The speedboat is about 3 miles offshore when a US Customs and Border Protection agent cuts the engine to drift on the current in quiet darkness, hoping for the telltale signs of immigrant smuggling — a motor’s whirr or sulfur exhaust fumes.
“It’s like trying to find a needle in a haystack, and the haystack is the Pacific Ocean,’’ agent Tim Feige said, minutes before sunrise.
This is a new frontier for illegal immigrants entering the United States — a roughly 400-square-mile ocean expanse that runs from a bullring on the shores of Tijuana, Mexico, to suburban Los Angeles.
In growing numbers, migrants are gambling their lives at sea as land crossings become even more arduous and likely to end in arrest. Sea interdictions and arrests have spiked year-over-year for three years, as enforcement efforts are increased to meet the challenge.
While only a small fraction of border arrests are at sea, authorities say, heightened enforcement on land, and a bigger fence, is making the offshore route more attractive.
The number of Border Patrol agents doubled to more than 20,000 since 2003, and President Obama is dispatching the National Guard after clamor for a crackdown in the desert led to Arizona’s tough new immigration law.
“I think they found that going west through the ocean is probably their best bet,’’ said Michael Carney, deputy special agent in charge of investigations for US Immigration and Customs Enforcement in San Diego.
US agents arrested 753 suspected illegal immigrants on Southern California shores and seas between October and Aug. 24, up from 400 the previous 12 months and 230 the year before. They spotted 85 watercraft since October, up from 49 during the previous 12 months and 33 the year before.
The smugglers use old, single-engine wooden vessels known in Mexico as pangas. They’re several feet wide and about 25 feet long. If they are found on US waters, they are almost invariably smuggling people or drugs.
US authorities have stepped up sea patrols near the border, forcing pangas loaded with illegal immigrants and sometimes with marijuana farther offshore, with landings farther north.
An abandoned vessel was found in November in Laguna Beach, 85 miles north of Mexico. A boat with 24 people was found 43 miles off the San Diego coast in May.
Six boats have landed at Camp Pendleton Marine Corps base, more than 50 miles north of the border, since November, including two that were abandoned. The base, a short hike to Interstate 5, has stepped up security.
Authorities believe smugglers put their passengers ashore and return to Mexico, when possible, to avoid losing their boats and leaving evidence behind. But they also quickly abandon the boats and run for it if they sense they are about to be caught.
Smuggling on California waters dates back to the alcohol trade during Prohibition, but authorities noticed a change in late 2007 when pangas began traveling without lights at night with up to 25 people packed on open decks.
At up to $5,000 a person — roughly twice the fee to cross illegally over land — one overnight trip can generate $100,000.
Some arrests at sea may be a result of heightened enforcement. This year, the Orange County Sheriff’s Department joined in boat patrols on a 32-mile coastal stretch south of Los Angeles.
Only two immigrants are known to have been killed crossing in US waters, their boat overturning off San Diego in January. Two months earlier, eight were rescued atop an overturned boat that was adrift for a day. Smugglers have been arrested on both sides of the border, with those in the United States being sentenced to a year or two in prison.
In Mexico, the boats launch from a poor fishing village named Popotla, about 15 miles south of the border. It is between Playa de Rosarito’s high-rise hotels and condos that cater to American tourists and expatriates and next to the studio where the 1997 blockbuster “Titanic’’ was filmed.
Squatters live in about two dozen shanties crammed on a hillside. There is no electricity, paved road, sewage, or garbage collection. But it is easy to understand why smugglers are drawn to the village. It is out of view from the highway only 200 yards away, and it is the only public boat launching spot on a 50-mile stretch south of the border.
Authorities have failed to pierce the top ranks of smuggling organizations. The low-slung boats, when weighed down with people, float only about one foot above water, making them difficult to see on radar. Night-vision binoculars have limited reach.
“They’re beating us with low-tech,’’ said ICE’s Carney. “I’m not saying they can’t be detected, but I’m saying they’re very hard to detect.’’