Learning to live with the oyster drill

Keep away, avoid or combat?

Paul Vader, editor HZ Discovery

Photo: oyster drill drilling on an oyster shell

In recent years, oyster farming in Zeeland has increasingly been affected by damage from the Japanese oyster drill (Ocinebrellus inornatus). This predatory snail acts according to its name by drilling a hole in the oyster shell, killing the oyster and consuming the meat. Other shellfish such as mussels also fall prey to the oyster drill. The losses in yield are now such that traditional oyster farming at the bottom of the Oosterschelde is in danger. Can the tide be turned or should the farmers forget about cultivating on the sea bed and continue oyster farming on tables? The Aquaculture research group completed a study this summer aiming at finding ways to limit the predation pressure of the oyster drill. In any case, it is clear that the Pacific oyster drill, which was probably introduced into Dutch waters through shellfish transport, is here to stay. But what options do growers have for learning to live with this oyster predator?

The most obvious measure to control the oyster drill is to collect them through fishing. The growers know from experience that the oyster drills return when oysters are sown on a clean plot. However, not much is known about the speed at which this happens, because relatively little can be found in literature about the biology of the oyster drill. The researchers have therefore mapped a number of properties of this species, such as the speed at which they move and to what extent wind, currents and the presence of food play a role in this. They also found that the snails prefer a hard surface. Clean fishing and then regularly keeping the edges free from oyster drills is one of the recommendations for oyster farmers.

Predator’s choice It is known from the literature that oyster drills have a preference for smaller oysters (9 to 76 mm), the so-called oyster brood. The HZ researchers showed through field experiments in the Oosterschelde that the Japanese oyster is more predatory than the flat oyster. The oyster drill uses chemical signal substances in the water to locate its prey. A series of experiments in the laboratory showed that a small majority of oyster drills choose the ‘scent’ trail of oysters over that of mussels. Field experiments showed that mussels are also eaten, although the predatory snails have a slight preference for oysters.

Mussel edges A potentially successful method to keep the oyster drill away from the farming plots is to erect physical barriers in the form of mussel seed edges (small mussels) around marketable oysters. Extensive experiments have been conducted with this in the Yerseke oyster pits. The researchers found, not unexpectedly, that the widest edges (1.5 m) slowed the oyster drills the most in successfully passing the edge. It matters whether the edge has been in place for a certain time. Over time, the mussels form a layer of silt that the oyster drills prefer to avoid, whereby an older mussel edge better protects the oysters than a fresh one. The mussel edge works in three parts: it forms a physical barrier as described above, the mussels form an alternative prey for the oyster drills and the presence of the mussels ensures that the predatory snails no longer ‘smell’ their favourite prey. Which of the three plays the largest role has not become clear from the experiments. A practical objection to this solution is that the regulations currently do not allow mussels to be sown on oyster plots.

Recommendations for growers In September, the final symposium of this project financed by RAAK SIA was held in the Oesterij in Yerseke. The main recommendations that researchers Eva Hartog and Tony van der Hiele presented to the growers were:

  • cleaning plots before sowing the oyster brood and regularly cleaning the edges afterwards;
  • growing small oysters on tables out of reach of the oyster drills and then allowing them to grow further on bottom plots;
  • further research into the use of mussel edges as protection against predation and the possible changes in regulations that this may require.

The main recommendations of the project Learning to live with the oyster drill are described in four fact sheets. Further information and the fact sheets can be found on the Delta Expertise-site