Risk of marine mammal die-offs in the Southern Ocean

Abstract

Compared to the vast amount of data on the northern polar and sub-polar marine mammals, there is relatively little information about the species in the Southern Ocean. These waters have been estimated to contain about 50% of the world's seal population and 80% of the world's biomass of pinnipeds (Laws 1984). Four species of seals - Weddell Leptonychotes weddellii, Ross Ommatophoca rossii, crabeater Lobodon carcinophagus, and leopard Hydrurga leptonyx - inhabit the pack-ice. In addition, the southern elephant seal Mirounga leonina and Antarctic fur seal Arctocephalus gazella, which breed farther to the north, forage southward into the marginal ice zone (Costa and Crocker 1996). Although these species are well adapted to the harsh polar environment, reproductive success depends on predictability of both ice conditions and food resources. Antarctic waters also are critical summer feeding grounds for many species of cetaceans, including six species of baleen whales and several species of odontocetes (Brown and Lockyer 1984; Costa and Crocker 1996; Moore et al. 1999). Marine mammal die-offs appear to have increased in frequency in the past two decades (Geraci et al. 1999). With a few notable exceptions, such as mortalities due to oil spills, these events have been associated with outbreaks of infectious disease, harmful algal blooms, or oceanographic anomalies or unusual weather conditions that result in large-scale starvation or trauma. We can assume that these same factors are most likely to underlie future events in the Southern Ocean. However, we must be cautious when using information gained from conditions or events in the northern hemisphere, where contiguous coastlines may promote unusual migrations of marine animals as well as passive transport of algae and other organisms from one area to another, and where input of nutrients, contaminants, and pathogens from terrestrial sources is significant.