Hours before the release began, a store manager named Wang Jinglong in one of the Chinese capital's biggest seafood markets told AFP there had already been a "major impact" on his business, especially tuna sales.

"We used to get some fresh Japanese fish, but due to customs bans we stopped receiving them two months ago," Wang said, referring to import controls imposed last month.

Wang showed AFP frozen Japanese seafood products that he will be unable to restock once sold -- if customers are still interested.

"There's a large gap in our sales volume compared with before. In the past, such as during the pandemic, we had to kill three to five tuna every week," Wang said.

"Now we kill very few fish and they are not from Japan, but from Australia, New Zealand and Spain."

The 53-year-old said the quality of those products is "very poor, and not comparable to that of Japan".

He said he has little choice in the face of "great resistance" from the public to Japanese products.

"This pollution topic is being closely followed."

The release plan has been endorsed by the International Atomic Energy Agency -- the United Nations' nuclear watchdog -- which said it meets international standards and "will not cause any harm to the environment".

The overriding consensus among international experts is that the operation is safe.

But shortly after the discharge of wastewater began on Thursday, China said it would suspend the import of all Japanese aquatic products.

Some Beijing shoppers responded to the release by rushing to supermarkets and buying table salt in large quantities, Chinese media reported.

Chinese shoppers snapped up large amounts of salt in the wake of the 2011 Fukushima disaster, based on groundless rumours that the iodine in it could prevent radiation poisoning.

State-backed Beijing News said "certain stores had seen a rush on table salt after opening" on Thursday morning, prompting the capital's commerce bureau to say that "salt reserves are abundant, market supplies are secure, and citizens have no need to hoard salt".

Elsewhere in the Beijing market, workers said the impact of the water release plan had been significant.

Many recently stopped selling all seafood from Japan.

"The plan to release the water is causing trouble for Japan and all other countries," said Huang Xiaohao, the boss of a store advertising imported products.

"If you look around at what we're selling, you'll find that most of these things are actually domestic products," he said.

Pressure has come from both official customs restrictions, others said, as well as from consumers who worry about the impact of Japanese seafood products on their health.

One merchant who declined to be named told AFP that tuna from places other than Japan -- where he usually sourced products -- are simply not as good.