Authorities are still searching for a 63-year-old man after he fell into the ocean on Sunday after hooking a "huge" fish off the coast of Hōnaunau, Hawaii.

Mark Knittle, who stands at 5 feet, 10 inches tall, and 185 pounds, was fishing for ahi tuna four miles beyond the Hōnaunau Boat Ramp when he vanished, said Hawaiian police.

"The friend heard Knittle say, 'the fish is huge,' then saw Knittle go overboard into the water," according to a press release. His friend tried to grab his line, but Knittle quickly disappeared; the friend jumped into the water to search but couldn't find him, the news release said.

Ahi, Yellowfin tuna can weigh up to 400 pounds and can reach six feet, according to NOAA Fisheries. Deep sea fishing experts said that standard tuna weighs about 130 pounds, and anything over 200 pounds is considered "big."

"If there's a fish that's actually pulling him around, you don't know where he could go," Darwin Okinaka, Hawaii County Fire Department assistant chief of operations told The Associated Press.

"When hooked, Ahi tuna usually do not jump, but they will make sudden sideways turns and swim in large circles," Whipsaw Sportfishing, a charter fishing company based in Oahu, said on their website. "Considering their weight, this can be an exhausting experience, and you will have to have patience while trying to reel them in."

The Coast Guard and the Hawaii Fire Department have searched for Knittle for 72 hours. The search will conclude on Thursday.

"If you don't have your gear right, or you get your hand wrapped in that line, those fish are all muscle," Paul Appleblom, owner of Kauai Apex Charters, told Insider. "I've been trying to even figure out what the hell happened with this guy."

"If you're hand-lining and wrapping your hands incorrectly, you could get pulled over and be along for the ride, and they're strong," he added.

While deep-sea fishing is inherently risky, such overboard incidents are rare.

In 2017, a young man was dragged off his boat near the coast of Australia after having caught a large marlin fish. He remained entangled with the fish for six hours before he was rescued, according to BBC. 

"These fish are fighting for their lives, their adrenaline is pumping. They're in survival mode," Appleblom said. "Anything can happen."