Royal Netherlands Institute for Sea Research

Problems with fyke nets

The fyke nets are attached to stakes. These wooden stakes are attractive to the naval shipworm (Teredo navalis), which drill holes in wood (in our case oak poles) in search of shelter. The shipworm is not actually a worm, but a type of shellfish.

These wooden stakes are attractive to the naval shipworm (Teredo navalis).

Drill head

The shells of the shipworm are situated on either side of a wormlike body in the form of a drill head. At the other end of the animal are two shells (pallets) to close off the inhalant and exhalent siphons. They do not eat wood but filter small plankton from the surrounding water. There are several species of shipworm. The theory that different species of shipworm were brought to the North Sea in the 17th century by The East Indies by Dutch merchant ships seems to be untrue. Shipwrecks older then the 15th century already show shipworm holes and fossil shells of the shipworm are also found in our area. But increasing trade on our waters will have increased the infection rate of all kinds of wooden structures. The Lowlands secured by dikes built up around wooden frames were particularly in peril from 1730 onwards and could only be saved by the use of big stones imported from the Baltic countries. Merchant ships were covered by copper plates or creosote for protection but it was difficult to keep things running.


For the fyke nets, the poles are gradually getting weaker and weaker and will have to be replaced before they break. The only solution is to take the stakes out in October and let them dry for a few months, so that the young shipworm die. The poles will then be safe until June when the young larvae swarm out in search of a piece of wood in the neighborhood on which to settle. Female shipworms can collect sperm in their bodies and keep the eggs inside until the free-swimming veliger larvae hatch and are able to leave the drilled tunnel and enter the big dangerous sea.