How farming giant seaweed can feed fish and fix the climate

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Giant kelp can grow up to 60cm a day, given the right conditions.
Joe Belanger/

Tim Flannery, University of Melbourne

This is an edited extract from Sunlight and Seaweed: An Argument for How to Feed, Power and Clean Up the World by Tim Flannery, published by Text Publishing.

Bren Smith, an ex-industrial trawler man, operates a farm in Long Island Sound, near New Haven, Connecticut. Fish are not the focus of his new enterprise, but rather kelp and high-value shellfish. The seaweed and mussels grow on floating ropes, from which hang baskets filled with scallops and oysters. The technology allows for the production of about 40 tonnes of kelp and a million bivalves per hectare per year.

The kelp draw in so much carbon dioxide that they help de-acidify the water, providing an ideal environment for shell growth. The CO₂ is taken out of the water in much the same way that a land plant takes CO₂ out of the air. But because CO₂ has an acidifying effect on seawater, as the kelp absorb the CO₂ the water becomes less acid. And the kelp itself has some value as a feedstock in agriculture and various industrial purposes.

After starting his farm in 2011, Smith lost 90% of his crop twice – when the region was hit by hurricanes Irene and Sandy – but he persisted, and
now runs a profitable business.

His team at 3D Ocean Farming believe so strongly in the environmental and economic benefits of their model that, in order to help others establish similar operations, they have established a not-for-profit called Green Wave. Green Wave’s vision is to create clusters of kelp-and-shellfish farms utilising the entire water column, which are strategically located near seafood transporting or consumption hubs.

Read more: Seaweed could hold the key to cutting methane emissions from cow burps

The general concepts embodied by 3D Ocean Farming have long been practised in China, where over 500 square kilometres of seaweed farms exist in the Yellow Sea. The seaweed farms buffer the ocean’s growing acidity and provide ideal conditions for the cultivation of a variety of shellfish. Despite the huge expansion in aquaculture, and the experiences gained in the United States and China of integrating kelp into sustainable marine farms, this farming methodology is still at an early stage of development.

Yet it seems inevitable that a new generation of ocean farming will build on the experiences gained in these enterprises to develop a method of aquaculture with the potential not only to feed humanity, but to play a large role in solving one of our most dire issues – climate change.

Globally, around 12 million tonnes of seaweed is grown and harvested annually, about three-quarters of which comes from China. The current market value of the global crop is between US$5 billion and US$5.6 billion, of which US$5 billion comes from sale for human consumption. Production, however, is expanding very rapidly.

Seaweeds can grow very fast – at rates more than 30 times those of land-based plants. Because they de-acidify seawater, making it easier for anything with a shell to grow, they are also the key to shellfish production. And by drawing CO₂
out of the ocean waters (thereby allowing the oceans to absorb more CO₂ from the atmosphere) they help fight climate change.

The stupendous potential of seaweed farming as a tool to combat climate change was outlined in 2012 by the University of the South Pacific’s Dr Antoine De Ramon N’Yeurt and his team. Their analysis reveals that if 9% of the ocean were to be covered in seaweed farms, the farmed seaweed could produce 12 gigatonnes per year of biodigested methane which could be burned as a substitute for natural gas. The seaweed growth involved would capture 19 gigatonnes of CO₂. A further 34 gigatonnes per year of CO₂ could be taken from the atmosphere if the methane is burned to generate electricity and the CO₂ generated captured and stored. This, they say:

…could produce sufficient biomethane to replace all of today’s needs in fossil-fuel energy, while removing 53 billion tonnes of CO₂ per year from
the atmosphere… This amount of biomass could also increase sustainable fish production to potentially provide 200 kilograms per year, per person, for 10 billion people. Additional benefits are reduction in ocean acidification and increased ocean primary productivity and biodiversity.

Nine per cent of the world’s oceans is not a small area. It is equivalent to about four and a half times the area of Australia. But even at smaller scales,
kelp farming has the potential to substantially lower atmospheric CO₂, and this realisation has had an energising impact on the research and commercial
development of sustainable aquaculture. But kelp farming is not solely about reducing CO₂. In fact, it is being driven, from a commercial perspective, by sustainable production of high-quality protein.

A haven for fish.
Daniel Poloha/

What might a kelp farming facility of the future look like? Dr Brian von Hertzen of the Climate Foundation has outlined one vision: a frame structure, most likely composed of a carbon polymer, up to a square kilometre in extent and sunk far enough below the surface (about 25 metres) to avoid being a shipping hazard. Planted with kelp, the frame would be interspersed with containers for shellfish and other kinds of fish as well. There would be no netting, but a kind of free-range aquaculture based on providing habitat to keep fish on location. Robotic removal of encrusting organisms would probably also be part of the facility. The marine permaculture would be designed to clip the bottom of the waves during heavy seas. Below it, a pipe reaching down to 200–500 metres would bring cool, nutrient-rich water to the frame, where it would be reticulated over the growing kelp.

Von Herzen’s objective is to create what he calls “permaculture arrays” – marine permaculture at a scale that will have an impact on the climate by growing kelp and bringing cooler ocean water to the surface. His vision also entails providing habitat for fish, generating food, feedstocks for animals, fertiliser and biofuels. He also hopes to help exploited fish populations rebound and to create jobs. “Given the transformative effect that marine permaculture can have on the ocean, there is much reason for hope that permaculture arrays can play a major part in globally balancing carbon,” he says.

The addition of a floating platform supporting solar panels, facilities such as accommodation (if the farms are not fully automated), refrigeration and processing equipment tethered to the floating framework would enhance the efficiency and viability of the permaculture arrays, as well as a dock for ships
carrying produce to market.

Given its phenomenal growth rate, the kelp could be cut on a 90-day rotation basis. It’s possible that the only processing required would be the cutting of the kelp from the buoyancy devices and the disposal of the fronds overboard to sink. Once in the ocean depths, the carbon the kelp contains is essentially out of circulation and cannot return to the atmosphere.

The deep waters of the central Pacific are exceptionally still. A friend who explores mid-ocean ridges in a submersible once told me about filleting a fish for dinner, then discovering the filleted remains the next morning, four kilometres down and directly below his ship. So it’s likely that the seaweed fronds would sink, at least initially, though gases from decomposition may later cause some to rise if they are not consumed quickly. Alternatively, the seaweed
could be converted to biochar to produce energy and the char pelletised and discarded overboard. Char, having a mineralised carbon structure, is likely to last well on the seafloor. Likewise, shells and any encrusting organisms could be sunk as a carbon store.

Once at the bottom of the sea three or more kilometres below, it’s likely that raw kelp, and possibly even to some extent biochar, would be utilised as a food source by bottom-dwelling bacteria and larger organisms such as sea cucumbers. Provided that the decomposing material did not float, this would not matter, because once sunk below about one kilometre from the surface, the carbon in these materials would effectively be removed from the atmosphere for at least 1,000 years. If present in large volumes, however, decomposing matter may reduce oxygen levels in the surrounding seawater.

Large volumes of kelp already reach the ocean floor. Storms in the North Atlantic may deliver enormous volumes of kelp – by some estimates as much as 7 gigatonnes at a time – to the 1.8km-deep ocean floor off the Bahamian Shelf.

Submarine canyons may also convey large volumes at a more regular rate to the deep ocean floor. The Carmel Canyon, off California, for example, exports large volumes of giant kelp to the ocean depths, and 660 major submarine canyons have been documented worldwide, suggesting that canyons play a significant role in marine carbon transport.

These natural instances of large-scale sequestration of kelp in the deep ocean offer splendid opportunities to investigate the fate of kelp, and the carbon it contains, in the ocean. They should prepare us well in anticipating any negative or indeed positive impacts on the ocean deep of offshore kelp farming.

The ConversationOnly entrepreneurs with vision and deep pockets could make such mid-ocean kelp farming a reality. But of course where there are great rewards, there are also considerable risks. One obstacle potential entrepreneurs need not fear, however, is bureaucratic red tape, for much of the mid-oceans remain a global commons. If a global carbon price is ever introduced, the exercise of disposing of the carbon captured by the kelp would transform that part of the business from a small cost to a profit generator. Even without a carbon price, the opportunity to supply huge volumes of high-quality seafood at the same time as making a substantial impact on the climate crisis are considerable incentives for investment in seaweed farming.

Tim Flannery, Professorial fellow, Melbourne Sustainable Society Institute, University of Melbourne

This article was originally published on The Conversation. Read the original article.


How climate change threatens to make our bread less tasty

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Increasing carbon dioxide is impacting some of our favourite foods.

Glenn Fitzgerald, University of Melbourne

Climate change and extreme weather events are already impacting our food, from meat and vegetables, right through to wine. In our series on the Climate and Food, we’re looking at what this means for the food chain. The Conversation

The concentration of carbon dioxide in our atmosphere is increasing. Everything else being equal, higher CO₂ levels will increase the yields of major crops such as wheat, barley and pulses. But the trade-off is a hit to the quality and nutritional content of some of our favourite foods.

In our research at the Australian Grains Free Air CO₂ Enrichment (AGFACE) facility, we at Agriculture Victoria and The University of Melbourne are mimicking the CO₂ levels likely to be found in the year 2050. CO₂ levels currently stand at 406 parts per million (PPM) and are expected to rise to 550PPM by 2050. We have found that elevated levels of CO₂ will reduce the concentration of grain protein and micronutrients like zinc and iron, in cereals (pulses are less affected).

The degree to which protein is affected by CO₂ depends on the temperature and available water. In wet years there will be a smaller impact than in drier years. But over nine years of research we have shown that the average decrease in grain protein content is 6% when there is elevated CO₂.

Because a decrease in protein content under elevated CO2 can be more severe in dry conditions, Australia could be particularly affected. Unless ways are found to ameliorate the decrease in protein through plant breeding and agronomy, Australia’s dry conditions may put it at a competitive disadvantage, since grain quality is likely to decrease more than in other parts of the world with more favourable growing conditions.

Increasing carbon dioxide could impact the flour your bread.

Food quality

There are several different classes of wheat – some are good for making bread, others for noodles etc. The amount of protein is one of the factors that sets some wheat apart from others.

Although a 6% average decrease in grain protein content may not seem large, it could result in a lot of Australian wheat being downgraded. Some regions may be completely unable to grow wheat of high enough quality to make bread.

But the protein reduction in our wheat will become manifest in a number of ways. As many farmers are paid premiums for high protein concentrations, their incomes could suffer. Our exports will also take a hit, as markets prefer high-protein wheat. For consumers, we could see the reduction in bread quality (the best bread flours are high-protein) and nutrition. Loaf volume and texture may be different but it is unclear whether taste will be affected.

The main measure of this is loaf volume and texture, but the degree of decrease is affected by crop variety. A decrease in grain protein concentration is one factor affecting loaf volume, but dough characteristics (such as elasticity) are also degraded by changes in the protein make-up of grain. This alters the composition of glutenin and gliadin proteins which are the predominant proteins in gluten. To maintain bread quality when lower quality flour is used, bakers can add gluten, but if gluten characteristics are changed, this may not achieve the desired dough characteristics for high quality bread. Even if adding extra gluten remedies poor loaf quality, it adds extra expense to the baking process.

Nutrition will also be affected by reduced grain protein, particularly in developing areas with more limited access to food. This is a major food security concern. If grain protein concentration decreases, people with less access to food may need to consume more (at more cost) in order to meet their basic nutritional needs. Reduced micronutrients, notably zinc and iron, could affect health, particularly in Africa. This is being addressed by international efforts biofortification and selection of iron and zinc rich varieties, but it is unknown whether such efforts will be successful as CO₂ levels increase.

Will new breeds of wheat stand up to increasing carbon dioxide?

What can we do about it?

Farmers have always been adaptive and responsive to changes and it is possible management of nitrogen fertilisers could minimise the reduction in grain protein. Research we are conducting shows, however, that adding additional fertiliser has less effect under elevated CO₂ conditions than under current CO₂ levels. There may be fundamental physiological changes and bottlenecks under elevated CO₂ that are not yet well understood.

If management through nitrogen-based fertilisation either cannot, or can only partly, increases grain protein, then we must question whether plant breeding can keep up with the rapid increase in CO₂. Are there traits that are not being considered but that could optimise the positives and reduce the negative impacts?

Selection for high protein wheat varieties often results in a decrease in yield. This relationship is referred to as the yield-protein conundrum. A lot of effort has gone into finding varieties that increase protein while maintaining yields. We have yet to find real success down this path.

A combination of management adaptation and breeding may be able to maintain grain protein while still increasing yields. But, there are unknowns under elevated CO₂such as whether protein make-up is altered, and whether there are limitations in the plant to how protein is manufactured under elevated CO2. We may require active selection and more extensive testing of traits and management practices to understand whether varieties selected now will still respond as expected under future CO₂ conditions.

Finally, to maintain bread quality we should rethink our intentions. Not all wheat needs to be destined for bread. But, for Australia to remain competitive in international markets, plant breeders may need to select varieties with higher grain protein concentrations under elevated CO2 conditions, focusing on varieties that contain the specific gluten protein combinations necessary for a delicious loaf.

Glenn Fitzgerald, Honorary Associate Professor of Agriculture and Food, University of Melbourne

This article was originally published on The Conversation. Read the original article.

Farming in 2050: storing carbon could help meet Australia’s climate goals

Brett Anthony Bryan, CSIRO

Australia’s agricultural lands help to feed about 60 million people worldwide, and also support tens of thousands of farmers as well as rural communities and industries.

But a growing global population with a growing appetite is placing increasing demands on our agricultural land. At the same time, the climate is warming and in many places getting drier too.

Agriculture, and particularly livestock, is currently a major contributor to greenhouse gas emissions. But new markets and incentives could make storing carbon or producing energy from land more profitable than farming, and turn our agricultural land into a carbon sink.

How might these competing forces play out in changing Australian land use? Our research, published in Global Environmental Change, assesses a range of potential pathways for Australia’s agricultural land as part of CSIRO’s National Outlook.

Changing landscapes

The only constant in landscapes is change. Ecosystems are always changing in response to natural drivers such as fire and flood.

Humans have complicated things. Indigenous Australians manipulated the Australian landscape and climate through burning for millennia, sustaining a population of around 750,000 and underpinning a culture.

European colonisation brought a different and more pervasive change, clearing land, building cities, damming rivers and establishing an increasingly mechanised and industrialised agriculture.

These iconic but changed landscapes inspired the romantic art of Arthur Streeton and poetry of Banjo Paterson among many others — and helped forge a young nation’s identity.

‘Still glides the stream, and shall for ever glide’, 1890. Arthur Streeton. The Art Gallery of NSW describes the painting as ‘an idealised vision of the Yarra River at Heidelberg, with the Doncaster Tower in the middle distance and the Dandenong Ranges beyond’.

Change can happen surprisingly quickly. Often before we know it we’ve gone too far and need to scramble for fixes that are so often costly, slow and ultimately inadequate.

For example, in South Australia, researchers in the early 1960s raised the alarm that the feverish post-war period of soldier resettlement, land clearance and agricultural development threatened entire native plant and animal communities with extinction. The government’s response over the following 30 years was to expand greatly the conservation reserve network and eventually prohibit land clearing.

History repeating?

Agricultural lands produce a range of goods and services. But in many places the focus on agricultural productivity has come at the expense of ecosystems. Biodiversity, soil and water are all on downward trends.

Is the balance right? Opinion varies. Many would say no, and consider the status quo to be stacked strongly against the environment.

Others see agriculture as entering a boom time, driven by growing population and rising food prices. Substantial interest from overseas investors in Australian agricultural land reflects this opportunity.

Parts of Australia’s agricultural land continue to change fast. Lessons hard-learned by South Australia seem to have been forgotten. Rates of land clearance in Queensland are rising again since 2010 after a long-term trend of decline.

In the 1990s, new financial incentives led to the planting of over 1 million hectares of forest in southern Australia. Now a failed business model, many of these plantations are being returned to agriculture.

Demand for more secure sources of energy has generated rapid expansion of coal seam gas and wind power generation, and the development of northern Australia remains a bipartisan priority.

Worldwide, Australia is not alone — many international examples also exist of recent, massive, rapid and accelerating changes in how land is used.

Australia has historically taken a hands-off approach to managing land use change, instead focusing on increasing the productivity and competitiveness of agriculture. Apart from a handful of planning and environmental regulations, the use of land has been subject to minimal governance or strategic direction.

Where to from here?

What is it that Australians really want from our land? We know what we don’t want: wall-to-wall crops, pasture, buildings, gas wells, mines, wind farms or trees.

We can expect healthy debate around the margins, but, in general, diversity, productivity and sustainability seem to be widely valued. Most of us want to leave the place in decent condition for future generations.

Europe has had this conversation and knows what it wants from its landscapes — and it’s not afraid to pay for it (for instance, through agricultural subsidies). A deep aesthetic and cultural heritage is the central objective, with a balance of recreation opportunities, tourism, a clean and healthy environment and high-quality produce all being high priorities.

Once we know what we want, we can work out how to get there.

That’s where science can help. We now have the ability to project changes in land use in response to policy and global change, and the environmental and economic consequences.

CSIRO’s recent National Outlook mapped Australia’s potential future pathways. A companion paper in Nature found that it is possible to achieve strong economic growth and reduce environmental pressure, if we put the right policies in place now. It provides a glimpse of how our rural lands might respond to coalescing future change pressures.

Farming carbon

In our modelling, carbon sequestration in the land sector plays a key role of Australia’s future. Land systems can help with the heavy lifting required to hold global warming to 2℃ as recently agreed in Paris.

There are several factors that could drive this change, including climate, carbon pricing, global food demand and energy prices.

We modelled the economic potential for land use change and its impacts in over 600 scenarios (full data available here), combining a suite of global outlooks and national policy options.

A carbon price, which enables landholders to make money from storing carbon in trees and soils (often much more money than from farming), may increase pressure to shift farmland to restored forests.

Who knows? A pay rise while watching trees grow could be an attractive proposition for our ageing farmers. Complementary biodiversity payments could also help arrest declines in wildlife and help it adapt to climate change.

If we redouble our focus on productivity, by 2050 agriculture will produce more than today, even as farmland contracts. The least productive areas are less able to compete with reforestation and other new land uses, leaving the most efficient agricultural land in production.

But trade-offs are likely. Trees use a lot more water than crops and pasture, so we will need to think carefully about managing water resources.

Economic potential for land use change and sustainability impacts from 2013 to 2050 under national global environmental and economic conditions consistent with 2℃ warming by 2100

Australians care about their land and are more aware than ever about what is happening to it. While we can have some control over the future of our land, and we do exercise this control in certain circumstances (such as urban planning), our long-term approach to rural land has been to let environmental and economic forces play out and let the invisible hand of economics determine what will be.

Given the pace at which change can happen, a smarter approach will be to start the conversation, work out what it is we want from our land, and put the policies and institutions in place to get us there.

The Conversation

Brett Anthony Bryan, Principal Research Scientist, Environmental-economic integration, CSIRO

This article was originally published on The Conversation. Read the original article.

Goodies v baddies? Why labelling wild animals as ‘pests’ or ‘friends’ is holding farming back

Manu Saunders; Gary Luck; Rebecca Peisley, and Romina Rader, University of New England

It’s hard to keep wild animals out of farms. Birds, mammals and insects all affect crop yields, in positive ways (such as flies pollinating flowers) and negative ones (such as when birds damage fruit).

Agricultural research and management programs often deal with these interactions by focusing on simplistic “good” and “bad” labels: aphids are annoying pests, for example, whereas bees are little angels.

In reality, however, no animal is 100% a “goodie” or “baddie” – their effects on crop production vary with context. Interactions between animals and crops are influenced by seasons, landscapes, management practices, and other animals. They can also be affected by the social, cultural and economic values of the local farming community. The same species can be “good” in one system and “bad” in another.

It sounds complicated, because it is. But this is where ecological research can help. Understanding the interplay between these factors will help ensure that farms can protect wildlife while also providing us with food and other resources.

Good versus bad?

When we reviewed 281 papers that evaluated increases or reductions in crop yields due to wild birds or insects on farms, we found that the binary view of “good” and “bad” animals is still widespread.

Of the studies we looked at, 53% (mostly in the agricultural sciences) focused on identifying and managing the “baddies”, by weighing up costs that animals create for farmers by damaging crops. Another 38% (mostly ecology and conservation studies) calculated the impact of the “goodies”: benefits such as pollination and pest control. Only 9% of the studies we reviewed considered both costs and benefits in a single system.

This shows that most scientific studies are still taking an approach that is too simplistic. Attempting to link increases or reductions in crop yields with a single pest or helper species doesn’t usually tell the whole story. It doesn’t tell us about other factors that influence crop yields, like seasonal changes in animal activity, effects of different management practices, or interactions between different animal species.

Because so many studies have focused on quantifying the effect of one group of animals (such as bees), or focused on effects at one crop development stage (for example, using fruit set as an indicator of pollination efficiency), the overall body of knowledge on how wild animals affect crops has become disjointed and sometimes contradictory.

Moving forward

In a second paper, we suggest a new way to address these complex issues that considers the social and environmental contexts of crop production across the entire growing season. By looking at the interplay between the various positive and negative effects, we can gain a more realistic estimate of how crop yields are affected by wild animals.

Here’s an example. In Australian almond orchards, native birds are often considered pests because they can cause crop losses by pecking at developing fruit. But after harvest has finished, the same birds also remove the decaying “mummy” nuts left on trees. Growers sometimes use paid manual labour to remove these nuts, because they harbour disease and pests that can damage the trees.

A cost-benefit analysis of shows that the positive economic value of the birds cleaning up the mummy nuts outweighs the cost of crop losses from damaged almonds. Averaged across the entire plantation, the presence of the birds is a net positive for farmers. This means that letting birds do their thing could be more cost-effective for growers than deterring the birds and then paying people to remove the mummy nuts. But without this cost-benefit approach, it easy to imagine how farmers would persist in viewing the birds as crop pests and shooing them away.

Very few studies have considered how wild animals create this type of cost-benefit trade-off in farming ecosystems. Yet this approach is central to the study of ecology, and there are obvious parallels between natural and agricultural systems. Both, for instance, have pollination and pest control as key functions.

Farms are ecosystems too. So we need to find a way to maintain sustainable crop production while also protecting biodiversity and ecosystem function. Doing this means moving beyond simplified systems and intensive production.

Productive farms have complex cycles of interactions between crops, wild animals and people. These cycles need to be sustained, not isolated from the system. As with any ecosystem, understanding is the first step towards protection.

The Conversation

Manu Saunders, Post-doctoral Research Fellow (Ecology); Gary Luck, Professor in Ecology and Interdisciplinary Science; Rebecca Peisley, PhD Candidate, Institute for Land, Water and Society, and Romina Rader, Lecturer in Community Ecology, University of New England

This article was originally published on The Conversation. Read the original article.

How shrimp farming wreaked havoc on Sri Lanka’s coasts

Mark Huxham, Edinburgh Napier University

We are not far from the ocean here. The air smells of salt and sulphur, of marine life. But the square of black, cracked mud in front of us, bounded by its four crumbling walls of sand, is no place for living things. It was previously a pond for cultivating tiger prawns, the lucrative species that was the reason for cutting the lush mangrove forest that once covered this area. The recent history of this abandoned place is sadly representative of the story of thousands of hectares in this region in the west of Sri Lanka.

A swelling appetite for shrimps and prawns in America, Europe and Japan has fuelled industrial farming of shellfish in the past few decades. The industry now has a farm-gate value of $10bn (£6.4bn) per year globally and the prawn in your sandwich is much more likely to have come from a pond than from the sea. While the industry is dominated by the likes of China, Vietnam and Thailand, a large number of other countries have invested heavily in cultivation too.

One is Sri Lanka, which saw the industry as a passport to strong economic growth and widespread employment. Just outside the world’s top ten producers, it accounts for approximately 50% of the total export earnings from Sri Lankan fisheries. More than 90% of the harvested cultured prawns are exported, going mostly to Japan.

Yet the picture is decidedly mixed on a closer inspection. The country saw an explosion of unregulated aquaculture on the island in the 1980s and 1990s, bringing riches to a few and the hope of riches or at least an income to many more. But poor coastal management also brought white spot syndrome virus, a virulent disease that spreads in water and on the feet of birds, and can kill all the prawns in a pond in under a week.

Abandoned shrimp farm in western Sri Lanka
Estibaliz Diaz Tejedor

Crowding shrimp together in warm little pools full of nutrients creates the perfect conditions for an outbreak. It contributes to the fact that here and elsewhere in the tropics, most intensively farmed ponds remain productive for only five to ten years (the other main reason is the build-up of an organic ooze, rich in uneaten food and prawn faeces). Such ponds are then abandoned in favour of new areas of wetland to convert for another brief harvest. The disease kills off prawns in the wild in large numbers too.

A bird’s-eye view

To get a sense of how bad the problem has been in Sri Lanka, I was one of a group of researchers who studied the Puttalam area on the west coast, one of the first in the country where large-scale aquaculture was introduced.

Google Maps

We looked at satellite imagery from 1992 to 2012, which showed an explosion in prawn farms from less than 40ha in our study area to over 1,100ha (a rise of over 2,700%). This combined with a decline in natural habitats – mangroves lost some 36% of their area over the period. Yet most of these historic ponds are now unproductive or abandoned.

The evidence from the satellite images combined with interviews with local people suggest that a staggering 90% of ponds are lying idle. The story is unlikely to be quite as bad across the country as a whole, since Puttalam was one of the early areas to be cultivated. Detailed figures are thin on the ground, but certainly overall shrimp exports in 2012 were 65% below their 1999 peak.

Prawn aquaculture has been likened to slash-and-burn cultivation – find a pristine spot, remove the vegetation and farm it for a few years before moving on. But the analogy is misleadingly benign. Slash-and-burn systems on a small scale can be sustainable, since the cut plots can recover afterwards.

Don’t mess: mangroves
Wikimedia, CC BY-SA

In the case of prawn farming, a better phrase would be “slash and sink”. Mangroves are among the most carbon-dense of all ecosystems, often storing more than 2,000 tonnes of carbon per hectare in sediments beneath the forest floor, according to research that our group has yet to publish. Cut them down and this carbon is oxidised and emitted into the atmosphere as CO2.

We estimate that nearly 192,000 additional tonnes of carbon have been added to climate change as a result of these land-use changes in Puttalam, Sri Lanka alone. And that of course excludes any emissions during farm operations and the potential for the lost mangroves to capture carbon in future.

An additional issue is the sinking shoreline. In the face of global rising sea levels of more than 3mm a year, healthy mangrove forests are among the best protection since they bind together sediments and even elevate their soils to match the rising tide. Lose them and the chances of coastal subsidence, erosion and storm damage goes up.

In fact, mangroves are such useful ecosystems that destroying them almost never makes sense, even from a narrow economic perspective. A recent analysis in southern Kenya showed that conserving and restoring the forests was worth at least $20m more in present value than allowing current cutting to continue.

So what about Sri Lanka? A positive recent development was that the government announced that it would protect all of its remaining mangroves, totalling some 8,800 hecatares. It also promised to replace a further 3,900 – a task that will require careful restoration of the right tidal conditions and planting trees where necessary. Another positive sign is that there are now local movements that are coordinating production among zones and farms to avoid disease and achieve better sustainability. This is on the back of a commitment by the government in 2010 to expand the industry.

The country should also look to return some of its abandoned ponds to production, provided producers are supported to adopt best practice and work together to avoid disease outbreaks and pollution in future. As for us in the West who import these shellfish in vast quantities each year, we need to think harder about the real costs of that cheap prawn sandwich. Without knowing where it has come from and what farming practices have been used, we would do well to steer clear.

The Conversation

Mark Huxham, Professor of Teaching and Research in Environmental Biology, Edinburgh Napier University

This article was originally published on The Conversation. Read the original article.