Acidifying oceans and the future of molluscs: implications for food security?

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Some of the threats posed by climate change can appear rather esoteric or abstract. One of these is ocean acidification - it is not immediately obvious why we should care. A recent paper by Sara Cooley and colleagues give a good example of why the threat of changing ocean chemistry matters.

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Some of the threats posed by climate change can appear rather esoteric or abstract. One of these is ocean acidification - it is not immediately obvious why we should care. A recent paper by Sara Cooley and colleagues give a good example of why the threat of changing ocean chemistry matters.

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In my last blog I pointed out that most of the food produced by farming the sea is molluscs. This represents about two-thirds of the worlds global mollusc production, to which we must add the supply that comes from wild fisheries. Not only are these supplies important for those of us in developed countries who enjoy the luxury of eating oysters and mussels, they are also critical to the income, food and nutrition security of many, particularly, in Southeast and East Asia.

That shellfish farming and fisheries production might be under threat from climate change may be a surprise to lay readers, but not to marine chemists, who have shown in both laboratory and field studies that changes in atmospheric CO2 lowers the pH of the oceans and that this increased acidity makes a crucial difference for animals that build hard shells or skeletons. These animals find it difficult because, when seawater becomes more acidic, the concentrations of carbonate fall. That means that the animals have to spend more energy to take the chemical up and deposit shell or skeleton. This in turn means they have less energy to devote to growth and reproduction [1,2].

And this doesn't just mean that these animals simply grow a little less - it can also reduce the size of populations. Analysis of a long-term data set from the US, for example, has shown how mollusc populations have changed in response to fluctuations in pH [3].

Well that's all very well, but is the size of the likely impact really something we should worry about? That's the question Sarah Cooley and her colleagues asked in a recent paper where they examine how climate driven changes in ocean acidification might affect the long-term prospects for global mollusc production and the likely impact of these changes on food security [4].

The approach they took was to first look at today's mollusc production, consumption and export, and ask how vulnerable producer countries are to changes in production and how easily they could adapt and cope with changes. They then looked at present and future surface ocean chemistry forecasts and identified transition decades for different regions of the world when future ocean chemistry will be distinctly different from today. The basic idea is that countries with low adaptability, high nutritional or economic dependence on molluscs, rapidly approaching transition decades, or rapidly growing populations, will be most vulnerable to any declines in mollusc supply that are driven by ocean acidification.

Several important predictions arise from this work. The first is that some of the regions in which demand for molluscs is likely to rise the most are also regions where future acidification effects are likely to come soonest. The countries of the Western Central Pacific and the North Eastern Pacific (USA and Canada) fall into this category.

Of course, countries like the USA and Canada will no doubt be able cope with these effects, albeit perhaps at some cost to the aquaculture industry and to consumers. In contrast, and more worryingly, the authors also found that many of the countries that are most vulnerable and least able to adapt to mollusc declines - the countries of south and south east Asia and east Africa - are also in regions where transition decades are likely to arrive early. Some of these can expect problems within the next 15 years and will need to start thinking about coping strategies.

Among the strategies that these vulnerable countries can adopt, supporting local people to invest in viable mollusc aquaculture enterprises may be one of the most fruitful. They can do this, for example, by ensuring appropriate planning and environmental policies are in place, and providing technical support and extension services, infrastructure to connect producers to markets and access to credit.

Encouraging local mollusc farming in these countries is a promising strategy for two reasons. First, it removes dependency on the unpredictable and uncontrollable fluctuations that are likely to occur in wild populations. Second, it offers prospects for increasing overall supply that do not exist for fully exploited wild fisheries. Third, it requires relatively little capital investment, and is a viable approach for diversifying livelihoods for poor people. As an added bonus, mollusc culture is among the most environmentally benign forms of food production, relying as it does on the natural productivity of the ocean to supply the food on which they grow.

For some countries, especially those in the developed world, there is also the potential to tap into opportunities for innovation that culture based production allows. It is perfectly reasonable, for example, to expect that we might develop cultured strains that are less susceptible to acidification or to develop means for locally modifying water chemistry.

This kind of study is important because it seeks to explicitly and quantitatively link effects that are somewhat removed from our immediate concern - the thickness of a mollusc's shell - to much more concrete effects on human well-being  - how much food will be available. Making these connections is essential for developing the evidence base to help support policy dialogue and decision-making.

Of course, ocean acidification is only one of several stressors acting on marine ecosystems. Nutrient run-off from the land, increases in water temperatures, pollution from coastal developments, changes in freshwater run-off patterns -  all of these are likely to change in the coming years and affect the food supplies from our coastal waters. But fortunately, many of the coping strategies I described above are broadly similar no matter what the major stressor turns out to be, or where you live. So whether it's oysters for that special occasion, or a clam stew to feed the family, the answers are there - we just need to act.

[1] Talmage, SC, Gobler, CJ (2009) The effects of elevated carbon dioxide concentration on the metamorphosis, size, and survival of larval hard clams (Mercenaria mercenaria), bay scallops (Argopecten irradians), and Eastern oysters (Crassostrea virginica). Limnology and Oceanography 54, 2072-2080

[2] Miller, AW Reynolds, AC Sobrino, C, Reidel, GF (2009) Shellfish face uncertain future in high CO2 world: influence of acidification on oyster larvae calcification and growth in estuaries. PLoS ONE 4, e5661

[3] Wootton, JT Pfister, CA, Forester, JD (2008) Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset. Proceedings of the National Academy of Sciences 105, 18848-18853

[4] Cooley, SR Lucey, N Kite-Powell, H, Doney, SC (2012) Nutrition and income from molluscs today implies vulnerability to ocean acidification tomorrow. Fish and Fisheries 13, 182-215