Long-Term Population Trends of Patagonian Marine Mammals and Their Ecosystem Interactions in the Context of Climate Change

CRESPO, E. A.

Global Change in Atlantic Coastal Patagonian Ecosystems. Natural and Social Sciences of Patagonia

Springer

Año: 2022; p. 263 - 290

Marine mammals are among the top predators in the ocean. The Argentine Sea is home to the South American sea lion, the South American fur seal, the southern right whale, the southern elephant seal and other species of whales and dolphins. These species were hunted by native peoples that populated the coast until the Europeans colonized South America. Since then, they have been intensively exploited by the English, French, Dutch, Portuguese, Belgian and Spanish, and later by the Argentines (17th to 20th centuries). Marine mammal population reductions caused imbalances in the structure of the ecosystem. Protection reached the right whale in the 1920s and for the rest in the 1960s. Signs of recovery were only detected in the 1990s for right whales and sea lions, and in the 21st century for fur seals. However, the ecosystem was no longer the same. During the 1960s, at the time the exploitation of marine mammals ceased, incidental mortality appeared as a new problem for coastal species. Different type of fishing gear were implemented at sea. High seas fisheries in the 1980s, in particular bottom trawling, removed huge amounts of biomass from the ocean and produced remarkable changes in the marine community. Many commercial species extracted by fishing are mostly prey of mammals and birds. Sea lions have increased their population to one third of the original K and right whales to one tenth; that of fur seals and elephant remain unknown. The biomass released by the reduction of sea lions triggered the population growth of penguins, other birds and dolphins. During the 1990s, right whales have evidenced density-dependence through an increased mortality rate and a declining rate of increase, and sea lions a reduction in body size. The increase in pinniped populations possibly triggered the increase in killer whales population. Regarding the changes that have been detected linked to climate change, few have been documented in the populations that live in the waters of the Southwestern Atlantic, mostly related to the Antarctic Circumpolar Ocean. Antarctic krill growth and indirectly on krill-dependent predators are negatively related with temperature, with biomass reflecting variability in oceanographic conditions. Positive temperature anomalies in the tropical Pacific and the Georgia Islands showed that there are strong links between krill biomass during the previous winter period and reduced reproductive performance of Antarctic fur seals and gentoo penguins. There is a strong relationship between the reproductive success of right whales and SST anomalies in the South Georgia Islands in the fall of the previous year. Regarding the future of these high trophic level predators, it is pending to find an agreement in the near future that will make possible to find a sustainable use of fishery resources in terms of the amount of extractable biomass and the maintenance of top predator populations in a long run balance.