Cetaceans as indicators of historical and current changes in the East Antarctica ecosystem

Changes in the Antarctic ecosystem have been triggered by anthropogenic and natural factors. This paper reviews the scientific information of whales that could be indicative of changes in the East Antarctica ecosystem in the context of two hypotheses, the ‘krill surplus’ hypothesis in the middle of the past century and the recovery of krill-eater large whales since the 1980’s. There was an interest to investigate the effects of those events on other krill predators such as the Antarctic minke whale, which had not been exploited on a large scale. A review of the scientific information in East Antarctica (70°E-170°W) showed that the increased krill availability in the middle of the past century could have been translated into better nutritional conditions for some krill predators like the Antarctic minke whale, resulting in a decreasing trend in the age at sexual maturity of this species between approximately 1940 and 1970. A low age at sexual maturity favored an increase in the recruitment rate and total population size in a similar period. The evidence available since the 1980’s showed a sharp increase in the abundance of some species in East Antarctica such as the humpback and fin whales. The evidences also showed that the nutritional conditions of Antarctic minke whales have deteriorated as revealed by a decrease in energy storage and stomach content weight since the 1980’s. This observation is consistent with the stable trend of age at sexual maturity and recruitment after 1970’s. The stable trend in recruitment is consistent with the total abundance of Antarctic minke whale estimated by sighting surveys, which has been broadly stable since the 1980’s. The observations above suggest availability of krill for Antarctic minke whales could have decreased in recent years. Decrease in availability of krill for this species could result from competition with other recovering krill-eater large whale species, e.g. the reversal of Law’s ‘krill surplus hypothesis’. Environmental factors alone are unlikely to explain the observed changes in demographic parameters in Antarctic minke whales. An implication of this is that in East Antarctica, competition for space and food could better explain the pattern of changes in biological and demographic parameters observed among sea-based krill predators. However to further investigate the plausibility of this hypothesis it will be necessary to obtain information on krill biomass trends in the research area. There is some partial information based on past dedicated krill surveys but the information is scattered and needs to be combined with new surveys in a comprehensive and consistent way so that a time series can be obtained.