Radiation in all seafood caught off Fukushima Prefecture tested below the detectable level in November for the first time since the 2011 nuclear disaster.

Species including bass, rockfish and stone flounder–sales of which were banned by the central government–were tested between Nov. 11 and Nov. 28, and the prefectural government said they all fell below the detection threshold, meaning radioactive cesium was not detected in any samples.

The main reason is that most fish species have undergone a generation change over the past five years with the contaminated marine life dying out, said officials at the prefectural government’s fisheries experimental station.

In addition, the passage of time helped fish exude radioactive cesium from their bodies.

The prefectural government began the tests in April 2011 following the disaster at the Fukushima No. 1 nuclear plant the previous month.

Forty thousand fish and shellfish samples have been checked from 186 species over the past five and a half years.

The initial tests found that more than 90 percent of the samples were contaminated with radioactive cesium above the central government’s safety limit of 100 becquerels per kilogram.

The percentage of polluted fish and shellfish then declined annually.

The tests since April last year showed that the pollution in all samples was within the safety limit.

The monitoring covers seafood caught in 30 locations, in waters with a depth of 5 meters and at a distance of hundreds of meters from the shore, including the area in a 20-kilometer radius of the crippled plant.