The video shows Shepard struggling with the groupers, one of which grabs onto the fisherman's catch and briefly drags him along with it.
"I weigh 220 pounds, and it dragged me easily 20 feet or so before letting go," Shepard told National Geographic.
Shepard can be heard shouting unintelligible curses at the thieving fish. He said one of the groupers bent his spear "so badly it will no longer load into my speargun."
Experts said the groupers -- which can live for up to 50 years and grow to up to 8 feet long and 800 pounds -- were likely stalking Shepard and waiting for the right moment to ambush his catch.
"[Were they] following the diver around? No question about it. They probably know his boat by the sound of it," said R. Grant Gilmore, a fish ecologist at the Florida-based Estuarine, Coastal and Ocean Science, Inc.
National Geographic Explorer-in-Residence Sylvia Earle suggested looking at the incident from the fish's perspective.
"Thinking like an ocean, it is clear that the diver, not the grouper, is doing the stealing," she said.
Shepard, for his part, said he has forgiven the thieving fish.
"As frustrating as it is to lose my catch to them, I respect the laws protecting goliaths and will never physically harm or kill them," he said. "As apex predators, goliaths fill a niche in that ecosystem in which I am just a temporary visitor. We are targeting the same prey and it is well within their right to fight me for what is already theirs."
