DESCRIPTION The invention relates to the removal of oyster borers and other harmful snails from breeding beds of oysters and mussels.

In recent years, oyster borers (also oyster drills), including the Urosalpinx cinerea, Eupleura and Ocenebra, have widely spread in European waters and pose a very serious threat to oyster and mussel farming . The oyster borer drills through the shell of the oyster after which the oyster dies. The snail mainly affects young oysters, causing the breeding to stagnate and the stock to decrease.

The object of the present invention is to provide the solution to this problem by providing a device and method for removal of oyster borers and other harmful snails from breeding beds and by providing for a fertile soil whereupon oysters and mussels can be bred again .

According to the invention a device is provided for removing oyster borers and/or other harmful slags, hereinafter referred to as snails, from a breeding bed for oysters and/or mussels, etc., hereinafter called oysters, comprising a collecting tray that is arranged to be moved forward over the breeding bed, wherein the collecting tray comprises a bottom plate and, with respect to the direction of travel, on both sides comprises a side plate, on the rear side comprises a rear plate and on the upper side comprises partly an upper plate and partly a bar frame, the bars of the bar frame extend essentially between the front side of the bottom plate and (at least the front side of) the upper plate, wherein the distance between the bars is smaller than the smallest cross-section (size, dimensions) of (at least a substantial part of) the oysters, and is larger than the largest cross-section (size, dimensions) of (at least a substantial part of) the snails.

Because the bars extend (obliquely upwards) from the (low lying) front side of the bottom plate to the (high lying) upper plate, while the distance between the bars is smaller than the smallest cross section of the oysters, the oysters are moved upwards from the (sea) bottom over the sloping (smooth) bars, because the oysters cannot fall through between the bars. Eventually, when the device is towed further, the oysters will return onto the (sea) bottom behind the device. However, the snails, which are smaller in size than the oysters, like the oysters will be "scooped up" from the (sea) bottom by the front side of the device (by means of the front side of the bottom plate). Contrary to the oysters, the snails will fall through the spaces between the bars, because the spaces (distances) between these bars is smaller than the diameter/size of the oysters, but larger than the diameter/size of the (smaller) snails. Contrary to the oysters, as a result the snails end up and are collected in the collecting tray, i.e. in particular in the part that is defined and enclosed by the rear plate, the upper plate, the bottom plate and the side plates.

Preferably, the collecting tray at least is partly manufactured from perforated material, in particular perforated steel plate, having perforations that are smaller than the size of the snails. As a result the water (and partly also the sand that has come along) can drain via the perforations, while the snails remain trapped in the collecting tray.

Preferably, the bars are extending over (at least a part of) the upper plate, preferably at least until the rear plate. As a result, by means of these continuous, smooth bars, the oysters glide effortlessly over the towed device and return to the soil of the (sea) bottom by falling behind the device.

In order to be able to discharge the snails caught within the collecting tray, when the collecting tray is substantially full, preferably the rear plate is arranged to be able to be opened, after the device has been lifted out of the water, in order to be able to empty the contents of the space defined and enclosed by the rear plate, the upper plate and (parts of) the bottom plate and the side plates. Preferably, the device comprises an oyster dredge known per se for a vessel or cooperates with it.

The invention also relates to a method for removing oyster borers and/or other harmful slags, hereinafter referred to as snails, from a breeding bed for oysters and/or mussels etc., hereinafter called oysters, comprising the steps of:

- providing a device according to the invention;

- collecting the snails by means of the device according to the invention;

- heating or cooking the collected snails so that they are killed;

- depositing of the dead snails on the same or another breeding bed for oysters and/or mussels etc. During experiments, it has been found that dead snails deposited on a breeding bed for oysters etc. possess good properties as a basis for collecting oyster brood.

The device according to the invention, adapted for removing oyster borers and other harmful snails from breeding beds of oysters and mussels, is preferably connected to a known oyster dredge on the vessel. By fishing with the device that is attached to a known oyster dredge, fine material such as the oyster borer ends up in the colleting tray intended therefore, while larger material such as the oysters themselves slide over the device so that they end up on the oyster bed again. The device is lifted onto the vessel and the collected material can then be processed in a cooking kettle. After cooking, the dead snails maybe sown again to collect oyster brood. The main purpose of the processing is to eradicate the oyster borer that causes a lot of damage to oyster farming, as mentioned earlier.

The vessel maneuvers the oyster dredge with the attached device over the entire oyster bed so that the entire (sea) bottom of the bed can be treated. The collected material in the device is transported onto the vessel and then sorted and boiled in the cooking vessel and after that sown on the recently treated bed to serve as a raw material for collecting oyster brood. The invention will now be further discussed with reference to the figure description below.

Figure 1 shows an embodiment of a device according to the invention. Figure 1 shows, in views a, b and c, an exemplary embodiment of a device 1 for removing oyster borers and/or other harmful slags, hereinafter referred to as snails, from a breeding bed for oysters and/or mussels, etc., hereinafter called oysters. The device comprises a collecting tray 2 that is adapted to be displaced underwater at some distance above the level of the (sea) bottom 4 of the breeding bed by means of bottom slide elements 3. For instance, the device is towed by a vessel by means of a towing frame 5 and towing cables.

The collecting tray 2 comprises a bottom plate 6 and, with respect to the direction of towing, comprises a side plate 7 on both sides, on the rear side a rear plate 8 and on the upper side partly an upper plate 9 and partly a bar frame 10. The bars of the bar frame are extending substantially between, for example, a transverse rod 11 at the front side of the bottom plate 6 and the rear side of the upper plate 9. The distance between the bars 10 is smaller than the smallest cross-section of the oysters 12. By towing the device 1 by means of rod 11 at the front side of the bottom plate, the oysters are grubbed from the sand at the (sea) bottom 4. However, the distance between the bars is greater than the largest cross-section of the snails 13 which, like the oysters 12, are also grubbed from the (sea) bottom 4. For example, the distance between the bars 10 is approximately 32 mm.

For example, the collecting tray is made of perforated steel plate, with perforations that are smaller than the size of the snails 13, for example perforations of about 1 cm ² .

In the embodiment shown, the bars 10 extend over the upper plate 9, for instance up to the rear plate 8. As a result, the oysters 12 are effortlessly displaced over the collecting tray 2. As illustrated in Fig. lc, the rear plate 8 is arranged to be opened, after the device 1 has been lifted from the water, in order to be able to empty the contents (for an important part consisting of snails 13 caught by the collecting tray) of the space that is defined and enclosed by the rear plate 8, the upper plate 9 and parts of the bottom plate 6 and the side plates 7 (see arrow 14).

Figure la clearly shows that the towing frame 5 respectively can be formed by, or may cooperate with, an oyster dredge for a vessel known per se. Preferably, cables or chains are arranged between the frame 5 and the rear side of the collecting tray in order to be able to drag the collecting tray 2 correctly and stably over the (sea) bottom 4.

The removal of the snails 13 comprises: collecting the snails 13 by means of the device 1, unloading the collected snails, as shown in Fig. lc, and subsequently heating or cooking respectively of the collected snails so that they are killed. Then, these dead snails may be successfully deposited on the same or optionally on another breeding bed for oysters and/or mussels etc., where they form a breeding soil for a new generation of oysters.

Thus, the present invention provides a solution to the problem of the oyster borers, namely by providing a device and a method for removing oyster borers from the breeding beds and for creating a fertile soil where oysters and mussels can be bred again.