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During a four-'day period from 28 April to 1 May 1985, we observed bowhead whales breaking up through sea ice in order to breathe. Our observations were made from grounded sea ice approximately 10 km northeast of Point Barrow, Alaska, during the spring bowhead migration (14 April to 10 June). From acoustic and visual data, it was estimated that 665 whales passed the observation perches during this four-day period. However, only 117 (17%) whales were seen. The remaining whales either passed underneath the ice or were beyond the range of the visual observers. Whales used their heads, in the area of the blowholes, to push up against the ice (18 cm maximum thickness) and fracture it, creating a hummock of ice in which they were able to respire. Often during such breathing episodes, even at distances of only several hundred meters, the animal was not seen but its blows were clearly audible to the visual observers. Acoustic tracking of whales showed they avoided a large multi-year ice floe seaward of the observation perch. We hypothesize that bowheads use their calls to assess the thickness of ice in their migratory path. In assessing their calls, we suggest the whales can avoid areas where the ice is too thick to break through (to breathe) and/or too thick to provide clearance for them to swim beneath.

arctic

Observations on the Ice-Breaking and Ice Navigation Behavior of Migrating Bowhead Whales (&lt;i&gt;Balaena Mysticetus&lt;/i&gt;) near Point Barrow, Alaska, Spring 1985