Bowhead whales feed on plankton concentrated by estuarine and tidal currents in Academy Bay, Sea of Okhotsk

Academy Bay in the Sea of Okhotsk is an important summertime feeding ground for pelagic-feeding Bowhead whales (Balaena mysticetus) in the western subarctic North Pacific. The present work combines satellite observations with physical (CTD, currents, tides) and biological (zooplankton sampling) measurements. Data obtained aboard the RV Lugovoe in August–September 2003 and July–August 2004 show dense populations of zooplankton (such as copepods Calanus glacialis, Pseudocalanus sp., pteropods Limacina helicina, and chaetognaths Sagitta sp.) that are concentrated by physical mechanisms within this critically important ecosystem. We show that near-bottom, cold water intrusions from the northern Sea of Okhotsk associated with the estuarine circulation advect arctic herbivorous calanoids (Calanus glacialis) and mollusks Limacina helicina into the region.Satellite and in situ observations that include conductivity–temperature–depth (CTD) data and measurements of tidal currents are used to study the nature of the estuarine circulation. Results show an anticyclonic circulation, associated with the effect of earth's rotation. An analytical model shows horizontally convergent flows in the lower layer toward the western coast of the bay that push water upward along the steep slope. This together with zooplankton vertical migration leads to aggregation. Thus the combination of the residual eddy formation and swimming behavior act to concentrate zooplankton near the cape and to provide the necessary densities of planktonic food supply for Bowhead whales.