SOUTH PADRE ISLAND, TEXAS - JANUARY 22: Boats seized from Mexican fishermen are kept behind the Coast Guard's South Padre Island station until they're destroyed to make room for more confiscated equipment. Officers apprehend dozens of fishermen every year, but almost all of them are repatriated in less than 24 hours, resuming their cross-border fishing practices shortly thereafter. (Photo by Kevin Sieff/The Washington Post) (Kevin Sieff/WASHINGTON POST)

The chase began after darkness descended on this stretch of ocean, where the U.S.-Mexico border cuts through the Gulf of Mexico.

The shark fishermen turned off the lights of their skiff before sneaking north across the border, and the U.S. Coast Guard boat followed suit, leaving six officers to find the target using a single pair of night-vision goggles.

“Those guys are north of the line,” said Petty Officer Andrew Watzek, squinting at the 25-foot Mexican skiff and then at a radar screen, where the border is a bold line extending off the coast. “They’re definitely in American waters.”

Both boats bob quietly, about 100 yards apart, on this cool night in January, until an officer pulls the throttle. The chase is on.

On land, a few miles west, the United States has spent billions of dollars in the past decade to secure its southern border, building 670 miles of fencing and adding more than 10,000 border patrol agents. But just off the coast of south Texas, that border is wide open, unmarked and largely unpatrolled.

The men who cross it at night sometimes carry drugs and immigrants. But overwhelmingly, they’re looking for new bounty in American waters: sharks whose fins are bound mostly for China. The global trade in shark fins is worth more than a billion dollars, experts say.

Biologists estimate that Mexican fishermen annually catch more than 50,000 sharks illegally in the United States because the best shark fishing is north of the border.

“They have GPS devices. They know where they are in the gulf and what they’re doing. They’re violating the sovereignty of American waters,” said Lt. Mickey Lalor, whose fleet includes about 70 officers devoted largely to rebuffing shark fishermen.

The chase in January ended like many others — with fishermen taking off for Mexican waters and Coast Guard officers stopping just north of the border, searching for fishing gear they might have left behind.

“It’s the same game every day. They chase us, sometimes seizing our boats. And the next day we do it again,” said Eric Carillo, whose family runs a small shark fishing business just south of the border in Playa Bagdad, bringing in $5,000 to $10,000 a month. Most of their profits come from fins, but fishermen also sell shark meat to Mexico City markets.

Each year, the Coast Guard apprehends dozens, seizing thousands of dollars in gear. But the fishermen spend less than 24 hours in U.S. custody, and then they are sent back across the border.

This is hardly the first time efforts to patrol U.S. borders have moved offshore. Cartel activity has spiked in the Pacific Ocean, including a fleet of semi-submarines, and boats transporting drugs have been interdicted off the coast of Florida for decades.

But because the laws are tailored to drug traffickers and human smugglers, shark fishermen are able to operate with near impunity. Some scientists worry that years of unregulated fishing could soon take a toll on the gulf, potentially disrupting an entire ecosystem.

“Because these guys fly under the radar, they could be catching a lot more sharks than we think,” said Robert Hueter, a shark biologist at the Mote Marine Laboratory in Sarasota, Fla., who has studied the impact of shark fishing near the border. “The fact that much of their fishing might be unaccounted for — that’s the big concern.”

The fishermen launch their boats from Playa Bagdad, peppered with cabins that once accommodated tourists. But Bagdad has been battered by hurricanes in the past 10 years, and the splintering huts now house about 150 shark fishermen.

“When we fish here, we catch next to nothing. Little fish. And barely any of them,” said Carlos Guerra, a Mexican citizen, who started working on the beach in December, days after being deported from Chicago. “When we cross the border, we catch so much. We make a lot of money.”

His reasoning, shared by other fishermen, is that American regulations have left its shark population mostly intact while the lack of Mexican enforcement has caused years of overfishing. Fishermen endure long nights and rough seas to fish in U.S. waters — hauling in sharks they dub “gringos.”

Some toss hundreds of longlines and hooks in the water at a time. Others use miles of gill nets, which hold dozens of sharks: mostly juvenile black tips, but also sand bar sharks, hammerheads and other species. The fins — which eventually head to Asia, where they’re used in shark fin soup — are worth $35 per pound in Playa Bagdad. As the market for shark-based dishes has grown, more fishermen have converged on the beach.

Mexican biologists lobbied for heightened regulation in recent years, but there is little evidence of such a policy in Playa Bagdad.

“The enforcement is still weak,” said Jose Leonardo Castillo-Geniz, a biologist at Mexico’s National Fisheries Institute. “With the need to reduce the size of Mexico’s bureaucracy, the fishery sector has been one of the victims of the cuts.”

Thus the Coast Guard finds itself as the bulwark between U.S. and Mexican waters, and the cornerstone of an inadvertent conservation effort.

Neither group is from the region. The fishermen leave their homes in southern and central Mexico, drawn by Playa Bagdad’s reputation for healthy fisheries just across a porous border. The Coast Guard officers arrive for three-year stints from California, Florida and other stations.

“I heard that I was going to the border, and I expected drug trafficking and smuggling,” said Petty Officer Stephen Garon. “I didn’t expect shark fishermen.”

The fishermen show no sign of relenting. In the past two months, officers have seen another uptick in apprehensions. The objects of those pursuits were almost all familiar faces — they had been apprehended several times before.

“I’ve lost more than 13 boats to the patrol in the last 10 years. We’ve lost thousands of dollars in equipment,” Carillo said. “But we’ve made four times more money fishing across the border than we would have made otherwise. It’s worth the risk.”