Abel Martinez sat watching the early morning commuters being piggybacked across the cold, thigh-deep waters of the Rio Grande.

"I don't know whether I'm going or not," Martinez confessed. He had been "frightened," he admitted, when he was caught and deported by the U.S. Border Patrol in his first attempt at the illegal crossing three days before.

Long before dawn, hundreds of residents of this border city begin gathering on the river's concrete banks for their daily journey to jobs as maids and construction hands and field workers in greater El Paso. North of the chain link fence across the Rio Grande, U.S. Border Patrol agent Ben Robinson would intercept a few in what he called their "cat and mouse game," knowing that most would get across the following day.

But increasingly, the Border Patrol here is redeploying agents to airports, railyards and highway checkpoints beyond urban El Paso, places where they believe it is more likely to catch immigrants like Martinez who hope to work in the United States, not as day laborers or seasonal farm hands, but as permanent illegal residents.

"We are seeing a different breed of alien now," Robinson said.

"It's hard, there's no opportunity here, and on the other side you are always running from the migra," or immigration service, said Martinez, 18, a backpack-toting high school graduate with a middle-class cadence to his Guadalajara accent. He wondered aloud about his chances for a high-paying U.S. factory job.

Rafael Cuero, 24, who has parlayed strong shoulders and a pair of hip waders into a profession as a Rio Grande "mule," was getting impatient. "Are you coming?" he asked Martinez. Paying the $1 fee, Martinez clambered on Cuero's back. He stepped off on the northern bank and followed boys on bicycles to a gap in the ineffectual fence.

Patterns of Mexican emigration are perhaps best observed here in the twin metropolises of Juarez-El Paso on the Texas border, where in contrast to other big border crossings the vast majority of illegal immigrants detained by the U.S. border patrol are Mexican. On the far sides of the border in San Diego, Calif., and McAllen, Tex., every fifth alien detained is non-Mexican, patrol officers estimated.

Last year more than 250,000 Mexicans were stopped along the 360-mile border stretch monitored from El Paso, up from about 200,000 in 1984, estimated Larry Richardson, the Border Patrol Chief Agent for the region. Immigration and Naturalization Service projections for 1986 now range up to 375,000.

Ever since Chihuahua cowpunchers began working in Chicago stockyards at the turn of the century, the border around Juarez-El Paso border has been one of the best established Mexican immigration routes. Yet despite its history as a gateway for northbound Mexican job seekers, the area has long been characterized by preponderantly localized immigration, with most undocumented workers remaining in West Texas. Today, about 100,000 Juarez familes, nearly half the city's population, depend substantially on regular income earned on the other side of the border, according to a recent survey by Mexican researchers.

Recently, however, that pattern has been changing, Richardson and others contended.

"The number of aliens is increasing, but more important is the increase we are seeing in the percentage heading for the interior, for the North and Midwest," Richardson said.

Border Patrol agents here now catch about 4,000 undocumented Mexicans monthly on outbound trains and another 1,000 boarding commercial flights at the El Paso airport, he reported. Those who make it safely beyond the border zone, Richardson said, "are home free."

Alan C. Nelson, commissioner of the U.S. Immigration and Naturalization Service, said in a Senate Judiciary Committee hearing April 25 that detentions of undocumented aliens along the Mexican border this year have jumped by 43 percent above 1985's figures. About 1.8 million illegals will be apprehended on the border in 1986, up from 1.2 million last year, he predicted.

Part of this "great surge" in detentions, Nelson has said, is due to the recent one-third increase in Border Patrol agents along the Mexican frontier. But Nelson attributes most of the increase to Mexico's worsening economic crisis.

Many academic experts challenged Nelson's assertion that a 43 percent jump in Border Patrol apprehensions is evidence of an "alarming" increase of similar proportion in Mexican emigration. Emigration trends in Mexico "have been remarkably stable, with changes occurring gradually over the long term," said Francisco Alba, a Mexican demographer.

"This is crass political manipulation" of Border Patrol statistics, charged Manuel Garcia y Griego, a U.S. scholar from the University of California at Los Angeles who conducts research on immigration at Mexico City's Colegio de Mexico. "If you have had a one-third increase in the number of Border Patrol agents, you know beforehand that the number of apprehensions is likely to go up by a third or more."

"I'm skeptical of our numbers also," said Richardson, citing critics who "say that sometimes we are counting the same guy 17 times. And sometimes maybe we are." But Richardson said he has is convinced that Mexico's economic troubles are producing a new class of emigrant with more education and skills than has been seen in the past, people who he said are better equipped to survive in the U.S. job market.

Some Mexican analysts agree. This year another million youths will enter the job market at a time when nearly half of Mexico's 24 million-strong labor force is unemployed or underemployed. With plunging oil prices deepening a recession that is already four years old, the Mexican economy is expected to contract by about 4 percent in 1986, further increasing the jobless rate and pulling real per capita income below the levels of 15 years ago. For the first time, analysts say, Mexico is accumulating a large pool of unemployed skilled labor.

"It is a logical assumption that the economic crisis has accelerated the exodus of Mexican workers to the United States, though not to the extent that some have suggested," Alba said.

"The profile of the typical undocumented immigrant has changed," said Guillermina Valdez, a U.S.-trained social scientist who heads the Juarez branch of Mexico's state-supported Center for Northern Border Studies. Only about 15 percent of Mexican immigrants today are employed as field workers, as compared to more than 50 percent 15 years ago, it is estimated.

But while the economic crisis may be accentuating these trends, the shift in Mexican immigration patterns first took hold during the 1976-1981 oil boom years, the period of the fastest economic expansion in Mexico's history, many researchers emphasized.

Other changes, however, such as the flight of trained professionals, are more directly attributable to Mexico's economic troubles, it is believed. Under the U.S. immigration quota system, Mexico has 2,000 slots annually for professionals whose skills are needed in the United States. As recently as three years ago this quota was not filled; today, U.S. consular officials report, there is a backlog of applicants.

A much larger number of professional and middle-class Mexican immigrants are entering the United States with tourist visas or border resident passes, then remaining illegally, many experts say. "They just leapfrog across the border and disappear," Richardson said.