NO20201178A1 - Catch sorter and method for excluding undesired catch using such a catch sorter - Google Patents

Description

TITLE: Catch sorter and method for excluding undesired catch using such a catch sorter

Field of the invention

The invention relates to fishing in general and more specifically to a catch sorter and a method for selective fishing. The invention is relevant for mobile fishing gear, such as trawls and demersal seines, and stationary fishing gear, such as fyke nets and stationary traps. The invention relates to fishing of shrimps, prawns, and other crustaceans, bony fish, other mesopelagic fish, zooplankton and other aquatic organisms. More specifically, the invention relates to a catch sorter arranged for excluding undesired catch from a flow of water with fish through the catch sorter, and a method for excluding undesired catch with the catch sorter.

Background of the invention

In this document, the meaning of the word “fish” is any living aquatic or marine organism (bony and cartilaginous fish, shrimps, prawns, other crustaceans, molluscs etc.) utilized in fisheries.

Selective fishing aims at capturing targeted fish based on species and/or size, while allowing non-targeted fish or other aquatic or marine organisms to escape unharmed. This means avoiding unwanted bycatches, illegal fish and improving the chances for survival of escaping specimen, reducing sorting time on board, improving product quality, reducing drag and thus gas emissions by towed fishing gears and in general improving profit in the fishery.

Conventional fishing trawls or nets are not able to differentiate between fish species or size of fish including small or juvenile fish. Such bycatch can result in overfishing and have negative effect on the ecosystem.

Net panels of fishing gears have predetermined mesh sizes, often governed by local or global regulations, and are configured to retain the desired species and sizes of fish. In certain areas, using a sorting grid in fish and shrimp trawls and other fishing gear is mandatory. In trawls, sorting grids with a predetermined grid spacing are installed at the rear part of the trawls, in front of the trawl bag (cod-end).

Requirements for minimum bar spacing in the grid is for instance 19mm for shrimps and 55mm for fish (for example cod).

From prior art one should refer to the Sort-X, which is a size-sorting device. A grid is installed slanting downward in the trawl net such that the larger fish that does not pass through the grid is directed downwards away from the grid and passes under the grid towards the cod-end. The smaller fish passes through the grid and is guided towards an escape opening. A disadvantage is that undersize fish may be able to keep clear of the grid and thus is maintained collected inside the sack. Refer also to NO 173162 B.

Other variants of size-sorters are the Sort-V and the Flexigrid.

The access to fishing areas may be closed if the catches contain bycatch above requirements. Malfunctioning bycatch reduction affects the general ecosystem and closing of fishing areas incurs significant losses to the fishing industry.

There is therefore a need for a method and a device to overcome the abovementioned problems.

Objects of the present invention

A main objective of the present invention is to aid selective fisheries in retaining cleaner catches of target species and sizes. The present invention aims at enhancing sustainable fisheries and especially to comply with UN sustainable development goal 14: Life Below Water. In order to maximize animal welfare and the probability for survival of escaping specimen, it is important that the sorting process occur during the active fishing period. Use of the catch sorter according to the invention may reduce harmful discarding of non-target specimen.

It is thus an object of the present invention to reduce or eliminate bycatch, such as unwanted or illegal species or juvenile fish, and to make it easier to comply with regulations. It is also an object to increase fish welfare when fishing, and in particular during the sorting process.

In order to obtain efficient and successful sorting, it is important that the fish is in direct contact with the sorting area of a catch sorter. As such, it is an object of the invention to provide a catch sorter that guides the fish towards the sorting area of the catch sorter in a gentle and efficient manner. Furthermore, it is an object to reduce or possibly eliminate any bycatch by ensuring that all the fish becomes exposed to the sorting device and cannot escape the sorting process.

Summary of the invention

The objective is achieved according to the invention by a catch sorter as defined in claim 1.

A number of non-exhaustive embodiments, variants or alternatives of the invention are defined by the dependent claims.

The present invention attains the above-described objectives by a catch sorter arranged for excluding undesired catch, such as fish, shrimps, or molluscs, from a flow of water passing through relative to the catch sorter, for example arranged in a trawl net. The catch sorter comprises:

- a hollow cylindrical body with a cylindrical wall or frame,

- an axially directed forward inlet to said hollow cylindrical body, arranged to let fish and water enter from upstream, and

- an opposite, axially directed rearward outlet from the hollow cylindrical body for water and sorted fish above a selected lower size limit L,

- a helical guide arranged within said cylindrical body,

- said helical guide is arranged for guiding fish in a helical path from said inlet towards the outlet and at least partially radially outwards towards a grating barrier in the cylindrical wall,

- the grating barrier comprises apertures allowing fish of a size below the selected size limit L to escape.

The object is further achieved according to the invention by a method as defined in claim 17. A number of non-exhaustive embodiments, variants or alternatives of the invention are defined by the dependent claims.

The method comprises the steps of:

- arranging the catch sorter (10) such that the axially directed forward inlet (21) is connected to a rear end of a first forward, upstream part of a fishing net such as a trawl, and the axial rearward outlet (22) is connected to a front end of a second, rearward, downstream part of a fishing net such as a cod-end or trawl sack.

- providing a current of water with fish in through the inlet (21) such that fish and water enter from upstream,

- guiding fish by the helical guide (30) in a helical path from said inlet (21) towards the outlet (22) and at least partially radially outwards towards the grating barrier (40) in the cylindrical wall (40'), while

- allowing fish of a size below the selected size limit (L) to escape through the apertures (41) of the grating barrier (40).

Effects of the Invention

The technical differences over prior art is that the present invention comprises a helical guide that efficiently guides all the fish gently towards a grating barrier and ensures that all the fish will contact and be exposed to the grating barrier. No fish can escape the selection process of the catch sorter. In addition, the helical path through the catch sorter provides for an extended sorting area compared to a flat panel, wherein the fish, when moving along the helical path, will change positions relative to each other, and also relative to the apertures of the grating barrier, and thus provide for a more effective selection process. Even more, the helical surface that the fish meets and that guides the fish forwards and towards the grating barrier makes the impact for the fish very gentle.

It makes it possible to provide an enhanced and more efficient sorting process during the active fishing period, ensuring a cleaner catch, and also to ensure animal welfare during the fishing and sorting process. The sorting process does not interrupt the fishing activity and simplifies the process once the fishing gear and the catch is hauled onboard the vessel.

Description of the Reference Signs

The following reference numbers and signs refer to the drawings:

Description of the diagrams

Embodiments of the present invention will now be described, by way of example only, with reference to the following drawing Figures, wherein:

Figure 1 shows, in perspective view, an embodiment of a catch sorter (10) according to the invention,

Figure 2 shows a side view of an embodiment of a helical guide (30) of the catch sorter (10) according to the invention, the helical guide having a continuous helically twisted surface (31),

Figure 3 shows an end view of the helical guide (30) of Fig.2,

Figure 4 shows a side view of an embodiment of a helical guide (30) of the catch sorter (10) of the invention comprising a partially or entirely stepped helically twisted surface (31'),

Figure 5 shows an end view of the helical guide of Fig.4,

Figure 6 illustrates a trawl net (100) with a catch sorter (10) according to the invention installed in front of a cod-end (200),

Figure 7 illustrates a side view of an embodiment with no axial stem in the helical guide, and

Figure 8 shows an end view of the embodiment of Fig.7.

Description of embodiments of the invention

Referring to fig.1, the invention is a catch sorter 10 arranged for excluding undesired catch, such as fish, shrimps, or molluscs, from a flow of water through the catch sorter 10. The invention comprises:

- a hollow cylindrical body 20, which may be called a cylindrical surface cage body 20, with a cylindrical wall 40', actually a cage wall 40', wherein the cylindrical end surfaces are open, please see below,

- an axially directed forward inlet 21 of said hollow cylindrical surface cage body 20, arranged to let fish and water enter from upstream, and

- an opposite, axially directed rearward outlet 22 from the hollow cylindrical surface cage body 20, for exit of water and also exit of sorted fish above a selected lower size limit L,

- a helical guide 30 arranged within said cylindrical surface cage body 20, - said helical guide 30 is configured for guiding fish in a helical path from said inlet 21 towards the outlet 22 and at least partially radially outwards towards a grating barrier 40 in the cylindrical wall 40',

- the grating barrier 40 comprising apertures 41 allowing fish of a size below the selected size limit L to escape out through the grating barrier 40.

In an embodiment of the invention the helical guide 30 is fixedly arranged within the cylindrical body 20.

In an embodiment of the invention there are two helical guides (30, 30) arranged within the cylindrical body 20. Said two helical guides (30, 30) may in an embodiment be arranged with a 180 degrees phase shift within the cylindrical body, please see Fig. 2, Fig.4, and Fig.7.

The term cylindrical body 20, or cylindrical surface cage body 20, is also used herein for a truncated frustoconical body having a larger inlet 21 than outlet 22, without it changing the invention.

The helical guide 30 thus forms a helical path 30P for which water and fish will follow (actually in an embodiment there are two helical paths (30P, 30P) as seen in Fig.2, Fig.4, and Fig.7, through the cylindrical body, through which helical path 30P will carry all fish above the lower size limit L through to the outlet 22, while undersize catch may escape through the cylindrical grating barrier 40.

The invention allows for small, juvenile fish and for example shrimps to escape through the grating barrier 40 when intending to keep larger size fish. The sorted fish that does not escape through the grating barrier 40, will continue through the outlet 22. The helical guide 30 ensures that the caught fish, shrimps etc. are guided towards and exposed to the sorting area, i.e. the grating barrier 40. The helical guide 30 may be fastened to the wall 40 of the cylindrical body 20, and the bars 42 may be fastened to an end ring at the inlet 21 and the outlet 22, or otherwise connected to a frame part of the cylindrical body 20. Relevant fastening methods include at least welding and gluing. The components of the catch sorter may be made of a range of materials such as stainless steel, aluminium, plastic, composite materials etc., having the required strength and which are not damaging to the environment and to the fish. It is desirable that the surfaces that the fish may come in contact with are smooth both for the fish welfare and for efficiency. The helical path 30P formed by the helical guide 30 provides for a significantly larger sorting area compared to the area of a flat, inclined panel, and while the fish is moved along the helical path 30P, they will change positions such that most or all fish are exposed to the grating barrier 40.

The helical guide 30 may also guide water in a partially radial direction, and water may be sucked in behind the helical, radially outer side edge 32.

The catch sorter 10 is intended for use in different types of mobile or stationary fishing gears made from net, herein referred to using the collective term “fishing net”, wherein there is a flow of water through the fishing gear. This term includes mobile fishing gear such as trawls and demersal seines, including bottom trawls, semi-pelagic trawls and pelagic trawls in single or multiple trawl configurations, and stationary fishing gear such as fyke-nets and fixed traps. For mobile fishing gear, the flow of water axially through the catch sorter 10 is a relative flow of water as a result of moving the fishing gear through the water. For the stationary fishing gear, the flow of water through the catch sorter 10 is a result of a water current, for example due to tidal water or pumping of fish. In both cases, fish will be transported through the catch sorter 10. The working principle of the catch sorter 10 is the same for any type of fishing gear where there is a relative flow of water through the device. The exact design of the catch sorter 10 may be varied in size and shape depending on which type of fishery, i.e. the target species of fish, the non-target undesired catch size and type, and the general design of the fishing gear for which the catch sorter 10 shall form a part.

The dimensions of the catch sorter can be adjusted to any type and size of mobile fishing gears, i.e. bottom trawls, pelagic trawls, semi-pelagic trawls or demersal seines and stationary fishing gears like fyke-nets and fixed traps or fish pumping pipes.

Referring now to fig.2 and fig.3. In an embodiment, the helical guide 30 comprises one or more generally helically twisted surfaces 31 having a first helical radially outer side edge 32 along an inner surface of the cylindrical wall 40', i.e. the cylindrical cage wall, and a second, helical opposite side edge 33 (or 32opp) relative to said first helical radially outer side edge 32.

Said second, helical opposite side edge 33 (or 32opp) may in an embodiment also be along the inner surface of the cylindrical wall 40’, thus said helically twisted surface 31 spanning the inner diameter of the cylindrical surface cage body, or the opposite edge 33 may in another embodiment as shown in Fig.1 be along an axial stem 23 in said cylindrical body 20. Said axial stem 23 may be arranged in the center of the cylindrical cage body 20. The helically twisted surface 31 may for example be fastened by welding to the axial stem 23 along the opposite side edge 33. The axial stem may in an embodiment be a buoyancy ballast tank filled with one or more buoyancy elements or air-filled. For this purpose, the hollow axial stem 23 may be provided with covers at each end for retaining said one or more buoyancy elements, and/or for preventing water through flow.

In an embodiment, there is no core stem axially in the guide 30, ref. Fig.7 and 8. One may imagine Fig.2 and Fig.4 wherein the axial stem 23 is reduced to zero diameter and the "inner" side edges 33, 33s of two helical guide halves joined so as for forming a diametrally spanning helically twisted guide 30 in every transverse cross section.

The one or more helically twisted surface 31 may be a continuous surface. In an embodiment, the helically twisted surface 31 may be an entirely or partially discontinuous surface for example made up of a surface with grating or small holes, bars arranged with small spacings allowing only water to pass through, etc.

Referring now to fig.4 and fig.5. In an embodiment, the helical guide 30 of the catch sorter 10 further is subdivided into a number of stepped surface sections 31s connected to form a stepped, generally helically twisted surface 31’, wherein the radially outer edge 32s of each surface section 31s coincide and together form said radially outer edge 32 along the inner surface of the cylindrical cage wall 40’ of the cylindrical cage body 20, and wherein a second, opposite edge 33s of each surface section 31s is arranged in a stepped, general spiral pattern along an axial stem 23 coaxial with said cylindrical body 20. In an embodiment the radially directed edges of pairs of surface sections 31s thus form triangular apertures 31a between the surface sections 31s. Water will flow generally only one way through each of said triangular apertures. Said stepped helically twisted surface 31s may contribute to an improved water flow through the hollow cylindrical body 20 with regard to bringing the undesired small catch more efficiently towards and through the grating wall. A significant advantage is that manufacturing a part-discontinuous stepped helical surface from mainly polygonal plate-shaped surface sections 31s is easier and less costly to produce than a continuous, smooth, helically twisted surface because the stepped helically twisted surface 31’ comprises a number of plane elements (surface sections 31s) that are easier to cut and fasten by for example welding to the inner surface of the wall 40’ and to the axial stem 23. The catch sorter 10 may also achieve an increased stiffness and torsional resistance in general.

In an embodiment, refer to fig.6, the axially directed forward inlet 21 is integrally connected to a rear end of a first forward, upstream part of a fishing net 100 such as a trawl net, and the axial rearward outlet 22 is integrally connected to a front end of a second, rearward, downstream part of a fishing net such as a cod-end 200 or trawl sack. The catch sorter 10 is preferably arranged in a section trailing the funnel-shaped belly section of the net, i.e. where the perimeter of the net is smaller, and in front of the cod-end 200. The inlet 21 should of course be fastened to the first part of the fishing net 100 by a peripherally extending connection, i.e. a connection where it is not possible for fish to escape out radially between the connecting parts of the net and the inlet 21, and correspondingly the outlet 22 to the second part of the fishing net 100, to ensure that all fish arriving through the rear part of the trawl net net 100 will have to enter the inlet 21, and all sorted fish, i.e. the fish that has not escaped through the grating barrier 40, will exit the outlet 22 and be collected in the second part of net, i.e. the cod-end 200. Such peripheral connection may be obtained by fastening each end of the hollow cylindrical body 20 (the inlet 21 and the outlet 22) to the first and second part of the net, respectively, with stiches or knots, twisted rope or chain, along the entire circumference of the inlet 21 and correspondingly the outlet 22, as illustrated in Fig.6.

According to another embodiment, the axial forward inlet (21) is (integrally) connected to a rear end of a first forward part of a pipe having a flow of fish and water inside, and the axial rearward outlet (22) is (integrally) connected to an inlet to a container or pipe for collection or transport of the sorted fish. Such an arrangement may sort fish pumped from one fish cage to another, or from a fish transport boat to land, or from one part of a fish transport boat to another, and separate two fish size classes above and below a size limit L.

In an embodiment, the cylindrical body 20 is actually a truncated frustoconical body with the widest part forwardly directed, and at least one helically twisted surface 31 or stepped generally helically twisted surface 31s is a truncated conical helix with the widest part of the helix arranged upstream in the truncated body.

According to an embodiment, the grating barrier 40 comprises bars 42 arranged axial-parallel, longitudinally along the perimeter of the hollow cylindrical body 20 in a spaced relation with a bar spacing that corresponds to the size limit L, although the bar spacing is not necessarily exactly equal to the size limit L. The bar spacing may in an embodiment be increased, gradually or step-wise, going from the inlet 21 towards the outlet 22, for example if there are arranged different closed channels downstream outside the cylindrical body, for different sub-classes of undersized catch.

The grating barrier 40 may in one embodiment comprise bars 42 helically arranged in a spaced relation along the perimeter of the hollow cylinder 20. The helical path of the bars 42 may then be arranged in the same direction as the at least one helically twisted surface 31 or the stepped generally helically twisted surface 31s and preferably arranged with a pitch equal to the pitch of the outer edge 32 of the at least one helically twisted surface 31 or the stepped generally helically twisted surface 31s. In another embodiment, the helical path of the bars 42 is arranged in an opposite direction relative to the at least one helically twisted surface 31 or the stepped generally helically twisted surface 31s and preferably arranged with an angle perpendicular to the outer edge 32 of the at least one helically twisted surface 31 or the stepped generally helically twisted surface 31s. The purpose of having helically arranged bars 42 in the grating barrier 40 is to reduce the risk of getting fish trapped between the bars 42 and the outer edge 32. Another advantage may be a gentler impact/ gentler passage for the fish towards the grating barrier 40 in the former embodiment.

In all embodiments, the grating barrier 40 may comprise bars 42 having a circular, elliptic, teardrop-shaped or polygonal cross-sectional shape. The shape of the bars should generally contribute to a gentler impact / gentler passage, and to reduce entrapment. A rounded impact surface on the bars may contribute to a gentler impact for the fish. A teardrop-shape may contribute to enhanced flow of water and thus reduced drag from the catch sorter on the fishing gear.

In an embodiment, the periphery of the helically twisted surface 31 or stepped surface 31s, along the edge 32, comprises indentations (as seen on fig 3 and fig.5) complementary to the inner surface of the cylindrical wall 40’ created by bars 42. Such indentations contribute to an easier and enhanced fastening of the helical guide 30 to the cylindrical cage wall 40’.

Two or more catch sorters 10 according to the invention may be arranged in series in for example a trawl net 100, in order to increase the selective capacity. The two or more catch sorters 10 may then be arranged with oppositely oriented rotation of the helical guides 30, such that the helical path of the water flow through the device 10 goes in opposite helical directions. This will reduce the torque reaction effect on the fishing gear resulting from the helical flow of water through the catch sorters. The two or more catch sorters 10 may have the same bar spacing. In an embodiment, a first catch sorter 10 may comprise a first bar spacing and the bar spacing of each subsequent catch sorter may be increased as counted in the downstream direction. Sorted-out catch from each subsequent catch sorter may then be led through separate channels outside the cylindrical body, for different subclasses of undersized catch.

The catch sorter 10 according to the present invention is suitable as a supplement to conventional fishing gear meeting the local or global requirements for the relevant fishing gear. Other equipment or devices, such as mandatary shrimp sorting grid, may be used in addition to the present invention. Similarly, the requirements for mesh width of the net may also be maintained according to conventional practice.

In an embodiment, a system for excluding undesired catch from a flow of water with fish, such as for example a flow of water through a trawl net, comprises a shrimp sorter of conventional type arranged upstream of the one or more catch sorter 10.

In an embodiment, a cover in the form of a side net may be attached to the fishing net 100 or to the catch sorter 10 on the outside of the catch sorter 10, to collect the excluded fish. This cover may have a different mesh size than the main fishing net, and may be provided with a sorting device from prior art.

The invention also relates to a method for excluding undesired catch, such as fish, shrimps, or molluscs, from a flow of water with fish through a catch sorter 10 according to any of the preceding paragraphs, the method comprising:

- arranging the catch sorter (10) such that the axially directed forward inlet (21) is integrally connected to a rear end of a first forward, upstream part of a fishing net such as a trawl, and the axial rearward outlet (22) is integrally connected to a front end of a second, rearward, downstream part of a fishing net such as a cod-end or trawl sack.

- providing a flow of water with fish in through the inlet (21) such that fish and water enter from upstream,

- allowing fish to be guided by the helical guide (30) in a helical path from said inlet (21) towards the outlet (22) and at least partially radially outwards towards the grating barrier (40) in the cylindrical wall (40'), while

- allowing fish of a size below the selected size limit (L) to escape through the apertures (41) of the grating barrier (40).

The fish that does not escape through the grating barrier, i.e. the fish above the size L, will continue through the entire helical guide 30 and exit the outlet 22 of the catch sorter 10 and into the rear part of the fishing net.

The flow of water may be provided by towing the fishing gear through the sea, or by arranging a stationary fishing gear such that a flow of water, for example as a result of tidal water, will flow through the device.

In other words, the method comprises guiding fish of different sizes in a water flow from an inlet of a cage body through a helical duct within the cage body. The cage body allows fish of a size below a defined size limit to escape out of a wall of the cage body, while the fish having a size above the defined size limit continues through the helical duct and out through an outlet at the opposite end of the cage body.

Claims (16)

Claims

1. A catch sorter (10) arranged for excluding undesired catch, such as fish, shrimps, or molluscs, from a current of water with fish flowing through the catch sorter (10),

characterized by:

- a hollow cylindrical body (20) with a cylindrical wall (40')

- an axially directed forward inlet (21) of said hollow cylindrical body (20), arranged to let fish and water enter from upstream, and

- an opposite, axially directed rearward outlet (22) from the hollow cylindrical body (20) for water and sorted fish above a selected lower size limit (L),

- a helical guide (30) within said cylindrical body (20),

- said helical guide (30) arranged for guiding fish in a helical path from said inlet (21) towards the outlet (22) and at least partially radially outwards towards a grating barrier (40) in the cylindrical wall (40'),

- the grating barrier (40) comprising apertures (41) allowing fish of a size below the selected size limit (L) to escape.

2. The catch sorter (10) according to claim 1, wherein the helical guide (30) comprises one or more generally helically twisted surfaces (31) having a first helical radially outer side edge (32) along an inner surface of the cylindrical wall (40') and a second, helical opposite side edge (33) relative to said first helical radially outer side edge (32).

3. The catch sorter (10) according to claim 2, wherein said second, helical opposite side edge (33) is along an axial stem (23) in said cylindrical body (20).

4. The catch sorter (10) according to claim 2, wherein said second, helical opposite side edge (33) also is along the inner surface of the cylindrical wall (40’).

5. The catch sorter (10) according to any of the preceding claims, wherein said one or more helically twisted surfaces (31) is a continuous surface.

6. The catch sorter (10) according to any of the preceding claims, further comprising a number of stepped surface sections (31s) connected to form a stepped generally helically twisted surface (31’) of the helical guide (30), wherein the radially outer edge (32s) of each surface section (31s) coincide and together form said radially outer edge (32) along an inner surface of the cylindrical wall (40’) of the cylindrical body (20), and wherein a second opposite edge (33s) of each surface section (31s) is arranged in a stepped spiral pattern along an axial stem (23) in said cylindrical body (20), thus forming triangular apertures between the surface sections (31s).

7. The catch sorter (10) according to any of the preceding claims, wherein the axially directed forward inlet (21) is connected to a rear end of a first forward, upstream part of a fishing net such as a trawl, and the axial rearward outlet (22) is connected to a front end of a second, rearward, downstream part of a fishing net such as a cod-end or trawl sack.

8. The catch sorter (10) according to any of the preceding claims, wherein the grating barrier (40) comprises bars (42) arranged longitudinally along the perimeter of the hollow cylindrical body (20) in a predetermined spaced relation.

9. The catch sorter (10) according to any of the claims 1 to 8, wherein the grating barrier (40) comprises bars (42) helically arranged along a helical path along the perimeter of the hollow cylinder (20) in a spaced relation.

10. The catch sorter (10) according to any of the preceding claims, wherein the bar spacing increases, gradually or step-wise, as counted from the inlet (21) towards the outlet (22).

11. The catch sorter (10) according to claim 9 or 10, wherein the helical path of the bars (42) is arranged in the same direction as the at least one helically twisted surface (31) or the stepped generally helically twisted surface (31s) and preferably arranged with a pitch equal to the pitch of the outer edge (32) of the at least one helically twisted surface (31) or the stepped generally helically twisted surface (31s).

12. The catch sorter (10) according to claims 9 or 10, wherein the helical path of the bars (42) is arranged in an opposite direction relative to the at least one helically twisted surface (31) or the stepped generally helically twisted surface (31s) and preferably arranged with an angle perpendicular to the outer edge (32) of the at least one helically twisted surface (31) or the stepped generally helically twisted surface (31s).

13. A system comprising one or more catch sorters (10) according to any of the preceding claims, said one or more catch sorters (10) being arranged in series.

14. A system according to claim 13, wherein the first catch sorter (10) comprises a first bar spacing and wherein the bar spacing of each subsequent catch sorter increases as counted in the downstream direction.

15. A system according to claim 13 or 14, further comprising at least one shrimp sorter of conventional type arranged upstream of the one or more catch sorters (10).

16. A method for excluding undesired catch, such as fish, shrimps, or molluscs, from a flow of water with fish flowing through a catch sorter (10) according to any of the claims 1-12,

characterized by:

- arranging the catch sorter (10) such that the axially directed forward inlet (21) is connected to a rear end of a first forward, upstream part of a fishing net such as a trawl, and the axial rearward outlet (22) is connected to a front end of a second, rearward, downstream part of a fishing net such as a cod-end or trawl sack. - providing a flow of water with fish in through the inlet (21) such that fish and water enter from upstream,

- guiding fish by the helical guide (30) in a helical path from said inlet (21) towards the outlet (22) and at least partially radially outwards towards the grating barrier (40) in the cylindrical wall (40'), while

- allowing fish of a size below the selected size limit (L) to escape through the apertures (41) of the grating barrier (40).