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Ice Leads


McMurdo Sound is covered by sea ice for 10-11 months of the year. Fast ice is sea ice which forms and fastens to the coastline; the boundary between fast ice and open ocean is the fast ice edge. The fast ice edge is dynamic; sea ice cracks form and enlarge while ice floes (large relatively flat pieces of sea ice) break off and float away, joining the pack ice further offshore. The long cracks that form in the fast ice are called leads.

Mt. Erebus on Ross Island is in the background, with Mount Bird and Cape Bird on the left of Mt. Erebus.


A hunting pod of killer whales Orcinus orca travels down an opening lead or crack of ice.

Killer whales will travel up these sea ice cracks a considerable distance from the open ocean, hunting penguins and diving deep under the ice to hunt Antarctic cod.


A mother and calf are seen in an opening lead of ice.

By mid-summer in McMurdo Sound, the plankton bloom reduces underwater visibility dramatically. Killer whale calves nurse for twelve months and may remain with the mother for as long as ten years [1].


A pod of killer whales reaches the end of an opening lead in the ice of McMurdo Sound and rests, spyhopping to take a look around.

1: FAO Species Identification Sheets for Fishery Purposes : Southern Ocean (Fishing Areas 48, 58 and 88) (CCAMLR Convention Area). W Fischer & JC Hureau, eds. Rome : Food and Agriculture Organization of the United Nations, 1985

Text ©Peter Brueggeman. Photographs ©Norbert Wu. Photographs may not be used in any form without the express written permission of Norbert Wu. Norbert Wu no longer grants permission for uncompensated use of his photos under any circumstances whatsoever; want more info?