GB2473713A

GB2473713A - Dredge for harvesting shellfish

Info

Publication number: GB2473713A
Application number: GB1015081A
Authority: GB
Inventors: Paul Birnie; James Stewart Adam
Original assignee: OBAN SCALLOP GEAR Ltd
Current assignee: OBAN SCALLOP GEAR Ltd
Priority date: 2009-09-18
Filing date: 2010-09-10
Publication date: 2011-03-23
Anticipated expiration: 2030-09-10
Also published as: GB0916383D0; FR2950225B1; GB201015081D0; GB2473713B; FR2950225A1

Abstract

A dredge 10 for harvesting shellfish comprising a frame 12, a plurality of teeth 20 adapted to dig into the seabed, and a collecting bag 14 attached to the frame. The collecting bag comprises an upper panel 42 and a lower panel 40 connected to each other on the lateral sides and the rear edge, to form a forward facing opening. The lower panel comprises a screen of a flexible sheet material having apertures 48 typically an array of rectangular apertures. Typically the flexible sheet material is selected from natural rubber, artificial rubber and polyurethane. The scallop dredge of the present invention produces less impact on the seabed than traditional scallop dredges, which use chain links or chain mail for the lower panel. The lower panel of a flexible sheet material presents a smoother surface to the seabed, so that it glides more smoothly over the seabed, thereby disturbing the seabed less. It can deform around obstacles on the seabed, instead of snagging against the obstacles as happens with traditional chain link dredges, causing the obstacles to be dislodged and disturbing the seabed. The reduced weight of the dredge, compared to traditional chain link dredges, results in the collecting bag lifting more easily over obstructions, and causing less disturbance to the seabed.

Description

Apparatus for harvesting shellfish This invention relates to a scallop dredge used for digging out scallops from the seabed and collecting them in a collecting bag, and to the collecting bag itself. Dredges and collecting bags according to the invention can also be used for digging out and collecting other shellfish which can be harvested in a similar manner.

In the United Kingdom king scallops account for approximately 80% of the catch, while smaller queen scallops account for the remaining 20%. King scallop dredges are steel structures with teeth arranged to penetrate the seabed and dig out scallops, which are collected in a chain mail bag attached to the dredge. There has been environmental concern about the effects of scallop dredging on seabed habitats. In some environments, such as maerl beds, which are mixed sediments built by a surface layer of slow growing, unattached coralline algae, and reef systems particularly where the substrate is relatively soft, scallop dredging is likely to permanently change the habitat. Similar concerns arise with the harvesting of shellfish in France.

It is an object of the present invention to provide a dredge with a reduced environmental effect, which at the same time can harvest scallops efficiently.

According to a first aspect of the present invention there is provided a dredge comprising a frame; a plurality of teeth adapted to dig into the seabed; and a collecting bag attached to the frame and comprising an upper panel and a lower panel connected to each other on at least a part of the perimeter and forming a forward facing opening; wherein the lower panel comprises a screen of a flexible sheet material having apertures formed therein.

Preferably the screen comprises a sheet of material with substantially constant thickness.

Preferably the screen comprises a sheet of polyurethane, polymer, natural or artificial rubber, or other suitable plastics material.

The screen may comprise a reinforced sheet. The reinforcement may be fibreglass mat, individual fibres, cords or wires.

Preferably the apertures are substantially square or rectangular.

Alternatively the apertures may be circular or oval.

The apertures may be squares having sides of between 45 and 90mm, preferably between 50 and 85mm.

The upper panel may comprise metal links, for example chain mail.

Alternatively the upper panel may comprise plastic links, for example interlinked loops of a plastics material. Alternatively the upper panel may comprise a net.

The upper panel may comprise a screen having apertures therein. The upper panel may be of the same material as the lower panel.

The screen of the lower panel may comprise two sheets of a flexible sheet material joined together in a hinged connection, for example by providing a transverse pivot bar which links two rows of eye bolts provided on opposing edges of the two joined sheets.

According to a second aspect of the present invention there is provided a collecting bag for a scallop dredge, comprising an upper panel and a lower panel connected to each other on at least a part of the perimeter and forming a forward facing opening; wherein the lower panel comprises a screen of a flexible sheet material having apertures therein.

Preferably the screen comprises a sheet of polyurethane, natural or artificial rubber, or other suitable plastics material.

The screen may comprise a reinforced sheet.

Preferably the apertures are substantially rectilinear, that is substantially square or rectangular. Alternatively the apertures may be circular or oval.

The lower panel may comprise fibreglass, wire or cord reinforcement, or other fibre reinforcement. Preferably the reinforcement is embedded within the flexible sheet material.

The upper panel may comprise metal links, for example chain mail.

The screen of the lower panel may comprise two sheets of a flexible sheet material joined together in a hinged connection.

The invention will now be described with reference to the figures in which: Fig 1 shows a dredge according to the invention viewed from below; Fig 2 shows a vessel towing a number of dredges according to the invention; Fig 3 shows a collecting bag according to the present invention viewed from underneath; Fig 4 shows a partial view of the upper panel of the collecting bag of Fig 3; Fig 5 shows the lower panel of the collecting bag of Fig 3; Fig 6 shows another embodiment of a lower panel of a collecting bag according to the present invention; Fig 7 shows a partial elevation on the lower panel of Fig 6 in the direction of arrow A; and Fig 8 shows a partial elevation of the lower panel of another embodiment of the present invention.

Referring to Fig 1 there is a dredge 10 which comprises a frame 12 to which is attached a collecting bag 14 at the rear of the frame 12. The dredge is towed from a towing point 16 at the front of the dredge 10. The frame includes a spring-loaded transverse pivot bar 18 to which are connected a plurality of spring teeth 20, which act as shock absorbers to enable the dredge to be towed over mixed stone and rocky substrates.

The pivot bar 18 is connected to the frame by pivot pins (not shown) and two tie rods 22 which each have bearing plates 24 fixed to the end thereof.

The bearing plates 24 are guided on longitudinal frame members 26, 28 so that they can move towards the rear of the dredge 10 against the biasing action of a spring 30 which surrounds the rod 22. When the dredge is dragged along the seabed the spring teeth 20 and pivot bar 18 can pivot rearwards against the biasing force of the springs 30 when they meet an obstruction. The longitudinal frame members 26, 28 are connected by a transverse frame member 32.

The collecting bag 14 comprises a lower panel 40 comprising a planar panel with an array of apertures 48 and an upper panel 42 of chain links.

The upper panel 42 can extend all the way to the transverse frame member 32 or, as shown in Fig 1, can extend to the transverse pivot bar 18, and a separate net 44 can extend from the front edge of the upper panel 42 to the transverse frame member 32 of the frame 12. The lateral sides of the upper panel 42 and the lower panel 40 are connected by links 46, best seen in Fig 3. Fig 3 shows the collecting bag 14 from underneath. The lower panel 40 is of reinforced polyurethane and has a number of square apertures therein. The upper panel 42 is omitted in Fig 3 for clarity, but in practice would be visible through the apertures 48.

Other materials, such as natural or artificial rubber, or any other plastics material which can be provided in flexible sheet form, may be used instead of polyurethane.

Fig 4 is a partial view of the chain link of the upper panel 42. Typically the chain link comprises large circular links 52 interconnected by smaller links 54. However, other arrangements of chain links or chain mail are possible. For example the upper panel may comprise a lighter plastic mesh or net, or a mesh of a combination of metal and plastic.

The rear end of the collecting bag may be closed by a weighted bar 60 which includes a flange 62 connected by bolts or rivets 64 to the upper and lower panels. Alternatively the weighted bar may be omitted if the dredge has sufficient weight without it. The front edge of the lower panel 40 is connected to a transverse bar 34 which is fixed to the frame 12, just above the pivot bar 18. The upper panel 42 and the lower panel 40 are thus connected to each other on three of their four sides, and form a forward facing opening on the fourth side above the spring teeth 20. The weighted bar 60, if used, serves to hold the lower panel 40 in a flat position on the seabed while it is being pulled.

The arrangement of apertures 48 in the lower panel 40 can best be seen in Fig 5. In the example of Fig 5 the apertures are formed as 75 mm squares with 25 mm spacing. However, the dimensions and spacing of the apertures may vary, and it has been found that the apertures may be between 50 and 100 mm square, most preferably between 75 and 80 mm square. Rectangular apertures may be used instead, with a length of between 60 and 120mm and a width of between 50 and 90mm. Typically the rectangular apertures may be arranged with their longer dimension extending transversely across the lower panel.

Typically the lower panel 40 may be a rubber screen mat, for example an ISEGUMTM rubber screen supplied by TEMA lsenmann Limited, or a polyurethane panel of typically 40 mm thickness. The panel 40 may contain cord reinforcement encapsulated within the rubber. Typically the warp is polyester and the weft is cotton, although other fibres may be used, for example metal or synthetic fibres or wires. The panel may be of natural or synthetic rubber, for example an NR/BR polymer blend, or of a suitable plastics material such as polyurethane. Typically the specific gravity is between 1.00 and 1.20, so that the panel sits on the sea bed, without the need for a weighted bar 60. The material of the panel is selected to provide sufficient resilience and flexibility to enable it to deform around obstacles, but with sufficient hardness so that it does not wear out when dragged over rocks and stones. Typically the bottom Shore hardness is between 75 and 95.

Scallop dredges according to the invention may be towed in gangs of up to ten or more dredges attached to an outrigger 70, which is towed behind a vessel 72. Typically a vessel 72 may tow two outriggers 70, one on each side of the vessel, as shown in Fig 2. The dredges 10 may be towed on warps 74 of different length to avoid entanglement. In the example of Fig 2 the front outrigger 70 is 9 metres ahead of the rear outrigger, while the outriggers overlap by 1 metre.

Referring to Figs 6 and 7. there is shown an alternative embodiment of the lower panel 140 of the invention. The lower panel 140 is also of polyurethane, natural or synthetic rubber, or any other plastics material which can be provided in flexible sheet form, may be used instead of polyurethane.

The upper panel 42 and the lower panel 140 are connected to each other on three of their four sides, and form a forward facing opening on the fourth side above the spring teeth 20. The optional weighted bar 60, if used, serves to hold the lower panel 140 in a flat position on the seabed while it is being pulled. However instead of using links 46 to connect the upper and lower panels, the lower panel 140 is provided a row of eye bolts on each lateral side, a row of eye bolts 162 on the rear edge, and a row of eye bolts 164 on the forward edge. The eye bolts can be cast into the panel 140 or fitted afterwards using suitable fixing means such as bolts (not shown). The eye bolts 160, 162, 164 can be fixed to the links 52, 54 of the upper panel 42 by any suitable means, for example heavy duty cable ties or wrapped wire, to join the upper panel 42 and the lower panel on the lateral and rear edges. The eye bolts 164 at the front edge of the lower panel 140 are secured to the pivot bar 18, for example by heavy duty cable ties or wrapped wire In the example of Fig 6 the apertures 148 are formed as 75mm squares with 25mm spacing. However, the dimensions and spacing of the apertures may vary, and it has been found that the apertures may be between 50 and 100mm square, most preferably between 65 and 85mm square. Rectangular apertures may be used instead, with a length of between 60 and 120mm and a width of between 50 and 90mm. Typically the rectangular apertures may be arranged with their longer dimension extending transversely across the lower panel.

Typically the material of the lower panel 140 is similar to that described above for the lower panel 40 of Fig 3. The lower panel 140 is formed from a front sheet 150 and a rear sheet 152, connected by a hinged connection.

In the illustrated example of Figs. 6 and 7, the hinged connection is achieved by providing a row of eye bolts 166 at the front of the rear sheet 152 and at the back of the front sheet 150. Two rods (not shown) are inserted through the eyes of the eye bolts 166, one rod for each row, and the two rods are connected, for example by chain links, to form a hinged connection. However Fig. 8, which shows a partial elevation corresponding in position to that of Fig. 7, shows an alternative form of hinged connection, in which a row of eye bolts 168 is secured to the front edge of the rear sheet 152, and another row of eye bolts 170 is secured to the rear edge of the front sheet 150, and a pivot bar 172 is threaded through both sets of eye bolts 168, 170, which are arranged on a common axis. Other hinged connections may be used, for example linking the row of eye bolts 166 at the front of the rear sheet 152 and the row of eye bolts 166 at the back of the front sheet 150 by heavy duty cable ties or separate links, or attaching a first transverse rod directly to the front of the rear sheet 152, for example by clamping, and a second transverse rod to the back of the front sheet 150 and coupling the transverse rods together, or any other suitable method. It has been found that providing the flexibility of a lateral hinged connection in the lower panel 140 increases the durability of the panel and reduces damage on the sea bed.

Tests by the inventors have shown that the scallop dredge of the present invention produces less impact on the seabed than traditional scallop dredges, which use chain links or chain mail for the lower panel. The lower panel 40, 140 of the present invention presents a smoother surface to the seabed, so that it glides more smoothly over the seabed, thereby producing less disturbance to the seabed. In addition, the rubber or plastic material of the lower panel 40, 140 is resilient and is able to deform to some extent when contacting obstacles such as rocks or stones on the seabed. The lower panel 40, 140 can deform around the obstacles, instead of snagging against the obstacles as happens with the chain links of traditional dredges, causing the obstacles to be dislodged and disturbing the seabed. The working life of a dredge according to the invention can be up to three times that of a traditional chain link dredge.

Tests have shown that it is area of the lower panel 40, 140 containing the three rear rows of apertures 48, 148 which is subject to most wear, and the invention offers the advantage of allowing the lower panel 40, 140 to be reversed front to back after a period of use, to extend the working life.

A further advantage arises from the reduced weight of the dredge, since the lower panel 40, 140 of the present invention is lighter and has a smaller displacement than the traditional lower panel which comprises chain mail or chain links. The dredge 10 can be made even lighter if the upper panel 42 is also made of rubber or plastic material, for example a polypropylene net. This results in the collecting bag lifting more easily over obstructions, and causing less disturbance to the seabed.

The scallop dredge of the present invention has a further advantage in that it is used in exactly the same way as traditional scallop dredges, and requires no special equipment for the operator of the vessel other than the equipment already used for traditional scallop dredges. The efficiency of harvesting is not adversely affected.

Although the invention has been described with respect to the harvesting of scallops, it is to be understood that the dredge of the invention can be used for harvesting other shellfish which can be found just below the surface of the seabed. The aperture dimensions described are optimum for capturing king scallops, but smaller or larger apertures may be provided for capturing smaller or larger molluscs. For example apertures of between 45 and 55mm square have been found to be suitable for queen scallops. Although the apertures are described as rectilinear, it is to be understood that other shapes are possible. The corners may be rounded, and the apertures may be substantially oval or circular. The size and shape is selected to ensure that the shellfish being harvested are unable to pass through the apertures, while smaller material can pass through.

Modifications and improvements may be incorporated without departing from the scope of the invention.

Claims

Claims 1. A dredge for harvesting shellfish comprising a frame; a plurality of teeth adapted to dig into the seabed; and a collecting bag attached to the frame and comprising an upper panel and a lower panel connected to each other on at least a part of the perimeter and forming a forward facing opening; wherein the lower panel comprises a screen of a flexible sheet material having apertures formed therein.
2. A dredge according to claim 1, wherein the screen comprises a sheet of a material selected from natural rubber, artificial rubber and polyurethane.
3. A dredge according to any preceding claim, wherein the apertures are substantially rectilinear.
4. A dredge according to claim 3, wherein the apertures are squares having sides of between 45 and 90mm.
5. A dredge according to any preceding claim, wherein the lower panel comprises embedded reinforcement, selected from fibreglass, fibres, wire and cord.
6. A dredge according to any preceding claim, wherein the upper panel comprises links.
7. A dredge according to any of claims 1 to 5, wherein the upper panel comprises a screen of a flexible sheet material having apertures therein, wherein the screen comprises a sheet of a material selected from natural rubber, artificial rubber and polyurethane.
8. A dredge according to any preceding claim, wherein the screen of the lower panel comprises two sheets of a flexible sheet material joined together in a hinged connection.
9. A collecting bag for a scallop dredge, comprising an upper panel and a lower panel connected to each other on at least a part of the perimeter and forming a forward facing opening; wherein the lower panel comprises a screen of a flexible sheet material having apertures therein.
10. A collecting bag according to claim 9, wherein the screen comprises a sheet of a material selected from natural rubber, artificial rubber and polyurethane.
11. A collecting bag according to any of claims 9 to 10 wherein the apertures are substantially rectilinear.
12. A collecting bag according to claim 11, wherein the apertures are squares having sides of between 50 and 90 mm.
13. A collecting bag according to any of claims 9 to 12, wherein the lower panel comprises embedded reinforcement, selected from glassfibre, fibres, wire and cord.
14. A collecting bag according to any of claims 9 to 13, wherein the upper panel comprises links.
15. A collecting bag according to any of claims 9 to 13, wherein the upper panel comprises a screen of a flexible sheet material having apertures therein, wherein the screen comprises a sheet of a material selected from natural rubber, artificial rubber and polyurethane.
16. A collecting bag according to claim 15, wherein the apertures in the upper panel are rectilinear.
17. A collecting bag according to claim 15, wherein the apertures in the upper panel are squares having sides of between 45 and 90mm.
18. A collecting bag according to any of claims 15 to 17, wherein the upper panel comprises embedded reinforcement, selected from fibreglass, fibres, wire and cord.
19. A collecting bag according to any of claims 9 to 13, wherein the upper panel comprises a net.
20. A collecting bag according to any of claims 9 to 19, wherein the screen of the lower panel comprises two sheets of a flexible sheet material joined together in a hinged connection.