SYDNEY, Australia — On an August morning aboard the Nathaniel B. Palmer research vessel floating at the bottom of the world, Christian Reiss was listening for acoustic signals bouncing off krill, a pinkish, feathery-limbed crustacean that is the lifeblood of the Antarctic ecosystem.
It was the last month of the Southern Hemisphere winter, and conditions were good: There was no thud from sea ice pancakes bumping together to distort his tests in the clear waters of the South Shetland Islands, about 500 miles south of Cape Horn.
Dr. Reiss, an oceanographer with the National Oceanic and Atmospheric Administration, and his team were studying where krill live in winter.
