Using Grids to Reduce Discards

08 April 2012, at 1:00am

In the last in a series ofarticles, Mike Montgomerie,Gear Technologist at Seafish, discusses the use of flexibleand rigid grids. With the increasingimportance of social, economicand political factors as a bigpart of the issue Seafish geartrial work over the last 15years is increasingly relevantand is now being rolled out incommercial fisheries.

Flexi grid in the top of a cod end extension to act in a similar manner to a square mesh panel

In the last in a series of articles, Mike Montgomerie, Gear Technologist at Seafish, gives his perspective on the issue and the work Seafish, along with industry, has been doing to help develop new selective technologies and net-based fishing activities to reduce discards. In this article he discusses the use of flexible and rigid grids.

A variety of different flexible is limited due to the large amount of fish being caught. If the fish are scattered and the net is in effect filtering the water, the fish will probably get easy access to the grid. However, it is more usual in pelagic trawling to target shoals of fish, where a large volume of fish will be passing beneath the grid at one time and only the ones at the top of the net will get exposure to the grid, thereby the opportunity to escape.

Flexible grids with several gap widths from 15mm to 25mm have also been trialled in the Nephrops industry as a size selection device for Nephrops. These grids were fitted in the lower panel of the extension of the trawl, the intention being to riddle out the small Nephrops. It was found that very few Nephrops were released, even when a grid with larger gaps was used. This was probably due to the shape and overall size of the Nephrops as they tumble down the trawl with legs and claws sticking out, combined with them not showing any escape mechanism at this point in the capture process. As would be the case with many species, they would need to be forced out of the gaps by water flow, or by some design feature in the trawl. French fishermen have been working on a grid for selecting out the smaller Nephrops. This grid is inclined at 45 degrees and dropped into the bottom panel of the trawl. This allows all the Nephrops exposure to it, releasing the small ones and deflecting the larger ones upwards and into the codend.

Stiffened netting panels fitted in the sides of a codend extension to allow the release of small fish

Rigid Grids

Rigid grids are used in many fisheries throughout the world and have proved to be a very efficient tool for the separation of species in trawls. Because they are a rigid structure it is fairly easy for legislators to regulate the grids overall dimensions, bar spacing and method of fitting. This helps to ensure consistency in its selectivity.

The rigid grid is fitted in the trawl ahead of the codend, inclined at an angle of approximately 45 degrees, with the bar spacing designed to allow the target species to pass through between the bars and into the codend. The larger bycatch is directed up and out of the net through an escape hole in the top panel of the trawl.

Rigid grids are most efficient when there is a big difference between size of the target species and the size of the fish or shellfish that are to be excluded from the trawl, as in the turtle excluder device and sea lion grids. Inclined grids are also used in many shrimp fisheries to separate and release the fish while retaining the shrimps.

Many rigid are made from metal and can be cumbersome to handle, particularly on the smaller classes of vessels which use net drums and power blocks to handle the gear. Some types of rigid grids are made using several hinged sections which makes them more manageable onboard, but allows them to retain their effectiveness in species separation in the water.

Plastic grid with vertical bars and gaps at the bottom

A wide variety of grid designs are already in use, or are being trialled. These include grids with different bar spacings, vertical bars and horizontal bars, and some with a gap at the bottom to allow the retention of bottom fish and prevent the build up of benthic debris. The size and design of any grid needs to be modified to suit the specific fishery, taking into account the vessels handling arrangements and the species to be discarded. With careful consideration of the fishery and the grids design, 100% of the larger fish have been shown to be released.

Recently several manufacturers have been trialling grids made from semirigid plastic materials. This enables the grids to bend as they are wound onto a net drum, but return to their flat form once the net is shot. This will also make the grid much easier and safer to use in the tight confines on the stern of a fishing vessel. Regardless of what material the grid is made from it must be made to fit well into the shape of the trawl, and cover a large enough surface area to allow good separation of the relevant species.

Fitting rigid grids The extension and last tapered sections of an average trawl will have a cross sectional shape similar to an oval or circle with a circumference of approximately 2.1 metres when being towed. To fit a rigid grid at 45 degrees in this section the grid would need to be in the region of 750mm long by 500mm wide. Any bigger and the meshes will be forced further open in the vicinity of the grid and liable to allow the escape of target species. This would also ruin the streamlined shape of the net.

Plastic grid with horizontal bars and gaps at the bottom

To be its most effective a rigid grid really needs to be fitted further up the trawl, where there is sufficient mesh in the circumference to allow the grid to be at sensible proportions. A larger grid gives the catch longer exposure to the grid, and therefore should improve its efficiency in separating the catch.

If a square or rectangular grid is fitted into a standard two panel net there will be distortion of the meshes around the corners - it is like fitting a square peg in a round hole! This distortion will allow target species to escape and, in the longer term, will cause stretching and abrasion of the netting.

Ideally an oval-shaped grid should be used in a two panel net. For a rectangular grid, it is better to fit a four panel (or boxed) section into the trawl to accommodate the grid without distortion of the meshes. A rectangular grid will fit the cross-sectional shape taken up by a four panel (or boxed) section of netting. To get maximum benefit from a rectangular grid it may be better to re-cut the aft end of a two panel trawl into four panels to fit the grid into.

Most grids are designed to release by-catch that is larger than the target species, but in some fisheries a portion of the by-catch may be of similar size to the target species and will pass through the grid. It may be necessary to combine the use of a grid with some other discard reduction device to allow the escape of the smaller fish. This could be as simple as larger mesh codends, or a larger mesh size in the square mesh panel. It is important to assess that a grid is going to be the most efficient and practical discard reduction device before using it in a specific fishery.

April 2012

Further Reading

- Go to our previous news item on this story by clicking here.