BANGKOK (Reuters) - The number of migrants leaving Myanmar and Bangladesh by boat in the past four months has plummeted because of Thai and Bangladeshi crackdowns on human smugglers, the United Nations and a rights activist said on Friday.

Migrants, who were found at sea on a boat, collect rainwater during a heavy rain fall at a temporary refugee camp near Kanyin Chaung jetty, outside Maungdaw township, northern Rakhine state, Myanmar June 4, 2015. REUTERS/Soe Zeya Tun

Thai police launched a sweeping campaign against smuggling gangs in May following the discovery of 30 bodies in graves near a human-trafficking camp close to the Malaysian border.

The police operation led to traffickers abandoning 4,000 migrants from Myanmar and Bangladesh at sea, sparking a chaotic round of “maritime ping-pong” as Indonesian, Malaysian and Thai navies pushed migrant boats away from their shores.

Tens of thousands of Rohingya Muslims have fled poverty and persecution in western Myanmar since religious violence erupted there in 2012.

Most have headed for Muslim-majority Malaysia, but many have made landfall first in southern Thailand or been intercepted and held for ransom in camps hidden deep in Thailand’s jungles.

The region is now in its “sailing season”, with calmer seas after the rains, the busiest time for smuggling and trafficking ships plying the Bay of Bengal.

But Chris Lewa from the Arakan Project, a Rohingya advocacy group which tracks migration, said this year the number of people sailing was much lower due to action against smugglers in Thailand and Bangladesh.

About 1,500 people sailed from Bangladesh and Myanmar between September and December, said Lewa, compared with 32,000 people tracked during the same period in 2014.

“Thailand is closed and cannot be used for disembarkation,” Lewa told Reuters. “The few brokers that still seem to be involved are the ones that have pre-existing ‘orders’ for people to be brought over, that’s what we are being told at departure.”

“Anti-trafficking operations are going on in Bangladesh as well.”

Vivian Tan, regional spokeswoman for the U.N. refugee agency, said preliminary data suggested fewer people took to boats in the last quarter of the year.

“Based on our interviews with affected communities, it could be that smugglers and potential passengers are taking a wait-and-see approach following last year’s scrutiny and crackdowns,” said Tan.

A major human-trafficking trial in Thailand involving 91 defendants allegedly involved with smuggling gangs that were trafficking Rohingya Muslims and Bangladeshis, is underway.

Activists have called on authorities to step up witness protection in the trial, which involves an army general and police investigators.

Hundreds of people who made the trip last year remain in immigration detention centers and shelters in southern Thailand and 11 Muslim Rohingya escaped from such a shelter on Friday, the head of the shelter told Reuters.