Southern Ocean

The Southern Ocean encircles the world, flowing in a continuous, clockwise stream known as the Antarctic Circumpolar Current. Isolating the continent of Antarctica and the South Pole from the nearest land mass of South America by the notorious Drake Passage, the Southern Ocean contains weather and climate systems that influence the conditions of the whole southern hemisphere and the planetary climate. As well as weather systems, the Southern Ocean is comprised of complex and massive ocean systems, with currents cycling nutrients, minerals and life through the world’s oceans.

Until recently, there was ongoing cartographical controversy on whether the Southern Ocean is a unique body of water, or rather the southern regions of three of the major world oceans – the Atlantic, Indian and Pacific Oceans. Since 2000 the Southern Ocean has been recognised as the body of water that extends from the Antarctic continent to latitude 60 degrees south. This precise boundary belies the extent and influence of the Southern Ocean system which, with massive, ice-bound Antarctica at its heart reaches from the Screaming Sixties latitudes through the Furious Fifties, Roaring Forties and beyond.

Captain James Cook, with the crews of the Resolution and the Adventure, was the first navigator to successfully sail around the base the world following the Southern Ocean current, proving its continuity and the lack of a land bridge to Antarctica during his second ‘voyage of discovery’ of 1772-75. At this time, the continent of Antarctica’s existence had not been confirmed but was inferred from the actions of winds and ocean currents experienced by navigators sailing through the higher latitudes. Cook seemed to have been awe-struck by the Southern Ocean. In Slicing the Silence – Voyaging to Antarctica (Harvard University Press, 2007, p. 43) historian Tom Griffiths describes Cook’s response to his circumpolar journey: “He voyaged distastefully through seas ‘pestered with ice’ and amongst icebergs that left his mind ‘filled with horror’…” The two ships were the first in history to cross the Antarctic Circle at latitude 66⁰ 33’ 39” south of the equator. Cook reported in his diary of the voyage of seeing icebergs after crossing below 51⁰ S, and thereafter encountered them frequently in the high latitudes.

Icebergs are not as common a sight in the Subantarctic latitudes of the Southern Ocean, as least during the short summer. The Subantarctic region could be considered to be encompassed between 42⁰ and 58⁰ south of the equator. The defining elemental forces of the Subantarctic are the Antarctic Polar Frontal Zone and perpetual westerly winds, the notorious Roaring Forties and Furious Fifties that encircle the 40th and 50th southern parallels of latitude. The winds were named by the sailors that first encountered them on exploratory and commercial voyages.

Propelled by the spinning of the earth, known as the Coriolis effect, the westerly winds are an essential element of the Southern Ocean. The winds mix oxygen, carbon and heat from the atmosphere into oceanic surface waters. This warmed water moves south towards Antarctica, and as icebergs form trapping fresh water, the surface water with its oxygen, carbon and heat from the atmosphere becomes more saline, and so more dense. This heavy, oxygen rich water cools and sinks in the high latitudes, becoming what is known as Antarctic Bottom Water, or ABW. Cold Antarctic Bottom Water slowly moves northwards over the ocean floor, transporting oxygen and nutrients including carbon through the deep sea. As the water moves north it slowly warms, and surfaces in the lower latitudes. This upwelling of cold water brings essential oxygen and nutrients, enriching the ocean ecosystems. Now flowing near the surface again, and influenced once more by wind and atmosphere, the water which began its journey in Antarctica follows a long traverse back towards the Southern Ocean, to repeat the cycle. This complex global process can take up to 1000 years per cycle, and is known as thermohaline circulation, or the Ocean Conveyor Belt. The Conveyor Belt links the Southern Ocean to the three major world oceans, driving the world’s climate and ocean systems.

Capturing and storing carbon dioxide from the atmosphere is one of the most valuable actions of the Ocean Conveyor Belt. While the processes of carbon capture in the Southern Ocean, and its potential role in climate change mitigation are still being studied, the crucial role of oceanic carbon in the food web is well established.

The Southern Ocean food web begins with the smallest of organisms: microscopic bacteria, phytoplankton (plants) and protozoa (animals). These single celled organisms make up the vast bulk of life in the Southern Ocean, with an estimated combined biomass in the billions of tonnes. Phytoplankton capture and process carbon dioxide through photosynthesis, and from there it enters the food web. Phytoplankton are the main food source for zooplankton – krill, copepods and salps, the grazers and herbivores of the ocean. To see krill in action, take a look Krill Cam, from the Antarctic marine research aquarium. These small but multitudinous invertebrates are a key-stone species in the Southern Ocean, the main food source for larger marine animals from squid and fish to penguins to whales, including the majestic blue whale.

Even for a large predator that does not directly feed on krill or other invertebrates, the link to krill is essential – their food source depends on these small species. An example of this would be the Killer Whale, which is known to feed on Elephant seal, which feed mainly on squid, which in turn feed on krill. In the case of the largest animals in the Southern Ocean, like the Southern Right Whale and Humpback Whale, the link between them and their tiny prey is a direct one. These and other species of whales feed by filtering enormous amounts of seawater through the baleen plates in their mouths, which capture krill.

Whales and krill in the Southern Ocean are not just linked by the food web. Both groups of animals have a shared history of exploitation by humans.

Unlike the environmentally, politically and emotionally fraught history and present state of whaling in the Southern Ocean, the environmental impacts of harvesting krill has until recently barely been considered in the public sphere. Krill harvesting has been an active industry in the Southern Ocean since the 1970’s, producing end products such as fish food for aquaculture and the aquarium trade, and more recently dietary supplements to fuel health food fads.

In 1982 in response to concerns about the fast growth of the Antarctic krill fisheries, the Commission for the Conservation of Antarctic Marine Living Resources, or CCAMLR, was established by international convention. CCAMLR covers almost 10% of the earth’s surface, has 25 member states, including Australia, and creates legally binding conservation measures for member states active in the Antarctica fisheries. CCAMLR focuses on ecosystem-based management of sustainable fisheries in Antarctica, and supports the work of scientist to understand the enormous and complex dynamics of krill and other species populations in the Southern Ocean. The size and extent of the Antarctic krill population is not precisely known. CCAMLR has a key role in monitoring catches and environmental impacts of krill fisheries on marine life, to better understand how much krill there is and how much can be sustainably harvested.

The concurrent rise of krill-oil health food fads and social media has seen campaigns to raise public awareness about the impacts of krill fisheries. While CCAMLR’s position is that the current level of Antarctic krill harvesting is sustainable, the global reliance on krill for livestock and aquaculture feed, human and livestock health supplements and pet food makes krill sustainability important to us all.

Campaigns like like Greenpeace’s License to Krill have an important role to play in communicating the existence and impact of fisheries in the Southern Ocean to the general public, who otherwise may never give a thought to where our krill oil supplements or Patagonian Toothfish sushi come from.

Patagonian Toothfish are a prized catch in the Southern Ocean and CCAMLR convention area, along with Antarctic Toothfish and Mackerel Icefish. Renowned for their firm, oily flesh, these fish are highly sought after in Europe, America and Asia and are becoming more popular in Australia. The Southern Ocean fisheries are monitored by CCLAMR and its member states, but there is still regular occurrences of Illegal, Unreported and Unregulated (IUU) fishing in the Southern Ocean, including the CCLAMR area. IUU fishing for in-demand species like Patagonian Toothfish can be highly lucrative, and while Australia prides itself on the control and elimination of the practice both in our waters and in neighbouring countries, it continues and will likely continue as long as it is profitable, with billions of dollars to be made each year.

Illegal, Unreported and Unregulated fishing is no doubt one the biggest current threats to world fish stocks, but the fact is much of the world’s ocean and marine life has already been heavily depleted by legal fishing over the past 250 years. The health benefits of eating fish are well known, but it is vital that as consumers we are informed about where our fish is coming from, and if it has been caught in a sustainable manner. This means not only that the target species is being harvested in such a way that it can replenish its stocks, but also that the bycatch and impact on non-target species is reduced as much as possible.

Bycatch is fish and other marine animals that are not targeted by the fishing vessel but are caught on lines or in nets aimed at the target species. In the Southern Ocean albatrosses and petrels are caught in long-line and trawling fishing equipment. The international community is working to reduce the bycatch of seabirds through the Agreement for the Conservation of Albatrosses and Petrels and there have been some successes with reducing the number of birds caught in recent years.

As consumers, there are online resources we can use to determine if our seafood choices are sustainable. Websites like the Marine Stewardship Council provide info on different types of seafood and seafood products, with a products’ sustainability rated through their certification scheme. However as it is not compulsory for seafood producers and suppliers to identify the source of their products, the origins of much of Australia’s seafood remains hidden. There is a growing movement to lobby the Australian government to introduce seafood labelling laws to require seafood producers and sellers to include the species and origin of their products in all labelling, but so far the legislation has not been updated to require this.

So while the vast, wild Southern Ocean is still one of the most remote and extreme environments on the planet, it is by no means removed from our everyday experience. Products from Southern Ocean species can be found in most Australian homes, with fish food for our small aquariums and omega 3 supplements for our joints frequently on the shopping list, and wild caught sushi or fish-farm fish and chips on the weekend just part of life. From gale force winds and sweeping waves the small, domestic products we all enjoy are produced, and our ever increasing demand must be monitored and checked to ensure the vast oceans can continue to sustain us.