Japan's Fisheries Agency says the levels of tritium in fish caught off the damaged Fukushima Daiichi nuclear power plant were too low to be detected, after the plant's operator began releasing treated and diluted water from the plant into the ocean.

The agency has been conducting regular monitoring of radioactive materials in fishery products from Fukushima and neighboring prefectures since the accident at the plant.

Officials plan to conduct sample testing of tritium levels in fish caught within waters 10 kilometers from the nuclear plant daily for about one month, following the start of the water discharge on Thursday. They will publish the results one or two days later.

The Fisheries Agency on Saturday published its first analysis results, after examining an olive flounder and a gurnard caught the previous day.

The results show that the tritium levels in the fish were "not detectable," as they were less than around 10 becquerels per kilogram -- the minimum level the government says is detectable in the testing.

The agency also plans to analyze fish caught in a wider area in an effort to prove the safety of Japan's marine products.

The Fukushima Daiichi plant suffered a triple meltdown in the 2011 earthquake and tsunami. Since then, water used to cool molten fuel at the plant is mixing with rain and groundwater, seeping into the damaged reactor buildings.

The water is being treated to remove most radioactive substances, but still contains tritium. Before the release, the operator is diluting the treated water to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water.