By Kevin Buckland

TOKYO (Reuters) - Japan's fisheries agency said on Saturday fish tested in waters around the wrecked Fukushima nuclear plant did not contain detectable levels of the radioactive isotope tritium, Kyodo news service reported.

Nets were set up on Thursday when plant operator Tokyo Electric Power Company (Tepco) began releasing treated radioactive water into the Pacific, angering fishermen and many others in Japan, alarming consumers in neighbouring countries and prompting China to ban Japanese aquatic products.

The agency plans to announce test results daily. Tepco said on Friday seawater near the plant contained less than 10 becquerels of tritium per litre, below its self-imposed limit of 700 becquerels and far below the World Health Organization's limit of 10,000 becquerels for drinking water.

Calls to the fisheries agency for comment were not answered on Saturday.

After lengthy debate, the government of Prime Minister Fumio Kishida decided on Tuesday to allow the release of 1.3 million tons of treated water from the Fukushima plant, destroyed by a 2011 tsunami, because Tepco was running out of storage space.

The utility filters most radioactive elements out of the water, but it dilutes tritium, an isotope of hydrogen, which is difficult to separate from water.

(Reporting by Kevin Buckland; Editing by William Mallard)