GENEVA (Reuters) - The World Trade Organization's Appellate Body ruled against the United States on Friday in a dispute with Mexico over tuna labeling, upholding an earlier ruling issued in April that found U.S. rules discriminated against Mexico."The Appellate Body concludes that the United States has not brought its dolphin-safe labeling regime for tuna products into conformity with the recommendations and rulings of the (WTO's dispute settlement body)," the WTO's appeals judges said at the end of their 144-page ruling on the case.The appeal ruling is final, and could lead to Mexico making a claim for retaliation against U.S. exports if it believes the United States has not brought its rules into line with the WTO ruling. “The United States is disappointed with this most recent report,” Tim Reif, general counsel at the U.S. Trade Representative's office, said in a statement. The U.S. government would consult closely on the next steps with members of Congress, American fishermen and conservation NGOs, he said. Mexico has been fighting for more than 20 years over rules the country argues have frozen its fishing industry out of a U.S. imported canned tuna market worth $680 million in 2014. Mexico has about a 3.5 percent share.The clash arose because yellowfin tuna swim with dolphins in the eastern tropical Pacific, where Mexico's fleet operates, using speedboats to herd the dolphins and large purse seine nets to catch the tuna swimming beneath them.Millions of dolphins were killed before international conservation efforts set standards to protect dolphins and put professional observers on ships to record each tuna catch.Mexico argued the agreements had cut dolphin deaths to minimal levels - below the thresholds allowed in U.S. fisheries - and that tuna from other regions did not face the same stringent tests, with ship captains allowed to self-certify that no dolphins were harmed.Reif said the ruling did not mean the United States would have to reduce the protection of dolphins or lower its labeling requirements, and the WTO Appellate Body had recognized the United States had already gone some way towards complying with Mexico's original concerns."The Obama Administration is committed to an all-of-government approach that strengthens wildlife conservation – including through high-standard trade agreements," he said, adding that the Trans-Pacific Partnership deal signed last month would promote marine mammal conservation among all 12 members.