YARMOUTH, Mass.—Dave Lamoureux's kayak, named Fortitude, must be the only one in Massachusetts registered as a motor vessel. That's because a powerboat registration is required to get a permit to fish for tuna here.

Apparently, it never occurred to the authorities that someone might be crazy enough to want to catch a bluefin while sitting in what amounts to a floating plastic chair and enjoying what Melville called a “;Nantucket sleigh ride.”;

Since the end of July, Lamoureux has caught three bluefins this way, paddling a couple of miles off Race Point, at the tip of Provincetown, hooking a tuna and holding on, the rod clipped to a harness on his chest, while being towed at speeds up to 15 miles an hour before the fish exhausts itself.

His most recent catch, on Nov. 5, was a 157-pound bluefin, a record tuna for an unassisted kayak fisherman, and a near record overall, topped only by a 183-pound halibut caught by Howard McKim, an Alaskan kayaker, in 2004.

Reeling in a halibut, though, has been likened to hauling in a load of plywood, and some of Lamoureux's admirers consider landing a bluefin, known for its power and ferocity, the greater feat. He is a hero at bait shops up and down Cape Cod. On the fishing blogs, a few grumblers call him a dangerous idiot.

Until about 10 years ago most kayak fishermen knew each other by name. Lately the sport has enjoyed a growth spurt, but it is still not recognized by the International Game Fish Association, the official record keeper for saltwater anglers. So kayakers keep their records informally and on Internet forums.

There is an honor system. Some kayakers allow themselves to be towed out and back by a mother ship. Lamoureux's record required paddling alone and bringing the fish into shore.

Lamoureux is 42 and friendly, with a big smile and a ready laugh, and lives most of the year in Chicago, where he is a futures and options trader. He also has a place in Boston and access to his parents' summer home here.

“;My personality—I trend toward risk and danger,”; he said last week, explaining that he used to rock climb and do extreme skiing.

But kayak fishing entailed “;measured risk, not being-crazy risk,”; he added, and compared it to trading. “;Being a trader, you like risk. You're comfortable with it. You have to weigh the reward versus the other side, which in this case is your life.”;

Lamoureux's 12-foot Heritage FeatherLite isn't even a fishing kayak. It's a recreational kayak he found in the family garage and has modified with additional equipment, the exact nature of which he will not disclose.

“;I can't be revealing all my secrets,”; he said, “;or else guys who are younger and in better shape will be breaking my records.”;

When Lamoureux climbs into his kayak, wearing a wet suit or a dry suit, he is loaded down with safety gear: life jacket, whistles, strobe lights, a signaling mirror, a compass, two GPS device, two radios, two cell phones, and two knives, in case he is dragged too far out to sea and needs to slash the line.

He hasn't yet capsized, but Lamoureux still prepares himself psychologically to wind up in the drink.

“;I actually consider myself safer than the average boater because all the safety equipment is attached to my person,”; he said. He also carries dive fins in case he has to swim home. “;I don't plan on calling the Coast Guard or the commercial fishermen for help,”; he said. “;I think that's irresponsible.”;

When Lamoureux first showed up in their fishing grounds, commercial tuna fishermen figured he was lost or in distress. Now he has befriended several of the commercial guys, to whom he will turn over a fish too big for him to manage. In August he reluctantly did this with a bluefin that eventually escaped but that on the fishing boat's sonar looked to be about 800 pounds.

“;That just broke my spirit,”; he said. “;They told me, 'That fish is so big, it doesn't even know you're here.”;'

Two years ago, Lamoureux began kayaking for stripers and bluefish. This summer he started looping through the tuna grounds on his way home, and at the end of July he hooked a bluefin. It proved too big for his striper rig and broke his line, but made him think catching a really big fish was at least possible.

Lamoureux consulted with George Lewis, a longtime mentor at Truman's Bait and Tackle in Yarmouth, with the staff of Nelson's Bait and Tackle in Provincetown, and with Austin Proudfoot at Goose Hummock, a shop in Orleans that coincidentally specializes in kayaks and tuna—though until Lamoureux came along, not in both at once.

Proudfoot fixed Lamoureux up with Van Staal rods and Fin-Nor reels, heavy duty-spinning equipment, and came up with the idea of using frozen ballyhoo, a sort of miniature swordfish, as bait.

“;You wrap the beak with wire and you rig it so the tail can move; that's the attraction,”; Proudfoot said, adding, “;When Dave first came in and said what he wanted, I sort of giggled and I thought, That's impossible. Now I tell him, 'Whatever you're doing, you're doing it right.”;'

Learning on the job, Lamoureux hooked 14 tuna before he finally caught one. There are five steps, he explained.

First you have to hook the fish, which strikes suddenly and violently—“;sort of like a raging bull,”; he said. Then there's the ride, which is the scariest part but also “;the most fun thing I've ever done.”; After that you have to fight the fish until it dies of exhaustion.

This can take hours and entails steering the tuna—Lamoureux won't say how—and controlling its speed with the drag on the reel.

Bluefins are powerful enough, he said, that if given too little line, they can cause a kayak to flip end over end. When they get close enough, commercial fishermen harpoon a tuna, but Lamoureux right away realized that that would be a disaster from a kayak.

“;Even I'm bright enough not to do that,”; he said, laughing.

At the end, the fish must be attached to the kayak and towed home, which is harder than it sounds, especially if, as Lamoureux hopes one day soon, it is a 300- or 400-pounder—enough weight to drag someone under. He has that part, too, all figured out, but don't hold your breath waiting for him to tell you how.