Whales in the Antarctic ecosystem

Abstract

The Antarctic whale fishery started at the beginning of this century and caused the successive depletion of the humpback, blue, fin, and sei whale stocks. These whales all migrate into the Antarctic to feed during the summer months on krill. Sperm whales have been a secondary target, and current whaling activity is dependent on the minke whale. Direct biological evidence, supported by official catch data, suggests that there has been an approximate doubling in the pregnancy rates of blue and fin whales, dating from the time of the major depletions of these populations in the 1930s. Sei whales show a similar change from 1945, which predates the main reduction in the numbers of this species in the 1960s. Evidence from earplug age readings of fin whales, supported by direct analysis of catch samples, suggests that the age at sexual maturity has also declined, from around 10-6 years, since the 1930s. Smaller changes are indicated in sei and minke whales, again before their direct exploitation. These changes imply that the whales are now growing faster than in earlier years, in addition to reproducing more frequently. This has been taken as evidence of a response by the reduced stocks having relatively more food. However, there is no evidence of increased recuitment, and doubts have been raised about the validity of the trends. Estimates have been made of the krill no longer consumed because of the reduced whale stocks, perhaps 147 × 10 6 tonnes. But other members of the Antarctic ecosystem, such as seals and birds, may be increasing as a result, and so it is unclear if the whales can recover even under full protection.