The hole in the ozone layer may be responsible for a dramatic decline in krill numbers in the Antarctic Ocean, according to U.S. and German scientists working on a Japanese Fisheries Agency ship. The krill population off the Antarctic Peninsula south of Tierra del Fuego has dropped by about 75 percent since the mid-1980s, according to the researchers. They say ozone-layer damage has increased the ultraviolet rays reaching the earth’s surface, and as a result plant plankton on which the krill feed is being killed off. Krill, shrimp-like creatures of up to 6 centimeters in length, are the staple food of many whales and penguins.