But pelagic fish and the Antarctic Icefish Champsocephalus gunnari were among the first types to be harvested commercially after the decline of whales. Some species of Antarctic fin fish have already been drastically over-exploited. As early as 1970-71 peak catches of over half a million tons were taken around South Georgia Island, but today this fishery is no longer commercially viable as the stock of this species has been reduced to less than 12.5 percent of its original amount. Ice Ages Ice ages are those periods during the earth's history when large portions of the earth were covered by icecaps. There have been some ten ice ages during the last one million years, occuring at intervals of about 100,000 years.
Geological records suggest that the ice ages developed slowly but disappeared relatively quickly.
Their origins have long puzzled scientists. It is now believed that a cycle of three main irregularities, in the Earth's orbit are the main factors initiating an ice age. The first irregularity is the tilt of the earth's axis from 22 to 25 which occurs in a cycle that repeats itself every 41,000 years. The second irregularity is the orbital procession which completes its cycle every 21,000 years. This determines the season at which the earth is closest to the sun during its orbit.