A retired British couple vacationing in B.C. has caught what their guide says might be one of the largest fish ever reeled in on the continent.

Michael and Margaret Snell from Salisbury, England, were out fishing for sturgeon on the Fraser River Monday in Chilliwack, east of Vancouver, when Michael saw his rod dip.

"You're there waiting for the rod to go and all of a sudden it plunges down then you strike, and it's on," Michael Snell said. "I got my wife, Margaret, to hang on to me. 'Hang on to my belt,' I said, 'Because if it pulls, I want to stay in the boat."

The white sturgeon, the largest freshwater fish species in North America, can live up to 100 years and is classified as an imperilled species in B.C.

After a 90-minute struggle, the Snells pulled in a white sturgeon that was nearly four metres long — about the length of their boat — and weighed roughly 500 kilograms.

Fishing guide Dean Werk, who was with the couple, said it was obvious to him the fish was huge.

"I knew it was a big fish because the way the fish took off after he was hooked up," Werk said. "I ran back to the back of the boat and I reeled up the rest of the rods and we got the anchor up and we started to ensue the chase.

Snell said it was quite a ride.

"All the time Dean's manoeuvring the boat, we're moving around, I'm winding like mad. You've got to put the rod down, heave it up, wind down."

Werk said the gigantic sturgeon was not tagged, meaning it has probably not been caught in at least 18 years.

But the only trophy they would take home was a photograph or two. The Snells released the fish back into the Fraser after posing for a few photos.

The last time the Snells were in B.C., they also caught a sturgeon that was about a 1.5 metres long.

With files from the CBC's Stephanie Mercier and The Canadian Press