After an unexpected surge early this year that alarmed refugee officials, the number of Vietnamese boat people arriving at other shores in the region has dropped off sharply in the second half of the year compared to the same period in 1980.

Various explanations exist for this development, not least the vagaries of the monsoon. But refugee officials here say a major factor is a crackdown on illegal departures because the exodus is severely damaging the country's economically vital fishing fleet.

Despite the crackdown, however, it is clear that Vietnamese are continuing to try to flee their homeland and that the rate of departures remains essentially unpredictable. It also appears that there is little other countries can do to discourage the Vietnamese from leaving. They keep coming out despite increasingly vicious pirate attacks and various aspects of "humane deterrence."

According to U.S. statistics, less than 17,000 Vietnamese boat people arrived in the region from July through October 1981 compared to a little more than 24,000 during the same period last year.

"I attribute this to a shortage of boats and enforcement of Vietnam's policy on illegal emigration," a U.S. refugee official said. He said there was no evidence that a U.S. decision this year to reduce the intake of Vietnamese refugees from 160,000 to 100,000 has deterred boat people from leaving their country.

The official said that aside from concern about the fishing fleet, he believed another Vietnamese motive in cracking down on illegal departures was that they continued to reflect badly on Hanoi's international image.

At the same time, officials said, the Vietnamese have allowed a sharp increase in legal emigration under the U.N.-sponsored Orderly Departure Program. The number of these departures started going up in September and reached a high of 1,723 in October -- more than double the monthly rate earlier in the year -- before dropping in November because of technical problems in arranging aircraft, a U.N. official said.

Some officials speculate that the rise in orderly departures might be linked to the concerns about keeping boats in Vietnam.

According to Western diplomats, Vietnam has been hurt not only by losses of boats, but by the disappearance of engines, fuel and skilled people.

Because of the tightened restrictions against boat-people departures, the Vietnamese have not had the freedom to fish that they had before, diplomats said. In addition, they said, Vietnamese authorities have been pressing ahead with efforts to "collectivize" fishing by requiring fishermen to turn over their catches to the government.

"All this has hurt the fishing industry," one diplomat said. He noted that fish constitutes "a principal source of protein" for the Vietnamese. Combined with shortages of other foods, notably rice, the decline of the fishing industry thus threatens more widespread malnutrition in Vietnam, diplomats said.

Western diplomats and Thai officials also report an influx of Vietnamese fishermen into neighboring Cambodia, which is occupied by as many as 200,000 Vietnamese troops.

Vietnamese fishermen have been going into Cambodia for at least a year and have established fishing settlements on the shores of the Tonle Sap Lake and on both banks of the Mekong and Tonle Sap rivers, according to refugee reports quoted by the officials.

According to one report, the Cambodian deputy director of the Fisheries Department in the Phnom Penh government's Ministry of Agriculture said earlier this year that one reason she defected and fled to Thailand was that the Vietnamese were taking over the Cambodian fishing industry. The former official asserted that fishing equipment donated by international relief organizations was reaching these Vietnamese settlers.

Besides decimating its sea-going fishing fleet, the boat-people exodus has hit the Hanoi government with a brain drain. According to U.S. surveys, Vietnam has lost a lot of Viet Cong cadre as well as doctors and members of the middle class.

In one recent random sample of arriving boat people, most were middle-class city dwellers, and 30 percent had either worked for the U.S. government in Vietnam or had been closely associated with its policies and programs. Only 7 percent were farmers and fishermen.

Unlike in 1979 when most of those fleeing Vietnam were ethnic Chinese, this group accounts for roughly 30 to 40 percent of the boat people, officials said.

The boat people now also include a high proportion of unaccompanied young people. They are sent out by their families in the hope they will reach the United States and serve as "anchors" for relatives departing in the future. One reason for this practice is the high cost of an illegal departure, ranging from one to three taels of gold per person. A tael is slightly more than an ounce.

Earlier this year some refugee officials were alarmed by a sudden spurt in arrivals of boat people in the region, especially during the spring when the number increased 25 percent from last year. For the first half of the year, a total of 49,239 boat people arrived in various Asian countries compared to 40,587 in the first half of 1980, according to U.N. figures. Although the arrivals were far fewer than during the exodus of 1979, some officials saw a worrisome trend.

Nevertheless, overall boat-people arrivals in the region declined during the second half of the year, with Thailand showing an especially sharp drop.

The arrivals in Thailand during the first half of the year averaged 2,570 a month compared to 2,360 a month during the same period last year. But in the second half of this year the average has plunged to about 400 a month through the end of November, compared to 1,023 a month for the same period in 1980.

Refugee officials attribute this change primarily to fear of Thai pirates, whose attacks on Vietnamese refugees have been growing more frequent and brutal. A second reason, they said, is a tougher Thai government policy designed to discourage Vietnamese refugees.

Thailand announced several months ago that after Aug. 15, arriving Vietnamese boat people would no longer be eligible for resettlement but would be placed indefinitely in austere camps.

These factors have not staunched the flow of boat people, but have made many of them try to avoid Thailand. In fact, according to refugee officials, a number of Vietnamese arriving on Thai shores have asked to be towed out to sea to make the short jump from Thai to Malaysian territory.

The Thai government evidently encourages this. It has instructed Thai district authorities to provide fuel, food and water and to repair Vietnamese boats -- up to a cost of about $300 per boat -- if the Vietnamese want to continue their "onward journey," refugee officials said.

Partly as a result, only 235 boat people arrived in Thai camps in October, compared to 2,384 in Malaysia, according to U.N. figures.

"The implication is that the Thais are dumping the problem on Malaysia," one refugee official said.

Another said the Malaysians were watching the situation closely but had not yet complained about it.

"It is a problem," a Malaysian official here said of the high number of Vietnamese arrivals in his country. But he said he was "not sure" that the Thai policy was responsible. He added that if the numbers increased, this would "almost certainly lead us to adopt a tougher policy" toward the boat people.