The organization said the giant fish, caught by Robert Pedigo aboard the Journeyman off the coast of Puerto Vallarta, Mexico, Thursday, is not eligible for the world record because Danny Osuna, a captain working as a deckhand aboard the Journeyman, touched Pedigo's pole while he was struggling with the tuna, FoxNews.com reported Friday.
IGFA rules state a world record fish must be caught without any help.
"We have caught a lot of fish in the high 300s, but this is the first time we've caught something like this," Osuna said.
He said Pedigo, who Fox said could not be reached for comment, was not too upset about losing the record.
"We're fine with that," Osuna said. "Actually, we never even thought the fish would be that big."
An IGFA spokesman said the incident puts the world record for a yellowfin tuna catch back to 405 pounds.