Commercial Tuna Fishing as a Source of Environmental DNA (eDNA)

This case study was prompted by reports that DNA (deoxyribonucleic acid) of undeclared tuna species had been found in cans labeled skipjack. Only one species of tuna is allowed in a can sold in the European Union if the muscle structure of the tuna is visible. This finding implied mislabeling in these processed tuna products. Given these issues, mislabeling may be considered a food fraud and/ or food protection issue. The purpose of this study was to determine the source of undeclared DNA in canned tuna. Samples from several steps of the cannery process were sent to several independent laboratories and analyzed by several molecular methods including Sanger and NextGen sequencing. Theses analysis found commingled environmental DNA (eDNA) from a variety of fish including Thunnus spp. in the catcher vessel brine or factory thaw water during processing. At least four species of Thunnini were found in the brine and five species of Thunnini were found in the thaw water by PCR and Sanger sequencing. Nine genera of fish, including four species of Thunnini, were found in the brine, and eight genera of fish and three species of Thunnini were found in the thawing water by next-generation sequencing (NGS). The DNA from these species or genera could easily accompany the muscle meat of the target species into the can or pouch. As such, unless a quantitative genetic analysis test is run that can identify how much DNA of various organisms is present in a sample, DNA should not be used as a proxy for tuna muscle tissue in retorted canned tuna processed in commercial tuna factory, because of the distinct possibility of adventitious eDNA from other species of tuna.