When nine-year-old Keegan Rothman set out to fish British Columbia's Fraser River with his dad, he expected his catch might be about a metre or so long – a sizable fish that would weigh about half as much as he does.

Instead, the New Jersey boy reeled in the largest fish he's ever landed and a rare catch on the river: a massive 270-kilogram (600-pound) white sturgeon.

"Am I going to lose it? When am I going to get it in? I'm so tired," Keegan, who weighs about 80 pounds himself, said when asked what he was thinking as he wrestled the fish for two hours. "If the drag didn't fit right, I would have gone overboard."

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Keegan's father, Dan Rothman, said the boy was at the stern of the boat when the three-metre-long fish jumped high in the air, its white underbelly catching their attention.

"We saw the fish right away, and once you see something that big, we're just hoping you can land it," Mr. Rothman said. "We chased it around with the boat the whole day. At one point, it wrapped itself around a tree stump at the bottom of the river."

Keegan began fishing at four years old and holds a youth record in the United States for the sheepshead fish, his dad said.

"He's caught his fair share, but nothing like this," Mr. Rothman said. "This is a once-in-a-lifetime fish."

Matthew Clive of Great River Fishing, the guide company the father and son hired for their trip, estimates the sturgeon was about 75 years old.

While it's not unusual to find such large fish swimming the Fraser River, Mr. Clive said he doesn't often see those fish being caught and landed. The most recent catch before Keegan's, Mr. Clive said, was last year when a man from Atlanta reeled in a sturgeon that was 10 feet, eight inches, or about 3.25 metres.

Sturgeon in the Fraser River are protected and part of a mark-and-recapture program jointly run by volunteer fishing guides and the provincial government.

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"We scan and tag all of the sturgeon that come to our boats," Mr. Clive said. "There's fish that you'll catch at one end of the river and a couple years later will be at the other end of the river."

Scientists use the data collected to track and understand fish migration and growth patterns, Mr. Clive said. A sturgeon caught and tagged in the river in 2012, for example, was discovered to have been born in 1885.

A total of 66,000 sturgeon have been tagged through the program, with approximately 122,000 instances of fish being recaptured and released, Mr. Clive said. About 10 per cent of fishing guides in the area participate.

Keegan and his father released the massive fish after taking pictures and video footage of the catch.

"If you kill them all there won't be a population any more," Keegan said. "There won't be any more to catch."