**1. Introduction**

The Antarctic ecosystem is very dynamic with changes in species composition and habitat occurring through time. Within this ecosystem, Antarctic krill (*Euphausia superba*) is a key prey species, supporting different species of baleen whales, pinnipeds, birds and fish. Changes in the ecosystem can result from human interventions or from natural causes. One example of human intervention is the large-scale harvesting of whales in the first half of the 20th Century, which has been discussed by several authors [1, 2]. This harvesting started in the Antarctic Ocean in 1904. Several species of krill-eating large whales, such as the Antarctic blue (*Balaenoptera musculus intermedia*) and humpback (*Megaptera novaeangliae*) whales were heavily reduced in number by commercial whaling during the first half of the past century. Other species such as the fin whales (*B. physalus*) were reduced during the second half. Over more recent decades, the populations of some large whales have started to recover [3]. Changes in the biomass of whale species also seem to have had strong effects on the demography of other krill-eating predators in the Antarctic ecosystem [1, 2].

An example of the effects of natural causes is the increases in the chinstrap penguins (*Pygoscelis antarctica*) populations of the Scotia and Weddell Seas over the last four decades (1950's-1990's), which has been attributed to a gradual decrease in the frequency of cold years with extensive winter sea ice cover resulting from environmental warming [4]. However, more recent analyses in the Antarctic Peninsula and Scotia Sea conclude that the chinstrap penguin instead may be among the most vulnerable species affected by a warming climate [3].

In studying the changes in the Antarctic ecosystem, there needs to be differentiation between West and East Antarctic, as well between land-based and sea-based krill predators. The West Antarctic Peninsula represents one of the regions of the planet where the fastest warming has been observed in the last 50 years [5]. For this reason the studies documenting ecosystem changes in the West Antarctic have considered environmental variables in addition to demographic information on land-based krill predators (mainly penguin species) [3, 4], on which environmental factors could have a larger impact. Warming has not been reported for the East Antarctic, so that environmental factors would not be expected to play the predominant role in the ecosystem changes in this part of the Antarctic.

Here historical and current ecosystem changes in the Indo-Pacific sector of the Antarctic (involving mostly East Antarctic) are documented through the examination of biological and demographic parameters of sea-based predators (whales). These changes in parameter values are interpreted in the context of some established hypotheses.
