The number of immigrants illegally crossing into the United States from Mexico has declined according to a new study, and some California farmers are already seeing the effects on their crops.

"We've seen in the valley this year a reduction of labor that we haven't seen for five or six years," said Manuel Cunha, president of the Nisei Farmers League in Fresno, Calif.

"There could be crops that could go down as being damaged because they cannot be harvested fast enough. Many of the vegetables growing in the valley are going to be in competition for the labor as well," he said. "You're going to see rotting."

Net immigration from Mexico has fallen sharply, and may even be down to zero, according to a recent study by Princeton University demographer Douglas S. Massey. Massey credits several factors for the shift: smaller family sizes and more job opportunities in Mexico, rising border crime, and immigration crackdowns in the United States.

Experts say the deciding factor in the decision to migrate north is often an economic one - and with unemployment remaining high in the United States and Mexico's economy booming, many Mexicans are choosing to stay in their home country.

"We've heard that many people from Tijuana or Juarez at different times have migrated out of fear for their personal security or that of their families' to San Diego or El Paso, but those aren't typical migrants," said Robert Donnelly, program associate at the Mexico Institute in Washington, D.C., a research center that studies U.S.-Mexico relations. "I would think that in most parts of the country the leading factor is still economic."

The evidence of slowing illegal migration can be seen along the border: Border apprehensions of undocumented Mexicans have fallen more than 70 percent since 2000. Meanwhile, an increase in legal migration of Mexicans has brought in 64 percent more immediate relatives from 2006-2010 than during the previous five years.

On some farms across California, which produces half of the nation's fruits and vegetables, the drop in undocumented immigrants can already be felt.

"We have heard from some of our members that the labor market is tightening up," said Wendy Fink-Weber, spokesperson for Western Growers in Irvine, Calif., an agricultural trade association whose members grow, pack and ship the majority of the fruits, nuts and vegetables in California and Arizona.

Don Villarejo, founder of the California Institute for Rural Studies, agrees that California farms have seen a labor shortage this year. "We are seeing the indication of the movement of people away from where they feel they might be at risk," he said.

"This year, starting in January and February, there was a shortage of people planting vegetables in the Salinas Valley," explained Cunha, noting that the labor shortage "moved to the San Joaquin Valley as well."

"We've seen a reduction in crew where there are usually 20 to 25 people in a crew. Now they are down to 14 to 15, or 17 to18."

But not everyone has seen a drop in labor.

Maria Machuca, communications director of United Farm Workers, estimates that there are "approximately 450,000 farm workers that travel from farm to farm in California." She said that according to the Department of Labor, "at least 50 percent of them are undocumented." The UFW however estimates that it's even higher, as much as 80 percent.

"There are actually a lot of farm workers looking for jobs during the season (between May and September), so we haven't heard that people have jobs open," she said.

Some of this may be due to a decrease in farm jobs as more machines replace workers, noted Villarejo. "Owing to mechanization, shifts in cropping patterns," he said, "there has actually been a roughly 15 percent decline for hired farm workers in the past 15 years."

But mechanical practices can't fix the labor shortage, noted Cunha. "Vines cannot be picked mechanically."

Dave Kranz, spokesperson for the California Farm Bureau Federation, a private non-profit organization in Sacramento and the largest farm organization in California, said that he wasn't aware of a significant labor shortage this year, though it may be too soon to tell. Farmers would notice this during the peak season of the harvest in August and September, he said.

But he added, "hiring people who can come and go legally" has been a "long-term concern" for the agricultural industry.

"It's a hard thing to get farmers to talk about," added Fink-Weber, "even though we admit that 70 percent of our workforce as a whole is improperly documented."

"Here in California," she added, "we don't have state laws like Arizona or Georgia," that have each passed laws cracking down on undocumented immigration.

With anti-immigration bills being signed into law in various states, policies toward immigrants could have a direct impact on labor, according to Angelica Salas, executive director of the Coalition for Humane Immigrant Rights of Los Angeles (CHIRLA).

"Workers are making decisions on what states to move to based on the welcoming nature of that state," Salas said.

Georgia's onion-growers, for example, worry their state's new anti-immigration law could lead to a labor shortage in their fields. The Georgia Fruit and Vegetable Growers Association estimates that the losses stemming from Georgia's new law, HB 87, will total at least $250 million this year alone, according to the blog, Farmworkers Forum.

Fink-Weber of Western Growers added that she is hearing from her agricultural members that, "If they are going to expand, they aren't doing it in the U.S. - but in Mexico or other countries."