6 DECEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG Editor’s Note I’m sure every aquaculturist in the world would agree that growing aquatic animals in water characterized this way is undesirable, but this is how the quality of water in future oceans is being described in response to stress from the triple threat of warming, acidification, and deoxygenation. The combination of factors can narrow water quality tolerance limits or increase sensitivity to pH or dissolved oxygen, affecting growth and survival, especially for sensitive larval and juvenile life-stages. There is only recent appreciation for the interaction among factors that combine to produce oceanic “hot spots” of vulnerability. A recent report summarized findings and provided recommendations to policy makers from the Third Symposium on the Ocean in a High-CO2 World, and the implications for coastal aquaculture and food security are profound. The oceans are currently absorbing about one-quarter of CO2 emitted each year but the capacity of the oceans to absorb additional CO2 is expected to decrease as the oceans become more saturated with the gas. Acidity of the oceans has increased by 26 percent, with pH declining from 8.2 to 8.1, since the start of the Industrial Revolution. If CO2 emissions continue at the current rate, pH is expected to decrease to 7.8 by 2100. Molluscan shellfish are among the groups of organisms most sensitive to ocean acidification because of the direct and corrosive effect of acidic conditions on the capacity to form shells and skeletons. This is particular true for the sensitive early life stages and problems are most common and apparent with larval development. Reduced calcification in juveniles and adults, resulting in reduced growth and survival, represents a clear and present danger to the future of global shellfish aquaculture. An article on the Slate website earlier this year asked the question “Are oysters doomed?” Perhaps not yet, but they are certainly threatened. The contribution of shellfish to overall global production is prominent. According to the FAO, 14.4 million metric tons of mollusks were produced in 2011, or 23 percent of total global aquaculture production. Ocean acidification also threatens the capacity of cultured shellfish to provide nutrient extractive and waste treatment ecosystem services that preserve and enhance the quality of coastal waters. Off the west coasts of Africa, South America and North America, there are natural upwelling areas where deep, naturallyacidic water upwells onto continental shelves and into coastal areas. The stratification boundary between undersaturated deep waters and saturated shallow water is progressively moving towards the ocean surface, increasing the frequency and extent of upwelling events. In upwelling areas, where waters are naturally more acidic, acidification, warming and deoxygenation interact synergistically to a greater extent than elsewhere. One such upwelling phenomenon has affected Taylor Shellfish, the largest shellfish grower in the US with about 5,000 ha of production areas. Their Whiskey Creek Shellfish Hatchery is the largest oyster seed producer in the region, supplying 75 percent of all farms. In 2007-2008, upwelling caused losses of 6080 percent. Natural sets of oyster in Wallapa Bay, a major growing area, have failed or been paltry since 2005. In 2010, the company recognized that the problem was related to upwelling of acidic waters. The company has responded with short-, medium- and longterm approaches. In the short-term, to manage around the problem, hatchery practices were modified so that more larvae are produced when water quality is good (pCO2 < 400 ppm) and operations are curtailed when water quality is bad. Sodium bicarbonate was added to hatchery water to increasing buffering capacity. The depth of hatchery intakes was changed to withdraw water at 10 m rather than 30 m. This approach can avoid deep acidic water but increases the risk associated with harmful algal blooms. A monitoring program for CO2 and pH is key to inform short-term management decisions. In the medium-term, it may be possible to select for more resilient shellfish through a genetic improvement program, although the acidic selection environment is likely to be increasingly harsh. One of the problems with the effect of ocean acidification on shellfish aquaculture is that it is an assault that is superimposed on already acidic conditions in estuaries. Runoff and pollutants, especially from agriculture, are localized sources of acidification. The decomposition of organic matter in estuaries produces carbon dioxide, so increased organic matter in runoff can exacerbate the problem with ocean acidification. Estuaries are less buffered than seawater so the effects of ocean acidification are felt more profoundly there. Estuaries and coastal bays are also the locations for most shellfish aquaculture in the world and these productive zones are most vulnerable. Adding crushed shells to growing beds is one potentially effective approach to counteract the effects of localized, nearshore acidification. As is the case with many other effects of climate change, the most vulnerable human populations are located in coastal areas of developing countries, where there is a high dependence on marine resources for protein and livelihoods. How can management practices be modified to avoid impacts of ocean acidification and other stressors? In the long-term, reducing CO2 emissions is essential, although the effects of current CO2 levels are likely to be felt for many decades to come, even if CO2 emissions were stabilized at current levels. In the short to medium term, seawater monitoring programs, hatchery relocation and shell planting programs can be effective to work around the problem. Given current assaults on ocean health, growers around the world may be forced to become more active in defending their threatened livelihoods. Scientists too must stand up and be heard. — John A. Hargreaves, Editor-in-Chief “Hot, Sour and Breathless”
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