The pair were looking for mahi mahi — but instead found a great white shark

Don Gates has spent decades on the water in the Florida Keys angling for the variety of fresh fish that call the clear blue waters off the archipelago home.

For the past 20 years, he’s been tagging mahi mahi, also known as dolphin fish, for the Dolphinfish Research Program, a nonprofit that studies the movement of the highly migratory green and blue species that is popular among both restaurant diners and sport fishermen.

On Monday afternoon while mahi fishing with his neighbor, Angie Gonder — both live on Cudjoe Key in the Lower Keys — he spotted something he never expected to see swimming near the island chain: A huge great white shark.

“I have been fishing the Florida Keys for 30 years, and I have never seen a great white shark before,” Gates, 66, told the Miami Herald/FLKeysnews.com Thursday.

Gates put the rod and reel down before grabbing his GoPro camera, sticking it in the water and capturing stunning footage of the large pelagic fish swimming around his boat, seemingly uninterested in anything but cruising in the Gulfstream, just beneath the surface of that day’s calm water.

“It was very peaceful, and not at all in a feeding mood,” Gates said.

He and Gonder were about 23 miles off Cudjoe Key right before they saw the shark, which he said was likely a female, possibly pregnant, and nearly 20 feet long — a large specimen even for a species known for its massive average size.

Gates and Gonder had just stopped by a log they saw floating that had bait fish swimming underneath. Such finds tend to create perfect scenarios for catching mahi, which often dart in and out from under floating debris to target smaller fish.

They did catch a mahi, as well as a barracuda. But, it was the shark they saw immediately after landing those fish that they’ll always remember.

“We drifted with that log for about 45 minutes and then moved to the east two to three miles, and the shark stayed there as well,” Gates said. “When we left around 6:45 p.m., it was still in the area.”

White sharks are more commonly found hanging around in cooler temperatures north of the Mid-Atlantic East Coast than in the warmer water off South Florida.

But seeing one passing by off the coast of the Sunshine State is actually not all that unusual, said Mahmood Shivji, professor at Nova Southeastern University’s Department of Biological Sciences and director of the Guy Harvey Research Institute.

There have been several sightings by anglers and divers off the Keys over the past few years, and Shivji said that’s because the long-distance travelers “have long been known to move through South Florida waters during their migration.”

The large predators are “one of a handful of sharks that can keep their body temperatures warm,” allowing them to roam cooler waters, said Shivji, who calls white sharks “absolute marvels of evolution.”