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Twist + Bucketeffect = stickers
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As long as we have operated, fishermen have asked us what the reason might be for why often the fish they see enter the trawl opening never ends up in the cod end, and it does not only apply to pelagic trawling. There can be several reasons; one can be that the towing speed is too low so that the fish stays in the opening and escapes in front of the trawl. But the suspicion has always been that it is usually because of other reasons.
As more and more started to use Scanmar TrawlEye, pelagic trawls also in the belly, in combination with the TrawlSpeed sensor also in the opening and in the belly, we could eliminate some of the problems caused by too low towing speed. |
Stickers in medium large mesh gave us a hint; what happens further ahead where the mesh is so big that the fish easily escapes through it?
We had our suspicions: Bucket effect and Twist!
Scanmar’s TrawlSpeed, Symmetry and Grid sensors have had angle meters implemented since 1985, and especially Symmetry sensors in the trawl opening and TrawlSpeed sensor in the belly gave clear indications that the water flow and fish entrance affected the trawl’s shape in different ways. We were also familiar with thorough tests made in 1985 by the German Institute of Marine Research, which proved the presence of Bucket effect in a pelagic trawl.
We therefore decided to make some simple prototypes of angle meters for testing on trawl doors, trawl, bag, bottom gear etc. and we registered amongst other things that the trawl’s shape could change a lot with different directions of water currents, towing speed and fish entrance. In development of our new sensor generation SS4 ScanSense, multifunctional sensors, we therefore decided that Angle measurements would be implemented as a standard, not just as an additional function, but specifically in an individual sensor, which, granted, can be upgraded with other functions.
With the experience we have gained through 4 years with the use of the Twist sensor, we can determine that Bucket effects arise often with the result of water pressing through the open mesh, and fish escaping.
The Screen dump shows semi pelagic fishing for Alaska Pollack in shallow water (150 meters). Trawl and gear are heavy and thus, together with short warps, the doors are tilted more than 20˚. The change in trawl height is caused by wire being shot in order to achieve a larger vertical opening. The sensor boxes show: (1) mounted on the head line small twist, 2 ˚ to starboard, and 13˚ tilt, and (2) mounted in the middle of the trawl; good balance.
More surprising was it perhaps to register that “all” trawls and trawl bags have the ability to twist, and creates an unfortunate water current in the trawl, which again leads fish out through the open mesh. And the twist can be quite significant, 70° is not unusual. Now it turns out that the trawl straightens when the bag fills, but a lot of catch is lost before that happens.
The trawls we have done experiments with have also shown that the trawl usually has different heights on each side, which affects the water flow back into the trawl. And here the Twist sensor is of great benefit.