An ocean like no other: the Southern Ocean’s ecological richness and significance for global climate



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Ceridwen Fraser, University of Otago; Christina Hulbe, University of Otago; Craig Stevens, National Institute of Water and Atmospheric Research, and Huw Griffiths, British Antarctic Survey

In 2018, a map named after an oceanographer went viral.

The so-called Spilhaus projection, in which Earth is viewed from above the South Pole, was designed to show the connected nature of the ocean basins.

It is a perspective that comes naturally to those who live in the ocean-dominated southern hemisphere.

Map of the world's oceans
The Spilhaus map depicts the world’s oceans as a single body of water.
Spilhaus ArcGIS project, CC BY-ND

The Southern Ocean, also called the Antarctic Ocean (or even the Austral ocean), is like no other and best described in superlatives.

Storing heat and carbon

Let’s first look at the Southern Ocean’s capacity to store excess heat and carbon. The world’s oceans take up more than 90% of the excess heat generated by the burning of fossil fuels and a third of the additional carbon dioxide.

Southern Ocean, with open ocean and sea ice
The Southern Ocean is our planet’s primary storage of heat and carbon.
Crag Stevens, Author provided

The Southern Ocean, south of 30°S, is estimated to store about 75% of this global oceanic uptake of excess heat and about 35% of the global uptake of excess carbon from the atmosphere. It is the primary storage of heat and carbon for the planet.

The Southern Ocean connects all major ocean basins, except the Arctic. The link is the Antarctic Circumpolar Current (ACC) – the largest ocean current on the planet. It carries more than 100 times the flow of all the rivers on Earth and transports enough water to fill Lake Ontario in just a few hours.

A combination of strong winds and a nearly uninterrupted passage around Antarctica give the ACC its strong flows and speed.




Read more:
Explainer: how the Antarctic Circumpolar Current helps keep Antarctica frozen


Mixing global currents

The Roaring Forties, Furious Fifties and Screaming Sixties are all popular names for the strong westerly winds that blow, nearly uninterrupted, across the Southern Ocean, creating equally impressive waves. This results in a massively energetic – and hard to measure – ocean surface.

Ship crossing the Southern Ocean
Strong westerly winds and the circumpolar current create massive waves in the Southern Ocean.
Craig Stevens, Author provided

But the heat and carbon exchanges across this complicated interface are globally important, and oceanographers have designed tools specifically for this challenging environment.

Ocean currents with different properties mix, rise and sink.
Craig Stevens, Author provided

To really comprehend the Southern Ocean, one must think in three dimensions. Waters with different properties mix both horizontally and vertically in eddies.

Relatively warm subtropical water is mixed south, deep cool water from the North Atlantic rises back up toward the surface and colder polar water masses mix northward and sink back down.

This complex interplay is guided by the wind and by the shape of the seafloor.

To the north, there are only three major constrictions: the 850km-wide Drake Passage, and the submarine Kerguelan and Campbell Plateaus. To the south, the ACC butts up against Antarctica.

Here the ocean plays another crucial role in the global climate system by bringing relatively warm — and warming — Circumpolar Deep Water into contact with the ice fringing Antarctica.

Annual thaw and freeze of sea ice

The annual cycle of sea ice growing and melting around Antarctica is one of the defining rhythms of our planet and an important facet of the Southern Ocean. The two polar regions couldn’t be more different in this regard.

The Arctic is a small, deep ocean surrounded by land with only narrow exits. The Antarctic is a large landmass with a continental shelf surrounded by ocean. Each year, 15 million square kilometres of sea ice advance and retreat in these waters.

sea ice around Antarctica
The annual freezing and thawing of sea ice around Antarctica is the world’s largest seasonal change.
Shutterstock/Maxim Tupikov

In contrast to the clear and dramatic changes in the north, the rhythm of Antarctic sea ice has followed less obvious patterns. In the face of a warming ocean, it was actually slowly expanding northward until around 2016, when it suddenly started to contract.




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Why Antarctica’s sea ice cover is so low (and no, it’s not just about climate change)


Looking at the annual cycle of Antarctic sea ice, one might think it simply grows and melts in place as things get cold and warmer through the year. But in truth, much of the sea ice production happens in polynya – sea ice factories near the coast where cold and fast Antarctic winds both create and blow away new sea ice as fast as it appears.

This process brings us back to global ocean circulation. When the new ice grows, the salt from the freezing sea water gets squeezed out and mixes with the seawater below, creating colder and saltier seawater that sinks to the seafloor and drains northward.

Polynya are in effect a metro stop on a global transport system that sees water sinking at the poles, flowing north to be mixed upwards in a cycle lasting close to 1,000 years.

Not all ice shelves respond the same

Computer simulations have shown how the ice shelves at Antarctica’s fringe have waxed and waned over past millennia.

Because these floating extensions of the ice sheet interact directly with the ocean, they make the ice sheet sensitive to climate. Ocean warming and changes in the source of the water coming into contact with an ice shelf can cause it – and in turn the whole ice sheet – to change.

Riiser Larsen Ice Shelf, in Antarctica
Floating ice shelves act like a buttress to hold back Antarctica’s massive ice sheet.
Shutterstock/sirtravelalot

But not all ice shelves will respond to warming in the same way. Some ocean cavities are cold and slowly evolving. Others are actually described as hot – in polar terms – because of their interaction with Circumpolar Deep Water. The latter are changing rapidly right now.

We can observe many cryosphere processes from space, but to truly understand how far the ocean reaches beneath the ice we have to go hundreds of metres beneath the ice surface.

Making climate predictions requires an understanding of detailed processes that happen on short timescales, such as tidal cycles, in parts of the planet we are only beginning to explore.




Read more:
What an ocean hidden under Antarctic ice reveals about our planet’s future climate


Observing the Screaming Sixties

How do we sample something so big and so stormy? With robots.

Satellites have been observing the ocean surface since the 1980s. This technology can measure properties such as temperature and ocean surface height, and even be used to estimate biological productivity. But satellites can’t see beneath the surface.

When the game-changing Argo programme started in the 1990s, it revolutionised earth science by building a network of drifting ocean sentinels measuring temperature and salinity down to a depth of two kilometres.

Marine scientist deploying an ocean probe
Argo probes measure salinity and temperature as they drift with currents in the Southern Ocean.
NIWA/Daniel Jones, Author provided

The research vessel Kaharoa holds the record for the most deployments of Argo probes in the Southern Ocean, including its most recent storm-tossed, COVID-19-impacted voyage south of Australia and into the Indian Ocean.

The Argo program is only the start of a new era of ocean observation. Deep Argo probes dive to depths of 6km to detect how far down ocean warming is penetrating.

The past and future Southern Ocean

Earth hasn’t always looked as it does today. At times in the planet’s past, the Southern Ocean didn’t even exist. Continents and ocean basins were in different positions and the climate system operated very differently.

From the narrow view of human evolution, the Southern Ocean has been a stable component of a climate system and subject to relatively benign glacial oscillations. But glacial cycles play out over tens of thousands of years.

We are imposing a very rapid climate transient. The nearly three centuries since the start of the industrial revolution is shorter than the blink of an eye in geological context.

Calving ice shelf in Antarctica
Antarctica’s ice is changing as global temperatures rise.
Shutterstock/Bernhard Staehli

Future changes in the short (say by 2050) and long (by 2300) term are difficult to project. While the physics are relatively clear about what will happen, predicting when it will happen is more challenging.




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Simulation tools that get the ocean, atmosphere and ice processes right are only starting to include ice shelf cavities and ocean eddies. The most recent synthesis of climate models shows progress in the simulated workings of the Southern Ocean. But sea ice remains a challenge to simulate well.

This is the frontier: a global research community working to connect data with rapidly improving computer models to better understand how this unique ocean operates.

Life in a sub-zero ocean

At first glance, Antarctica seems an inhospitable and almost barren environment of ice and snow, speckled with occasional seabirds and seals.

But diving beneath the surface reveals an ocean bursting with life and complex ecosystems, from single-celled algae and tiny spineless creatures to the well-known top predators: penguins, seals and whales.

The Southern Ocean is home to more than 9,000 known marine species — and expeditions and studies keep revealing more.

Ship battling high waves
The RV Polarstern battles through a storm in the Southern Ocean.
Huw Griffiths, Author provided

It’s not easy to study life in the Southern Ocean. Waves can be more than 20 metres high, and icebergs and sea ice lurk among them.

The water temperature is often sub-zero – freshwater freezes at 0℃, but saltwater freezes at closer to -2℃. Although scuba diving is possible, a lot of research on life in the Southern Ocean is done through remote sampling.

Marine scientists use robotic tools such as remotely operated underwater vehicles to look at and collect samples, and grabs and dredges to bring up bottom-dwelling organisms. We take genetic samples from marine mammals by shooting tiny biopsy tubes (like needles), attached to a cord for retrieval, into the animal’s flesh from a distance.




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We can glean wider information on diversity from environmental DNA (eDNA). Traces of organisms are filtered from samples of water and analysed using genetic tools that can usually identify what sorts of species are or were present.

Every expedition reveals new species – some of which are potentially commercially valuable, and all of which are important parts of the Southern Ocean ecosystem. Our knowledge of the diversity of the region is growing rapidly.

Nonetheless, the Southern Ocean is vast, and much of it remains either unsampled or undersampled.

Down at the bottom of the food chain

In the Southern Ocean, primary producers (organisms at the start of the food chain) range from single-celled algae – such as diatoms with intricately detailed shells made of silica – through to large macroalgae like kelp.

Algae growing on the underside of sea ice in Antarctica.
Algae growing on the underside of sea ice.
Andrew Thurber, Author provided

Kelp and other large seaweeds generally only survive where icebergs don’t often scrape the seafloor. Diatoms are diverse, and some species thrive on the underside of sea ice.

Ice algae form an important food source for krill, small crustaceans that are a critical part of Southern Ocean food webs.

Antarctic krill
Antarctic krill is a key species in the Antarctic marine ecosystem.
British Antarctic Survey, Author provided

Astonishingly, the cold Southern Ocean is also home to hot hydrothermal vent systems. These communities, which include huge densities of crustaceans and echinoderms, get their energy from chemicals that seep out of Earth’s crust, rather than from the Sun.

An Antarctic hydrothermal vent on the East Scotia Ridge. The image was taken by a remotely operated vehicle during the ChEsSO expedition.
ChEsSO/NERC, Author provided

Antarctic invertebrates make up more than 90% of the species in the Southern Ocean. More than 50% are unique to this ocean.

These invertebrates are often much larger than their relatives in more northern, warmer waters. This phenomenon is know as “polar gigantism” and is found across many groups, with giant sea spiders, huge sponges and scale worms the size of a forearm.

A selection of invertebrates commonly found by scientists diving at Rothera Station, Antarctica.
A selection of invertebrates commonly found by scientists diving at Rothera Station, Antarctica.
British Antarctic Survey, Author provided

Nobody is quite sure why Antarctic invertebrates grow so large, but it may be related to high oxygen levels, slow growth rates or the absence of key predatory groups such as sharks and brachyuran crabs.

Colourful creatures that live on the seafloor.
Marine invertebrates on the seafloor off the Antarctic coast.
Alfred Wegener Institute, OFOBS team, Author provided

Higher up in the food chain

In the marine food chain, Antarctic krill swim between the algal primary producers and the iconic top predators we always associate with Antarctica.

Baleen whales get much of their energy from great gulps of swarming krill (10,000–30,000 individual animals per cubic metre), and the pink streaks in penguin and seal poo show they are also keen on these tasty crustaceans.

Chinstrap penguins on Deception Island
Chinstrap penguins on Deception Island. Many penguins pooh in pink, because their diet is rich in krill.
Michelle LaRue, Author provided

Fish and cephalopods (squid and octopus) thrive in the Southern Ocean, providing food for deep-diving marine mammals such as elephant seals. Some fish species are so well adapted to the oxygen-rich cold waters they no longer produce red blood cells but instead produce antifreeze proteins in their blood to help them survive in the subzero waters.

Minke whale in Antarctic waters
Many whale species depend on Antarctic ecosystems for summer feeding and migrate to warmer, lower latitudes for winter breeding. But Antarctic minke whales are resident all year round.
Huw Griffiths, Author provided

Protecting marine environments

Arguably the most voracious predators in the Southern Ocean are humans.

Antarctica might be remote, but in the 200 or so years since its discovery, the seas around Antarctica have been heavily exploited by people.

First came the sealers, then the whalers, driving species to the brink of extinction. Even penguins were harvested for their oil.

An abandoned whaling station.
An abandoned whaling station.
Ceridwen Fraser, Author provided

More recently, fish and krill (which is fished for food or dietary supplements) have been the main targets, and populations of some species have declined sharply as a consequence.




Read more:
Humans are encroaching on Antarctica’s last wild places, threatening its fragile biodiversity


When more indirect impacts like ocean warming and acidification combine with fishing, this can lead to declining populations of krill, which in turn leads to reduced numbers of top predators such as whales.

Graphic showing how people affect ecosystems in the Southern Ocean
Humans are changing Southern Ocean ecosystems in many ways, both directly (purple-blue arrows) and indirectly (red arrows).
From: Chown et al (2015) The changing form of Antarctic biodiversity. Nature, 522: 431-438, CC BY-ND

Fishing in the Southern Ocean can be hard to regulate because these waters do not belong to any one nation. To help manage the impact of fisheries, quotas that limit catches are now managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR).

This international body is also working to establish more marine protected areas.
Without these efforts to manage catches, critical parts of the food web (such as krill) could be exploited to such an extent that ecosystems could collapse.

Changing environments mean changing ecosystems

More than 21,000 tourists and scientists visit Antarctica each year, potentially bringing pollution, diseases and invasive species. To manage human impacts on Antarctic ecosystems, and to help with political negotiations, the Antarctic Treaty came into force on June 23, 1961.

The treaty regulates all activity south of 60°S and includes an environmental protection protocol.

The impacts of global climate change and ocean acidification are nonetheless evident in the Southern Ocean, with warming ocean temperatures, reduction in sea ice and collapsing ice shelves.

Ocean off the Antarctic coast
Antarctic ocean waters are warming dramatically.
Ceridwen Fraser, Author provided

Increasingly, research is showing that even the distant Southern Ocean is not truly cut off from the rest of the world, with warming, plastic pollution and non-native species making their way to Antarctic waters from beyond the mighty polar front.

Seals and seaweed on a southern beach.
Southern bull kelp does not grow in the Antarctic, but it floats well and recent research has shown that it can drift to Antarctica, travelling tens of thousands of kilometres across the Southern Ocean.
Author provided

Rafts of floating seaweeds from outside the Antarctic, some carrying animal passengers, are able to cross the Southern Ocean and reach Antarctic shores. At the moment, they don’t seem able to survive the extreme climate of Antarctica, but that could change with warming.

New species moving in and setting up shop will put a lot of pressure on Antarctica’s unique plants and animals.

Adélie penguins rest and breed on land, but go to sea to forage for food.
Michelle LaRue, Author provided

It’s not all doom and gloom, though. Over the several decades since the Antarctic Treaty came into force, we’ve seen that nations can work together to help resolve challenges facing the Antarctic. One example is the establishment of Antarctic Marine Protected Areas (MPAs).

This level of international cooperation should give us hope not just for the future of the Southern Ocean, but also for other key challenges the world faces.


This story is part of our Oceans 21 series

Five profiles open our series on the global ocean, delving into ancient Indian Ocean trade networks, Pacific plastic pollution, Arctic light and life, Atlantic fisheries and the Southern Ocean’s impact on global climate. All brought to you from The Conversation’s international network.The Conversation

Ceridwen Fraser, Associate professor, University of Otago; Christina Hulbe, Professor and Dean of the School of Surveying (glaciology specialisation), University of Otago; Craig Stevens, Associate Professor in Ocean Physics, National Institute of Water and Atmospheric Research, and Huw Griffiths, Marine Biogeographer, British Antarctic Survey

This article is republished from The Conversation under a Creative Commons license. Read the original article.

It might be the world’s biggest ocean, but the mighty Pacific is in peril



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Jodie L. Rummer, James Cook University; Bridie JM Allan, University of Otago; Charitha Pattiaratchi, University of Western Australia; Ian A. Bouyoucos, James Cook University; Irfan Yulianto, IPB University, and Mirjam van der Mheen, University of Western Australia

The Pacific Ocean is the deepest, largest ocean on Earth, covering about a third of the globe’s surface. An ocean that vast may seem invincible. Yet across its reach – from Antarctica in the south to the Arctic in the north, and from Asia to Australia to the Americas – the Pacific Ocean’s delicate ecology is under threat.

In most cases, human activity is to blame. We have systematically pillaged the Pacific of fish. We have used it as a rubbish tip – garbage has been found even in the deepest point on Earth, in the Mariana Trench 11,000 metres below sea level.

And as we pump carbon dioxide into the atmosphere, the Pacific, like other oceans, is becoming more acidic. It means fish are losing their sense of sight and smell, and sea organisms are struggling to build their shells.

Oceans produce most of the oxygen we breathe. They regulate the weather, provide food, and give an income to millions of people. They are places of fun and recreation, solace and spiritual connection. So, healthy, vibrant oceans benefit us all. And by better understanding the threats to the precious Pacific, we can start the long road to protecting it.


This article is part of the Oceans 21 series

The series opens with five profiles delving into ancient Indian Ocean trade networks, Pacific plastic pollution, Arctic light and life, Atlantic fisheries and the Southern Ocean’s impact on global climate. It’s brought to you by The Conversation’s international network.


The ocean plastic scourge

The problem of ocean plastic was scientifically recognised in the 1960s after two scientists saw albatross carcasses littering the beaches of the northwest Hawaiian Islands in the northern Pacific. Almost three in four albatross chicks, who died before they could fledge, had plastic in their stomachs.

Now, plastic debris is found in all major marine habitats around the world, in sizes ranging from nanometers to meters. A small portion of this accumulates into giant floating “garbage patches”, and the Pacific Ocean is famously home to the largest of them all.

Most plastic debris from land is transported into the ocean through rivers. Just 20 rivers contribute two-thirds of the global plastic input into the sea, and ten of these discharge into the northern Pacific Ocean. Each year, for example, the Yangtze River in China – which flows through Shanghai – sends about 1.5 million metric tonnes of debris into the Pacific’s Yellow Sea.

A wildlife killer

Plastic debris in the oceans presents innumerable hazards for marine life. Animals can get tangled in debris such as discarded fishing nets, causing them to be injured or drown.

Some organisms, such as microscopic algae and invertebrates, can also hitch a ride on floating debris, travelling large distances across the oceans. This means they can be dispersed out of their natural range, and can colonise other regions as invasive species.




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And of course, wildlife can be badly harmed by ingesting debris, such as microplastics less than five millimetres in size. This plastic can obstruct an animal’s mouth or accumulate in its stomach. Often, the animal dies a slow, painful death.

Seabirds, in particular, often mistake floating plastics for food. A 2019 study found there was a 20% chance seabirds would die after ingesting a single item, rising to 100% after consuming 93 items.

A turtle tangled in a fishing net
Discarded fishing nets, or ‘ghost nets’ can entangle animals like turtles.
Shutterstock

A scourge on small island nations

Plastic is extremely durable, and can float vast distances across the ocean. In 2011, 5 million tonnes of debris entered the Pacific during the Japan tsunami. Some crossed the entire ocean basin, ending up on North American coastlines.

And since floating plastics in the open ocean are transported mainly by ocean surface currents and winds, plastic debris accumulates on island coastlines along their path. Kamilo Beach, on the south-eastern tip of Hawaii’s Big Island, is considered one of the world’s worst for plastic pollution. Up to 20 tonnes of debris wash onto the beach each year.

Similarly, on uninhabited Henderson Island, part of the Pitcairn Island chain in the south Pacific, 18 tonnes of plastic have accumulated on a beach just 2.5km long. Several thousand pieces of plastic wash up each day.

Kamilo Beach is referred to as the world’s dirtiest.

Subtropical garbage patches

Plastic waste can have different fates in the ocean: some sink, some wash up on beaches and some float on the ocean surface, transported by currents, wind and waves.

Around 1% of plastic waste accumulates in five subtropical “garbage patches” in the open ocean. They’re formed as a result of ocean circulation, driven by the changing wind fields and the Earth’s rotation.

There are two subtropical garbage patches in the Pacific: one in the northern and one in the southern hemisphere.

The northern accumulation region is separated into an eastern patch between California and Hawaii, and a western patch, which extends eastwards from Japan.

Locations of the five subtropical garbage patches.
van der Mheen et al. (2019)

Our ocean garbage shame

First discovered by Captain Charles Moore in the early 2000s, the eastern patch is better known as the Great Pacific Garbage Patch because it’s the largest by both size (around 1.6 million square kilometers) and amount of plastic. By weight, this garbage patch can hold more than 100 kilograms per square kilometre.

The garbage patch in the southern Pacific is located off Valparaiso, Chile, extending to the west. It has lower concentrations compared to its giant counterpart in the northeast.

Discarded fishing nets make up around 45% of the total plastic weight in the Great Pacific Garbage Patch. Waste from the 2011 Japan tsunami is also a major contributor, making up an estimated 20% of the patch.




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Whales and dolphins found in the Great Pacific Garbage Patch for the first time


With time, larger plastic debris degrades into microplastics. Microplastics form only 8% of the total weight of plastic waste in the Great Pacific Garbage Patch, but make up 94% of the estimated 1.8 trillion pieces of plastic there. In high concentrations, they can make the water “cloudy”.

Each year, up to 15 million tonnes of plastic waste are estimated to make their way into the ocean from coastlines and rivers. This amount is expected to double by 2025 as plastic production continues to increase.

We must act urgently to stem the flow. This includes developing plans to collect and remove the plastics and, vitally, stop producing so much in the first place.

Divers releasing a whale shark from a fishing net.

Fisheries on the verge of collapse

As the largest and deepest sea on Earth, the Pacific supports some of the world’s biggest fisheries. For thousands of years, people have relied on these fisheries for their food and livelihoods.

But, around the world, including in the Pacific, fishing operations are depleting fish populations faster than they can recover. This overfishing is considered one of the most serious threats to the world’s oceans.

Humans take about 80 million tonnes of wildlife from the sea each year. In 2019, the world’s leading scientists said of all threats to marine biodiversity over the past 50 years, fishing has caused the most harm. They said 33% of fish species were overexploited, 60% were being fished to the maximum level, and just 7% were underfished.

The decline in fish populations is not just a problem for humans. Fish play an important role in marine ecosystems and are a crucial link in the ocean’s complex food webs.

A school of fish
Overfishing is stripping the Pacific Ocean of marine life.
Shutterstock

Not plenty of fish in the sea

Overfishing happens when humans extract fish resources beyond the maximum level, known as the “maximum sustainable yield”. Fishing beyond this causes global fish stocks to decline, disrupts food chains, degrades habitats, and creates food scarcity for humans.

The Pacific Ocean is home to huge tuna fisheries, which provide almost 65% of the global tuna catch each year. But the long-term survival of many tuna populations is at risk.

For example, a study released in 2013 found numbers of bluefin tuna – a prized fish used to make sushi – had declined by more than 96% in the Northern Pacific Ocean.

Developing countries, including Indonesia and China, are major overfishers, but so too are developing nations.




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Along Canada’s west coast, Pacific salmon populations have declined rapidly since the early 1990s, partly due to overfishing. And Japan was recently heavily criticised for a proposal to increase quotas on Pacific bluefin tuna, a species reportedly at just 4.5% of its historic population size.

Experts say overfishing is also a problem in Australia. For example, research in 2018 showed large fish species were rapidly declining around the nation due to excessive fishing pressure. In areas open to fishing, exploited populations fell by an average of 33% in the decade to 2015.

A plate of sushi
Stocks of fish used to make sushi have declined in number.
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So what’s driving overfishing?

There are many reasons why overfishing occurs and why it is goes unchecked. The evidence points to:




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Let’s take Indonesia as an example. Indonesia lies between the Pacific and Indian oceans and is the world’s third-biggest producer of wild-capture fish after China and Peru. Some 60% of the catch is made by small-scale fishers. Many hail from poor coastal communities.

Overfishing was first reported in Indonesia in the 1970s. It prompted a presidential decree in 1980, banning trawling off the islands of Java and Sumatra. But overfishing continued into the 1990s, and it persists today. Target species include reef fishes, lobster, prawn, crab, and squid.

Indonesia’s experience shows how there is no easy fix to the overfishing problem. In 2017, the Indonesian government issued a decree that was supposed to keep fishing to a sustainable level – 12.5 million tonnes per year. Yet, in may places, the practice continued – largely because the rules were not clear and local enforcement was inadequate.

Implementation was complicated by the fact that almost all Indonesia’s smaller fishing boats come under the control of provincial governments. This reveals the need for better cooperation between levels of government in cracking down on overfishing.

Man checks fishing haul
Globally, compliance and enforcement of fishing limits is often poor.
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What else can we do?

To prevent overfishing, governments should address the issue of poverty and poor education in small fishing communities. This may involve finding them a new source of income. For example in the town of Oslob in the Philippines, former fishermen and women have turned to tourism – feeding whale sharks tiny amounts of krill to draw them closer to shore so tourists can snorkel or dive with them.

Tackling overfishing in the Pacific will also require cooperation among nations to monitor fishing practices and enforce the rules.

And the world’s network of marine protected areas should be expanded and strengthened to conserve marine life. Currently, less than 3% of the world’s oceans are highly protected “no take” zones. In Australia, many marine reserves are small and located in areas of little value to commercial fishers.

The collapse of fisheries around the world shows just how vulnerable our marine life is. It’s clear that humans are exploiting the oceans beyond sustainable levels. Billions of people rely on seafood for protein and for their livelihoods. But by allowing overfishing to continue, we harm not just the oceans, but ourselves.

fish in a net
Providing fishers with an alternative income can help prevent overfishing.
Shutterstock



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The threat of acidic oceans

The tropical and subtropical waters of the Pacific Ocean are home to more than 75% of the world’s coral reefs. These include the Great Barrier Reef and more remote reefs in the Coral Triangle, such as those in Indonesia and Papua New Guinea.

Coral reefs are bearing the brunt of climate change. We hear a lot about how coral bleaching is damaging coral ecosystems. But another insidious process, ocean acidification, is also threatening reef survival.

Ocean acidification particularly affects shallow waters, and the subarctic Pacific region is particularly vulnerable.

Coral reefs cover less than 0.5% of Earth’s surface, but house an estimated 25% of all marine species. Due to ocean acidification and other threats, these incredibly diverse “underwater rainforests” are among the most threatened ecosystems on the planet.

A chemical reaction

Ocean acidification involves a decrease in the pH of seawater as it absorbs carbon dioxide (CO₂) from the atmosphere.

Each year, humans emit 35 billion tonnes of CO₂ through activities such as burning of fossil fuels and deforestation.

Oceans absorb up to 30% of atmospheric CO₂, setting off a chemical reaction in which concentrations of carbonate ions fall, and hydrogen ion concentrations increase. That change makes the seawater more acidic.

Since the Industrial Revolution, ocean pH has decreased by 0.1 units. This may not seem like much, but it actually means the oceans are now about 28% more acidic than since the mid-1800s. And the Intergovernmental Panel on Climate Change (IPCC) says the rate of acidification is accelerating.

An industrial city from the air
Each year, humans emit 35 billion tonnes of CO₂.
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Why is ocean acidification harmful?

Carbonate ions are the building blocks for coral structures and organisms that build shells. So a fall in the concentrations of carbonate ions can spell bad news for marine life.

In more acidic waters, molluscs have been shown to have trouble making and repairing their shells. They also exhibit impaired growth, metabolism, reproduction, immune function, and altered behaviours. For example, researchers exposed sea hares (a type of sea slug) in French Polynesia to simulated ocean acidification and found they had less foraging success and made poorer decisions.

Ocean acidification is also a problem for the fishes. Many studies have revealed elevated CO₂ can disrupt their sense of smell, vision and hearing. It can also impair survival traits, such as a fish’s ability to learn, avoid predators, and select suitable habitat.

Such impairment appears to be the result of changes in neurological, physiological, and molecular functions in fish brains.

A sea hare
Sea hares exposed to acidification made poorer decisions.
Shutterstock

Predicting the winners and losers

Of the seven oceans, the Pacific and Indian Oceans have been acidifying at the fastest rates since 1991. This suggests their marine life may also be more vulnerable.

However, ocean acidification does not affect all marine species in the same way, and the effects can vary over the organism’s lifetime. So, more research to predict the future winners and losers is crucial.

This can be done by identifying inherited traits that can increase an organism’s survival and reproductive success under more acidic conditions. Winner populations may start to adapt, while loser populations should be targets for conservation and management.




Read more:
Acid oceans are shrinking plankton, fuelling faster climate change


One such winner may be the epaulette shark, a shallow water reef species endemic to the Great Barrier Reef. Research suggests simulated ocean acidification conditions do not impact early growth, development, and survival of embryos and neonates, nor do they affect foraging behaviours or metabolic performance of adults.

But ocean acidification is also likely to create losers on the Great Barrier Reef. For example, researchers studying the orange clownfish – a species made famous by Disney’s animated Nemo character – found they suffered multiple sensory impairments under simulated ocean acidification conditions. These ranged from difficulties smelling and hearing their way home, to distinguishing friend from foe.

A clownfish
Clownfish struggled to tell friend from foe when exposed to ocean acidification.
Shutterstock

It’s not too late

More than half a billion people depend on coral reefs for food, income, and protection from storms and coastal erosion. Reefs provide jobs – such as in tourism and fishing – and places for recreation. Globally, coral reefs represent an industry worth US$11.9 trillion per year. And importantly, they’re a place of deep cultural and spiritual connection for Indigenous people around the world.

Ocean acidification is not the only threat to coral reefs. Under climate change, the rate of ocean warming has doubled since the 1990s. The Great Barrier Reef, for example, has warmed by 0.8℃ since the Industrial Revolution. Over the past five years this has caused devastating back-to-back coral bleaching events. The effects of warmer seas are magnified by ocean acidification.




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Cutting greenhouse gas emissions must become a global mission. COVID-19 has slowed our movements across the planet, showing it’s possible to radically slash our production of CO₂. If the world meets the most ambitious goals of the Paris Agreement and keeps global temperature increases below 1.5℃, the Pacific will experience far less severe decreases in oceanic pH.

We will, however, have to curb emissions by a lot more – 45% over the next decade – to keep global warming below 1.5℃. This would give some hope that coral reefs in the Pacific, and worldwide, are not completely lost.

Clearly, the decisions we make today will affect what our oceans look like tomorrow.The Conversation

The Pacific Ocean off the Taiwan coast
Our decisions today will determine the fate of tomorrow’s oceans.
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Jodie L. Rummer, Associate Professor & Principal Research Fellow, James Cook University; Bridie JM Allan, Lecturer/researcher, University of Otago; Charitha Pattiaratchi, Professor of Coastal Oceanography, University of Western Australia; Ian A. Bouyoucos, Postdoctoral fellow, James Cook University; Irfan Yulianto, Lecturer of Fisheries Resources Utilization, IPB University, and Mirjam van der Mheen, Fellow, University of Western Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.