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We present a method for identifying eddies in the ocean with a risk of contamination by Fukushima-derived radionuclides based on solving advection equations backward in time for tracers in the AVISO altimetric velocity field.The method consists of computing Lagrangian synoptic maps for different Lagrangian indicators in a broad area in the North Pacific ocean which enable to identify mesoscale eddies.Each eddy is crossed by a material line with a large number of synthetic tracers which evolved backward in time in a daily altimetric velocity field starting from a given date to the day of the Fukushima accident on 11 March 2011.Then we calculate how many times the tracers visited an area near the Fukushima Nuclear Power Plant for the month after the accident and compute a tracking map for each eddy that demonstratesby a density plot the probability for water parcels of that eddy to be contaminated.Using those recommendations,a track of the research vessel(R/V)``Professor Gagarinsky' cruise has been planned and conducted from 13 June to 10 July 2012 in such a way to cross some mesoscale eddies of the subarctic front.Measurement of 134 Cs and 137 Cs has been carried out in seawater samples collected in a broad area in the Sea of Japan,the Okhotsk Sea and the western North Pacific to the east off Japan coast.Increased caesium concentrations at the surface have been detected at sampling stations located in the centers of those mesoscale anticyclonic eddies which have been shown numerically to trap Fukushima-derived radionuclides with a higher probability.The highest concentrations of 134 Cs and 137 Cs have been found to exceed the background level in the ocean before the accident in ten times.The results of observations are consistent with the simulated results.It is shown that tracers with increased radioactivity really have visited just after the accident the areas with presumably higher level of contamination.