US requirements on "dolphin safe" tuna labelling still discriminate against Mexican-caught tuna, according to a World Trade Organization ruling published Tuesday.
The United States overhauled its requirements for slapping required "dolphin safe" labels on tuna products after the WTO's dispute settlement body in 2012 found its rules discriminated against Mexican tuna.
But in Tuesday's ruling, a WTO panel agreed with Mexican charges that the US labelling was still violating trade rules.
The panel found that the US rules were being "applied in a manner that constitutes unjustifiable and arbitrary discrimination contrary to" global trade laws.
For more than 20 years, the United States has required all tuna sold in the country to carry a dolphin-safe stamp, certifying that no dolphins were harmed or killed when it was caught.
The move came amid international efforts to protect the dolphins that often swim alongside tuna and were getting swept into large fishing nets and dying by the millions.
Mexico meanwhile has for the past two decades been challenging the US labelling rules as discriminatory.
The main problem, according to the WTO rulings, is that the US measures are not even-handed in the way they address the risks to dolphins from different fishing techniques in different areas of the ocean.
The global trade body found that US regulations in the eastern tropical Pacific, where the Mexican fleet fishes, are more restrictive than in other waters.
The stakes are high because the tuna trade with the United States is worth billions of dollars, Mexican officials have said.
The United States has 60 days to appeal Tuesday's ruling, a WTO spokesman said.
Once the two months are up, the procedure will be considered closed and Mexico will be permitted to take retaliatory action against the United States if it does not bring its rules in line with the WTO decision.