Rafael had tuna permits but was told catching tuna with a net is illegal.
Instead, it's got to be caught by handgear, such as rod and reel, harpoon or handline.
"We didn't try to hide anything," Rafael told The Standard-Times newspaper of New Bedford, a famous whaling-era port 50 miles south of Boston. "We did everything by the book. Nobody ever told me we couldn't catch it with a net."
A fish that big is hugely valuable, prized by sushi-lovers for its tender red meat. A 754-pound tuna recently sold for nearly $396,000.
Rafael's fish will be sold overseas, and he'll get no share of the proceeds if regulators find a violation, The Standard-Times reported. The money would instead go into the National Oceanic and Atmospheric Administration fund that also holds money collected for fishery fines.
Rafael said he thinks he's going to surrender his tuna permits now.
"What good are they if I can't catch them?" he said.
The tuna was likely inadvertently snagged as Rafael's crew set a net to catch bottom-dwellers, he said.