Marine mammals and birds as top predators consume a significant part of the primary production of the oceans. Harvesting organic material from the water column and breathing at the surface they constitute a carbon pump, transferring CO2 from the ocean into the atmosphere. However, by redistributing nutrients into the upper water layers they can also enhance the productivity, leading to an increased carbon absorption and sedimentation. Both effects are species dependent and calculating quantitative assertions requires the knowledge of the spatio-temporal distribution of the animals. But for many marine mammal species the uncertainty in population size still reaches the order of a magnitude. Traditional ways of counting animals visually during ship transects often suffer from low encounter rates, and especially the ice covered areas of the Southern Ocean are rarely surveyed during polar winter. Typically, on a transect across the Weddell Sea at most two or three blue whales are sighted, making it very hard to determine population trends. On the other hand, a hydrophone in the water picks up blue whale sounds almost continuously throughout the year. While visibility is limited to surfacing animals a few kilometers around the ship, their sounds travel several hundred kilometers through the water thus increasing the area survey-able by a hydrophone more than thousand fold, providing much better statistics. Moreover, as acoustic recorders can be deployed for several years in oceanographic moorings, polar winter conditions and heavy ice cover do not hinder permanent data acquisition. A network of about 20 recorders currently deployed by the Alfred Wegener Institute in the Weddell Sea and the acoustic observatory PALAOA at the ice shelf continuously record the acoustic environment. Advanced spectrogram visualisation tools allow graphing these multi-year long acoustic recordings into easily readable images and reveal detailed distribution and migration data for several species. So far, blue, fin, humpback, sperm, killer and possibly minke whales as well as Weddell, Ross, crabeater, and leopard seals have been detected and are easily distinguishable through their species specific vocalisations.