Pakistan has detained five Chinese trawlers on suspicion of illegal fishing near its strategic port city of Gwadar, as a wave of protests swept across the region led by fishers concerned about losing control of their fishing grounds to China.

The Chinese trawlers, loaded with fish, were taken into custody by the Pakistan Maritime Security Agency (PMSA), which has begun an investigation into their suspicious presence in Gwadar, where China has built a deep-sea port as part of its global belt and road initiative.

Hundreds of fishers have rallied against the Chinese trawlers since last week, accusing them of fishing in Pakistani waters and sending the catch back to China.

A group consisting of local authorities and fishers visited one of the Chinese trawlers on Sunday to determine whether its fish had been caught in provincial waters. “The fishers believe the Chinese trawler had caught fish on the Gwadar coast,” said Khudadad Waju, the president of Fisherfolk Alliance Gwadar. “They are visiting other Chinese trawlers today to learn more. We demand that these fish should be auctioned in Gwadar and not be taken to China.”

Akbar Raess, 70, has been a fisher for five decades. “For centuries we have been fishing here. My grandfather was a fisherman and my children are fishermen, too. This sea provides us a living. The Chinese trawlers are here to end our livelihood. We will protest against them until the government stops granting them fishing rights,” he said.

The port of Gwadar is considered the gateway to the China Pakistan Economic Corridor (CPEC), a multibillion-dollar Chinese megaproject that invests in Pakistan as part of Beijing’s belt and road initiative of global infrastructure projects. In 2017 Pakistan surrendered control of Gwadar’s port to a Chinese state-run company, China Overseas Port Holding Company (COPHC), under a 40-year lease agreement.

“Before the beginning of the CPEC and Gwadar port [deal], we were told that it would change our lives by bringing development and employment for us,” Raees said. “But we are losing our livelihoods and the sea.”

Younas Anwar, a fisher and the general secretary at Fisherfolk Alliance Gwadar, said the development of the port had led to local fishers being barred from their traditional fishing grounds. “We are not allowed to fish there due to security reasons,” he said.

Waju said concerns were sparked last year when 20 Chinese deep-sea trawlers arrived in the port of Karachi to fish in Sindh and Balochistan provinces.

“We expressed our fears to the COPHC chairman and other Chinese authorities a year ago in Karachi that if the Chinese trawlers would be allowed inside Balochistan it would damage our livelihood, but we were reassured it wouldn’t happen,” said Waju.

Zhang Baozhong, the COPHC chairman, denied that the detained Chinese trawlers were fishing in Gwadar, claiming they were sheltering from a storm.

“The fishing boats at Gwadar anchorage [called] for safety from the monsoon of [the] Indian Ocean and have followed the International Maritime Organization (IMO) rules. There is no foreign, including Chinese, fishing company fishing now, and no fishing company will be allowed to fish in Pakistan water in the future,” Zhong told the Guardian.

The local fishers said that explanation was not credible. “Initially the government had remained silent on the matter, but later in a statement said that the trawlers claimed that they had been caught up in a storm and were taking refuge.”

Anwar said that when the prime minister, Imran Khan, visited Gwadar on 5 July, he assured locals that no Chinese trawlers had been awarded licenses to fish within the 12-nautical-mile limit. But other reports suggest about 100 Chinese trawlers have been granted licenses.

Tahir Bizinjo, a senator from Balochistan whose National party has been protesting against the Chinese trawlers, says the government is now denying it has issued licenses because the protests are causing political heat.

He told the senate that the presence of the Chinese trawlers in Gwadar shows the government has indeed granted them fishing rights.

“The government has to stop entertaining Chinese trawlers. The trawlers don’t only snatch the livelihood of the indigenous fishermen, but they also destroy sea ecology, environment and wipe out seafood,” Bizinjo told the senate.

According to documents seen by the Guardian, a senate standing committee following Bizinjo’s remarks confirmed that the Chinese ships did not seek permission to enter Pakistan’s waters and “did not respond to the mandatory very high frequency and louder calls. It needs to be verified whether there was any ‘bad weather’ condition as claimed by [the] Chinese fishing vessels.”