lack of transparency that prevents environmentally conscious purchasing decision and/or may result in inadvertent consumption of unhealthy products (e.g. high heavy metal content, \[[@pone.0196641.ref034]\]). Progress in this area can be achieved through a robust international policy of accurate species-level labelling and coordinated governance efforts.

Supporting information {#sec008}
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###### Tuna samples collected in six European countries between 2012 and 2014.

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###### Commercial denominations of tuna in EU and six member states (ES, PT, UK, ROI, FR, GER).

(DOCX)

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###### DNA analysis methods used in this study by the participant laboratories.

(DOCX)

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###### Primers sequences and PCR protocols used in the study.

(DOCX)

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###### Number of samples of fresh and frozen tuna analysed and mislabelling results split by type of label.

(DOCX)

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###### Number of samples of canned tuna analysed and mislabelling results split by type of label.

(DOCX)

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###### Species which appear as substitute in the mislabelled samples.

(DOCX)

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###### Tuna species identification ring trials.

(DOCX)

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Click here for addition