Eric Ozolins punctures the blacktip shark's skin with a coffee-straw-size ID tag. The big fish writhes in wet sand as Oz reaches inside his tackle box for the DNA sampling kit. Although the 10-foot-long, 600-pound tiger sharks he catches along Texas' Padre Island National Seashore (PINS) – the world's longest undeveloped barrier island – would eat this five-footer for breakfast, Oz grins as if he's just landed a trophy mount.

"A two-year-old female," he says, clipping a fingernail-size sample of genetic material from the blacktip's main dorsal fin. "The perfect shark scientists are looking to tag."

Marine biologists working to save sharks are getting help from the least-expected source: fishermen. Oz's tackle box holds upward of $10,000 in cutting-edge tracking technology – computerized satellite tags, sonar echolocation equipment – that gives scientists at Corpus Christi's Harte Research Institute empirical ammo to determine how many giant sharks, from bulls and tigers to hammerheads and makos, roam the northern Gulf of Mexico – and why. If the data confirms their belief that the barrier island is a feeding or breeding ground, it could prompt officials to limit commercial fishing in the area or promote catch-and-release efforts during pupping seasons, measures that could return the increasingly endangered population to historical levels. The research will also help scientists identify similar sites across the world, which could lead to more widespread protection.

But wrangling a sofa-size shark from the sea, affixing it with a smart tag, and hauling it back to the water is a maneuver not taught in biology seminars. It requires the physical stamina of an NFL linebacker and the mental acuity of a battlefield surgeon. That's where Oz comes in.

Six feet tall and muscular, Oz, 33, resembles a farm-team hockey player on permanent spring break. He first fished Padre Island as a kid on family vacations, returning permanently after graduating from high school in Austin. Freelance computer work finances his true vocation: casting more than 100 days a year. Although regarded as the Gulf's top shark fisherman (two years ago, he reeled in five that measured eight feet or longer), Oz insists he's no "shark whisperer." Those who've seen him in action think otherwise.

"He's the Michael Jordan of the shark-fishing world," says Greg Stunz, marine biologist at the Harte Institute. "There is no better angler here or probably anywhere else."

Stunz and his colleagues sought out Oz after seeing clips and photos the angler had posted online of his catch-and-release feats. Their unusual partnership is emblematic of the Harte Institute's rogue ethos. Founded with $46 million in 2000 by Ed Harte, a local newspaper magnate, the institute was given a simple mandate: "Do science that makes a difference." Stunz helped to assemble a team of disparate specialists, including a geologist, a lawyer, and an economist, to prove the social and fiscal value of oceanic life. But locating breeding grounds was a prime objective. The team needed someone to tag mature female sharks and then release them back to the waves.

To get Oz onboard, Stunz had to first earn the blessing of the PINS fishing community, which has a deep-seated suspicion of scientists, afraid their research might lead to restrictions on recreational fishing. So, he approached Oz's mentor, Billy Sandifer, a cantankerous 65-year-old Vietnam vet with waist-long hair, famous for having killed monster sharks before converting to catch-and-release conservation – a switch he made after he encountered a pregnant bull shark years ago and instantaneously felt compelled to push her back into the sea.

"We've always been shy about suits," Sandifer says. "I said, 'Motherfucker, don't backstab us, because first time you do, you've lost us forever.'"

Sandifer put Stunz in touch with his protégé, whom he described as the only angler fearless enough to camp in PINS' severe, isolated terrain for days at a time. "PINS is savage," Sandifer says. "That bitch will bite you, cut your heart out, and laugh at you."

A two-mile-wide, 70-mile-long strip of white-sand beach and wildflower-wreathed dunes, PINS ranks among the country's more dangerous national parks, thanks to its proximity to Mexico's war-torn state of Tamaulipas. Bales of weed and coke regularly tumble ashore, dumped by traffickers fleeing the U.S. Coast Guard. These waters draw high numbers of giant sharks because of the solitude, the offshore currents that bring school fish, and a geology that propels the continental shelf's deep waters to the beach (sharks sometimes skid ashore while chasing prey).

Oz's routine here is well established. Before leaving, he asks friends about recent trips, scours surf sites for swell information, and loads up on supplies (PINS is completely pack-in/pack-out). He parks his SUV parallel to the beach, just beyond the high-tide mark, props a ladder against the vehicle's duneward side, and climbs atop a platform bolted to the roof. He puts three or four rods in upright metal holders and sets the shark alarm, a keychain siren rigged to wail when a line begins unspooling.

From the rods, a 100-pound test main line runs down to an eight-foot steel cable leader dead-ending at hooked bait. Oz paddles the weighted bait up to 700 yards out to sea before dropping it in one of the 15-foot-deep "gut" channels troughed between sandbars – "shark highways" the predators cruise for food. He then quickly returns to shore. In addition to the danger of being overturned and hooked amid five-foot waves, Oz has to deal with overeager sharks. A six-foot blacktip recently attacked the bait dangling off his kayak, nearly toppling him into its jaws.

Since Stunz wants data on the largest sharks possible, Oz's muscle is key. After a sometimes three-hour fight, he reels the shark into ankle-deep water. From there, in a span of about five minutes, he lassos the tail, hauls the shark onto sand, and takes its measurements with a tape roll (weight is estimated from these numbers), along with a DNA sample. All without even being on the Harte payroll.

Oz uses three types of tags, depending on instructions he receives from Harte. Passive tags – thin plastic strips bearing Harte's phone number – are put in nearly every catch, so that any angler who nabs the shark in the future will call in its tag number and location, giving Harte a read on migration patterns. (Oz has caught the same sharks twice.) Acoustic tags emit a unique sonic code that yields a location when the shark is within a mile of strategically placed Gulf receivers. Microphone-size satellite tags – which, at $5,000 a pop, are used sparingly – are attached to a dorsal fin through a tiny hole Oz drills, and remain there for six months, when an electrical charge fires and sends the severed beacon rocketing to the surface. Atop the waves, the tag relays the water temperature, depth, and location from its entire ride on the shark – a mother lode of data compared with the other tags' location-only snapshots.

Despite all this tech, Oz and Harte face an up-current battle against Mexican gillnetters, who are feeding the growing Chinese demand for shark-fin soup, a $100-a-bowl dish served at status-driven events. Last year, abandoned gillnets holding some 3,000 rotting sharks washed ashore. Recently, a bull shark satellite-tagged on PINS sent its data back after just a month. The shark had traveled up to Houston and then out along the continental shelf before heading south to Mexico, where it promptly dropped 700 feet to the ocean floor and became a flatline blip (tags deploy early if a shark doesn't move for five days). "It was probably finned," Stunz says. "It ended up in the heart of Mexico's longline fishery."

The sun is setting when Oz's shark alarm peals again. A 20-minute fight from atop the SUV brings a five-foot blacktip to shore. After tagging and measuring it, it's time to return it to the sea. There's a microcurrent of pulse as the shark touches saltwater, and, after a stunned moment, the blacktip's gills rise and fall. Its tail flicks and it disappears in knee-deep water, a flash of gray beneath the foam.

"That shark could go up toward Louisiana or down south and get whacked by the longliners," Oz says, rigging another hook with fresh-caught skipjack. "There's just no telling."