The battle against the Ninja Slug

Entrepreneur Jean Dhooge comes from a family of mussel and oyster farmers in the village of Yerseke in the province of Zeeland who have been plying their trade in the face of the vagaries of nature for 115 years. Today, one of hazards they face is the Japanese oyster borer, a Japanese intruder that was first discovered in Zeeland’s waters just over ten years ago.

“Initially, it was found in just one area,” says Dhooge. “But like the coronavirus, the sea snail spread throughout the oyster farms.” The entire oyster sector in Zeeland suffered a rapid decline in their yields. Dhooge: “In some beds, the oyster mortality rate rose to eighty percent or more. It was a disaster.” Action was needed. Unless something was done, companies could be bankrupted or, even worse, the entire industry could go under.

Twelve of Zeeland’s oyster farms – including Dhooge’s – saw in HZ University of Applied Sciences a partner to help tackle the problem. “We often commission research from the university,” says Dhooge. “The researchers are practical people who come up with constructive ideas and communicate well. Our role as oyster farmers is confined to sharing our knowledge and reporting the results of tests. The collaboration is excellent.”

HZ’s experts went to work in 2018, starting with a survey of studies carried out in other countries affected by the problem, including France and the United States, to learn whether they had been able to find a solution. They hadn’t. The researchers also studied the creature’s behaviour and discovered that the snail travelled about 2.2 metres over the sea floor every day, was mainly attracted to baby oysters – which have thin shells – and also feasted on mussel seed.

Armed with these findings, the researchers began the search for solutions together with the oyster farmers. They tested whether the oysters could be protected by erecting a barrier with mussel seed. The oyster borer could then satisfy its hunger with these small shellfish and would stay away from the oysters. Although it was a promising idea, it proved difficult to apply in practice, says Dhooge. “It only works if the seabed on which the oysters are growing is entirely denuded of oyster borers, which is almost impossible to achieve because they lie beneath the sand and are hard to find.”

The researchers also tried a method that involves preventing the eggs of the oyster borer from hatching. Submerging them in acetic acid, quicklime or an ultrasonic bath seemed to at least slow that process. But more research is needed to make the method feasible in practice. However, a technique that does work is to cultivate oysters on trestles raised above the sea floor.

Dhooge: “Oyster borers live on the seabed, so we now breed our baby oysters in baskets on trestles. After a year, when the shell is thick enough to protect them from the oyster borer, they are moved to the seabed to grow further. This method reduces the numbers of oysters lost to manageable proportions.”

The raised farming method has one drawback: the cultivation area is far smaller than on the seabed. Dhooge: “There are no restrictions with the bottom culture of oysters, since it doesn’t disturb anyone. With off-bottom cultivation, the trestles can form an obstacle for recreational and commercial vessels.” Nevertheless, for the time being it is the most effective solution, according to Dhooge.

“With HZ, we are now investigating the optimal growing conditions for the oyster. We are also testing floating systems for automatically shaking the baskets, which is now done manually. The shaking prevents the oysters from being overgrown with seaweed and slows the oyster’s growth. Consequently, the oyster is meatier with a more succulent bite.