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Blue whales and short-beaked common dolphins dominate the biomass of cetacean fauna in the most productive regions of the eastern tropical Pacific (ETP). We modeled their population densities in a hierarchical Bayesian framework as functions of the physical structure of the water column, based on the hypothesis that their year-round distributions are forced by thermocline shoaling and outcropping in areas of eddy-like circulation and wind-forced upwelling. Those features provide suitable conditions for the aggregation of low-trophic level prey (krill and small pelagic fish). We used cetacean line-transect data and thermocline depth measurements from CTD casts collected during 12 NOAA-SWFSC cruises in the ETP, spanning the period 1986-2006. The hierarchical Bayesian approach allowed us to propagate naturally the uncertainty associated to the detection of the animals and the effectively sampled areas into the final inference. Temporal trends of inferred abundances showed the forcing of inter-annual events within the main habitats for both species. Unlike previous studies, high densities were predicted in the eastern Pacific Warm Pool, particularly the Costa Rica Dome and the North Equatorial Counter Current.

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