a Based on data from the Food and Agriculture Organization of the United Nations FAO FISH-STAT-PLUS ver. 2.3 (http://www.fao.org).
bExcept whales and seals whose numbers are reported rather than biomasses. c Includes brown, red, and green algae. dIncludes crabs and lobsters.
e Includes clams, mussels, scallops, oysters, abalone, and conch. f Includes krill and shrimp. g Includes octopus and squid.
h Marine species that migrate from the sea into freshwater and back. ' Includes corals, barnacles, echinoderms, sea squirts, sponges, and other species. j Number of baleen and toothed whales. k Number of seals and walruses.
l Total catch based on species' biomasses excluding whales and seals.
ral krill consumption by whales (Table 10.1)—as well as by seals, penguins, fish, and squid (Fig. 9.2)—reveals that the annual production of krill exceeds several hundred million tons. Considering that the worldwide harvest of species from the sea is around 100 million tons per year (Table 10.4), Antarctic krill could become humankind's single largest source of protein from the sea—potentially as large as all other marine fisheries from around the world combined.
For the past two centuries, nations have been harvesting living resources in the Southern Ocean beyond the jurisdiction of any one nation. The larger more accessible resources, namely the seals and whales, were harvested first (Figs. 10.110.3). Smaller species of squid, fish, and krill subsequently were exploited, along with the decline of the larger animals and development of new harvesting technologies (Figs. 10.5 and 10.6). Although species were harvested independently, impacts from their depletion have propagated throughout the complex of relationships in Antarctic ecosystems (Figs. 9.3 and 9.4).
In particular, diminished baleen whale populations have increased the availability of krill (Fig. 10.3, Tables 10.1 and 10.2). With decreased competition for the keystone species in the Antarctic marine ecosystem (Chapter 9: Living Planet), fin and sei whales began feeding south of the Antarctic Convergence earlier each summer. Because of increased food availability, fin and sei whales decreased their mean ages at sexual maturity and increased their overall pregnancy rates along with those of the blue whale. Moreover, with fisheries selecting the larger, more accessible individuals first, population distributions and dynamics become skewed toward smaller and younger individuals. Fortunately for the harvested whale species, the decreased competition created a positive feedback for enhancing their average reproductive output and recovery (Figs. 10.7a and 10.7b).
The ecological vacuum created from depleting the whales has rippled through populations of competitor species at the same trophic level in the Antarctic marine ecosystem (Chapter 9: Living Planet). For example, prior to whaling, all of the seal species together consumed about one-third of the krill in the Antarctic marine ecosystem. Today, the seals (primarily the crabeater) have become the largest krill predators—annually consuming more than 80 million tons of krill, more than 1.5 times as much as the whales.
There also are suggestions that the crabeater seal has been reducing its age of sexual maturity (Fig. 10.7a) and increasing its circumpolar population size perhaps up to 40 million. Moreover, chinstrap (Pygoscelis antarctica) and Adelie (Pygoscelis adeliae) penguin populations appear to be increasing 3 to 12% per year around Antarctica (Plate 9). Population growth among these krill-eating species coincides with the diminished competition from whales and increased availability of their common prey.
Was this article helpful?