Live oysters – delicacy or invasive species carrier?

Published on
January 11, 2016

Are Australian oysters a bio-security risk for Japan?

Bill Court, a Tokyo-based importer of fresh bluefin tuna, thinks so. “Prior to consumption in Japan, Australian oysters may be put in the sea or into holding systems in Japan, such that there is a strong possibility that the Australian oyster would transmit any diseases or parasites they might contain to oysters in Japan,” he said.

Live oysters are recently being promoted by Australia in an effort to get more value for the producers. Currently only 3 percent of Australian production, about half a million oysters, is exported—over 80 percent of this to Hong Kong, Singapore and Japan, according to industry organization Oysters Australia.

Much of this export product is frozen raw in the half-shell, but some see more scope to add value in the live export segment, selling a dining experience (often with wine pairing), rather than just a product. For example, a 2014 report by Ben Ralston, a New South Wales oyster farmer and Nuffield Australia Farming Scholar, suggests increasing live exports while easing the current requirement for an approved specialized packing room and periodic audits, which, he says, add to running cost and red tape for producers.

The trouble is that movement of live oysters, including spat, has been responsible for spreading many invasive species and diseases around the world.

From Australia, the main threat is Pacific Oyster Mortality Syndrome (POMS), caused by the ostreid herpesvirus-1 (OsHV-1) micro variant. Disease outbreaks have occurred in three water systems in the country: the Georges, Parramatta and Hawkesbury Rivers, and the virus has been detected in Brisbane Water. It kills oysters at temperatures above 16 degrees Celsius (61 degrees Fahrenheit), with typical mortality rates of 75-100 percent for spat and juveniles, 30-50 percent for the intermediate stage and about 10 percent for the very mature stage.

Japanese law puts the inspection responsibility on the exporting country, since it is difficult in practice for an importing country to know where an oyster comes from. Oysters for raw consumption must be accompanied by a certificate attesting that the oysters have been collected from a designated water area meeting Japanese microbiological criteria.

These criteria mainly relate to control of norovirus. However, Australia has also placed a quarantine on movement of live oysters from the affected areas, as well as issuing guidelines on disinfection of equipment that is moved from affected areas. The risk is that fresh outbreaks may not be detected before newly affected oysters are exported. Live oysters, if re-watered for holding, can provide a route for further spread of OHV-1.

But OHV-1 is not the only threat from movement of oysters, and Australia is not the only source of concern. A 2014 study partially funded by the European Commission, titled “Oyster imports bring alien ‘hitchhikers’ and disease,” found that 48 invasive species “very likely” arrived in Europe with oyster imports from the north-west Pacific. Among them, 32 algae and 11 invertebrates were commonly found on oysters while remaining species were oyster pathogens. This study was focused on spat imports rather than food oysters, but it does beg the question: Hadn’t we better leave the shells behind?

Contributing Editor reporting from Osaka, Japan

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