We use a Lagrangian particles dispersal method to track where free floating material (fish larvae, algae, phytoplankton, zooplankton...) present in the sea water near the damaged Fukushima Daiichi nuclear power station plant could have gone since the earthquake on March 11th. THIS IS NOT A REPRESENTATION OF THE RADIOACTIVE PLUME CONCENTRATION. Since we do not know how much contaminated water and at what concentration was released into the ocean, it is impossible to estimate the extent and dilution of the plume. However, field monitoring by TEPCO and modelling by the Sirrocco group in University of Toulouse, France both show high concentration in the surrounding water (highest rate at 80 Bq/L and 24 Bq/L for respectively I-131 and C-137) . Assuming that a part of the passive biomass could have been contaminated in the area, we are trying to track where the radionuclides are spreading as it will eventually climb up the food chain.