On Dec. 24, 2012, U.S. Coast Guard Sector Corpus Christi and its partners in the Texas Parks and Wildlife Department had already seized more illegal long lines and gill nets from the waters north of the U.S.-Mexican maritime boundary than in any previous year – but a discovery on that day brought the record year to a grim conclusion: A response boat crew from Station South Padre Island, on patrol, spotted a floating gill net about four miles off the Texas coast and 17 miles north of the maritime border. The net was 5 miles long and loaded with 345 dead sharks – 225 blacktip, 109 bonnethead, and 11 bull sharks.

Because they indiscriminately kill any creature that becomes entangled in them, gill nets are illegal in Texas, whose maritime authority extends nine miles out to sea. Though it was unknown who set the illegal net, there was little doubt, given recent history, where it came from: For the past several years, incursions of Mexican lanchas – small open fishing vessels with outboard engines – into U.S. waters have increased dramatically, as fishermen from camps near the village of La Playa Bagdad, south of the border, boldly venture north to deploy illegal gear they will later check for catch. Typically, these fishermen target red snapper and shark.

The United States’ exclusive economic zone (EEZ) – the internationally recognized belt of waters extending 200 miles off a nation’s coast, over which that nation has special resource rights – is the largest in the world, and the Coast Guard is responsible for protecting it from incursion by foreign vessels. In 2012, the service reported that nearly 90 percent of the detected incursions into the U.S. EEZ – 3.4 million square miles of ocean, buffering 90,000 miles of coastline – were in this sliver of the Gulf of Mexico.

While the Dec. 24 find was shocking, it was minor compared to the September 2011 discovery of a 3-mile-long gill net in U.S. waters near the mouth of the Rio Grande River that held 3,000 dead juvenile sharks – the loss, Texas Parks and Wildlife officials said, of an entire generation of species.

Such dramatic cases of shark poaching are driven partly by the desire for shark meat, but shark carcasses are often found discarded, without fins. Rising affluence in China and other parts of East Asia has fueled a boom in demand for the consumption of shark fin soup – a mostly flavorless dish, but, at about $100 a bowl, prized as a status symbol. A 2005 study by the National Oceanic and Atmospheric Administration estimated that Mexican lanchas illegally catch between 38,000 and 57,000 sharks in U.S. waters annually, most of them blacktips.

Lt. Joshua Sagers, commanding officer of the Coast Guard’s Station South Padre Island, said the station’s patrols routinely discover illegal fishing gear, sometimes two or three times a week. This is surprisingly often, given how difficult such gear is to spot; nets and long lines are often rigged with small, often transparent buoys, and the lanchas themselves are such low-profile vessels that they’re nearly invisible at a distance.

Interdictions of lanchas and their three- or four-man crews, however, are beginning to happen more often as the stakes are raised. “They’re fairly bold,” said Sagers. “Typically what happens is the boat is seized, and their gear is seized, and they’re held until they’re turned over to Mexican authorities. And whenever we ask them: ‘Why would you come here, knowing that you’re fishing illegally in U.S. waters?’ the answer is always the same. It’s that there are no fish in Mexican waters, because they’ve fished them out. Indiscriminate use of gill net and long line has depleted their fish stocks completely.”

The methods of lancha crews are harmful to other species as well: Marine birds such as pelicans, for example, have been discovered entangled or hooked when they go after fish caught in long line gear. “It’s certainly worth noting,” said Sagers, “that marine birds are a protected species. We regularly find marine turtles, which are endangered, caught in these gill nets. We find dead dolphins. So there’s certainly an impact beyond the fisheries management mission.” Such regular trespass into the U.S. EEZ is additionally troubling, Sagers said, when you consider that the coastal Gulf is commonly used as a corridor for illegal narcotics transits.