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Background: The vaquita of Mexico is critically endangered, with a population less than 19 individuals in 2018. The population continues to decline and gillnet use continues in vaquita habitat., Methods: Nine vaquita carcases were examined from 2016 to 2018 to establish cause of death. Concentrations of polychlorinated biphenyls (PCBs), DDTs and polybrominated diphenyl ethers (PBDEs) in blubber (n=3) were determined using gas chromatography/mass spectrometry and faeces tested for domoic acid and saxitoxin., Results: Carcases were in good nutritional status and had lesions and full stomachs consistent with fisheries bycatch. PCB, DDT and PBDE concentrations ranged between 94 and 180 ng/g, 500 and 1200 ng/g and 97 and 210 ng/g lipid weight, respectively, which are low compared with other marine mammals. No saxitoxin or domoic acid was detected., Conclusion: These findings support the conclusion that bycatch is the primary source of vaquita population decline and emphasise the need for stronger measures to eliminate gillnets from the Upper Gulf of California, Mexico., Competing Interests: Competing interests: None declared., (© British Veterinary Association 2020. Re-use permitted under CC BY-NC. No commercial re-use. Published by BMJ.)