Like other rorqual whale species, blue whales (Balaenoptera musculus) evolved to specialize in consuming large numbers of small-bodied prey through engulfment filter feeding.1,2 Blue whales in the eastern North Pacific feed almost exclusively on ephemeral swarms of krill.3,4,5 The densest and most energetically profitable krill aggregations are often associated with bathymetric features5,6,7,8 and circulation phenomena,8,9,10,11,12 but how the whales find patchy krill swarms in a vast dynamic habitat remains largely unknown. Integrating animal- and satellite-borne sensing in the eastern North Pacific, we show that blue whales can precisely localize feeding in a favorable foraging habitat by sensing horizontal thermal gradients near the ocean surface. While tracking a dynamic upwelling plume front over hundreds of kilometers, blue whales repeatedly: (1) responded to frontal encounters by sharply changing course and conducting deep exploratory dives; (2) systematically transected across and along the variable local orientation of frontal gradients-particularly during nighttime search periods that preceded intensive daytime feeding; and (3) localized temperature minima before moving toward the warm side of the front while feeding. Such persistent orientation of movement relative to thermal gradients during day and night, with or without the result of feeding, is consistent with thermal sensory perception and memory serving as the primary search modality. This case study, enabled by a rarely achievable integration of high-resolution sensing, motivates a broader examination of how highly mobile marine species track resources relative to the dynamic physical gradients that structure marine ecosystems.
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