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The repartition of tracers in the ocean's upper layer on the scale of a few tens of kilometres is largely determined by the horizontal transport induced by surface currents. Here we consider surface currents detected from satellite altimetry (Jason and Envisat missions) and we study how surface waters may be trapped by mesoscale eddies through a semi-Lagrangian diagnostic which combines the Lyapunov approach with Eulerian techniques. Such a diagnostic identifies the regions of the ocean's upper layer with different retention times that appear to influence the behaviour of a tagged marine predator (an elephant seal) along a foraging trip. The comparison between predator trajectory and eddy retention time suggests that water trapping by mesoscale eddies, derived from satellite altimetry, may be an important factor for monitoring hotspots of trophic interactions in the open ocean.