Square Mesh Panels
The square mesh panel is one of the most commonly used and well known selective devices. In this, the diamond mesh is turned through 45 degrees to the water flow thereby ensuring the meshes in the panel remain fully open throughout the fishing operation, allowing for the release of small fish. It is sometimes referred to as T45 mesh.
Square mesh panels are generally fitted in the top panel of a trawl the release of small round fish. The size of fish that can escape depends very much on the size of mesh used for the panel. Square mesh panels in this position are most effective for fish that have an escape stimulus to swim upwards such as haddock and whiting. The effectiveness of the panel very much depends on the proximity of the fish to the panel as they pass down the trawl. The length and width of the panel will also make a difference to the panels effectiveness. In sea trials it has been found that, generally, the closer to the cod end the more effective a square mesh panel is to releasing round fish.
In a standard trawl constructed from normal diamond mesh, without a square mesh panel fitted, the mesh opening, in the area of the fishing circle, will only be about 30 - 40% of the mesh length. This means that a mesh of 100mm inside mesh will only be open to about 30 - 40mm at its widest point, not a very big area for fish to escape from. The meshes further down the net may only be open to about 20% of the mesh length (20mm for 100mm mesh or 16mm for 80mm mesh) in the cod-end and extension this figure can be as low as10% of the mesh length. With modern high powered vessels towing the gear faster and at a more consistent speed this mesh opening will be maintained throughout the duration of the tow in all but the poorest weather. There is not much chance of escape for small fish! Square mesh or T45 mesh (the mesh turned through 45 degrees) however will maintain its open shape the entire time it is in the water providing an escape area for the smaller fish. However they can only escape through the square mesh if;
1 - they can find the open meshes,
2 - they have a stimulus to escape through these open meshes,
3 - they are exposed to them for long enough time,
4 - they have the energy to escape through them.
The standard square mesh panel used in UK fisheries is 3 metres long (2m for lower powered vessels) fitted at the end of the tapered section of the trawl or in the parallel extension. In this area of the cross section of the trawl will be of an oval shape with a maximum width of about 500mm and a height in the region of 350mm allowing most of the fish exposure to the panel but in a very confined space. However with the modern vessels towing in the region of 3 knots a fish is only exposed to the square mesh panel for about 2-3 seconds. This is not much time for a fish that is already fatigued from being herded into the trawl and stressed from travelling down from the mouth of the trawl to find the escape route and escape through it.
Despite this the regulatory square mesh panels in UK fisheries can reduce catches of small haddock and whiting by as much as 75%. They do not have the same effect on cod as cod do not possess the same stimulus to escape by rising up in the water column. The effectiveness of a square mesh panel at releasing small fish can vary depending on many criteria such as the clarity of the water, daylight or darkness, depth of water, colour and type of netting used, twine thickness, how it is fitted into the trawl, speed of water flow through the trawl, size of fish and whether the fish are passing down the trawl in groups or as a steady flow of individuals. By using larger mesh sizes, thinner twine or increasing the area of a square mesh panel more small fish will be released.
Many fishermen are already using larger mesh than required for regulation. It is not so easy to decrease twine thickness due to the need to maintain strength and prevent stretch. More recently many skippers have been experimenting with a second square mesh panel, of much larger mesh size further forward in the trawl in an attempt to reduce their discard rates. Some have tried fitting square mesh as large as 300mm mesh almost right up to the fishing circle of the trawl but have found that although these do work to a certain extent they are not as good as expected, only releasing about 30% of the haddock and whiting and having very little effect on cod catches.
This could be for several reasons. This fish are stimulated by the approaching bosom of the trawl to rise up and attempt to escape this danger by swimming ahead and over the headline. If they don't succeed in doing this they will then pass down the body of the trawl, where the secondary square mesh panels are situated, without any awareness that they are trapped to stimulate them to try to escape. This will not be initiated until the trawl narrows down at the after end close to where the regulation square mesh panel is fitted. Generally it is only the fish that are in close proximity to the top large square mesh panel as they pass down the trawl that will attempt to escape through it. The fitting of these square mesh panels closer to the front of the trawl can require a bit of thought as the diamond meshes in this area will be more open than in the extension.
Estimating the actual mesh opening, and thereby the mesh length at this opening can be extremely difficult. There is no easy solution as the actual mesh opening will vary with the design of trawl and several other criteria will also have an influence on it. If there is a length differential between the diamond and the square mesh it could well affect the fishing efficiency of the trawl. If the square mesh panel is made too wide in relation to the number of diamond meshes across the point where it is fitted or the width is calculated with a presumption that the diamond meshes are more open that they will actually the square mesh panel will be too wide and tend to concertina and make it difficult for any fish to escape through it. It is necessary to carefully calculate the width of the panel in relation to the diamond mesh opening. Just ahead of the cod end or the extension of the trawl, the mesh opening will only be approximately 15% of the mesh length.
Catch Comparison Trials using a 400mm Square Mesh Panel
The effects of square mesh panel mesh size and position on the selectivity of high and low powered Nephrops trawlers